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<div class="slides">
<section id="title-slide">
<h1 class="title">Laboratorio di TNDS – Lezione 2</h1>
<p class="author">Maurizio Tomasi</p>
<p class="date">Martedì 1 Ottobre 2024</p>
</section>
<section id="esercizi-per-oggi" class="slide level1">
<h1>Esercizi per oggi</h1>
</section>
<section id="link-alle-risorse-online" class="slide level1">
<h1>Link alle risorse online</h1>
<ul>
<li><p>La spiegazione dettagliata degli esercizi si trova qui: <a
href="carminati-esercizi-02.html">carminati-esercizi-02.html</a>.</p></li>
<li><p>Come al solito, queste slides, che forniscono suggerimenti
addizionali rispetto alla lezione di teoria, sono disponibili
all’indirizzo <a
href="https://ziotom78.github.io/tnds-tomasi-notebooks/">ziotom78.github.io/tnds-tomasi-notebooks</a>.</p></li>
<li><p>Ricordo a quanti usano i computer del laboratorio di
<strong>configurare il compilatore</strong>. Seguite le istruzioni che
avevamo fornito settimana scorsa (<a
href="tomasi-lezione-01.html#/configurare-il-compilatore">link</a>).</p></li>
</ul>
</section>
<section id="esercizi" class="slide level1">
<h1>Esercizi</h1>
<ul>
<li><p>Lo scopo della lezione di oggi è introdurre il concetto di
«classe», che è un tipo di dato complesso del linguaggio C++, creando
una classe <code>Vettore</code> che implementa un array «intelligente»
di valori <code>double</code>.</p></li>
<li><p><a href="carminati-esercizi-02.html#esercizio-2.0">Esercizio
2.0</a>: creazione della classe <code>Vettore</code>.</p></li>
<li><p><a href="carminati-esercizi-02.html#esercizio-2.1">Esercizio
2.1</a>: completamento dell’esercizio 2.0.</p></li>
<li><p><a href="carminati-esercizi-02.html#esercizio-2.2">Esercizio
2.2</a> (da consegnare per l’esame scritto): è lo stesso tipo di
esercizio della scorsa lezione, ma ora occorre usare quanto si è scritto
per gli esercizi 2.0 e 2.1.</p></li>
</ul>
</section>
<section id="velocità-del-codice" class="slide level1">
<h1>Velocità del codice</h1>
</section>
<section id="calcolo-della-varianza" class="slide level1">
<h1>Calcolo della varianza</h1>
<ul>
<li><p>Attenzione a non far fare al vostro codice calcoli
inutili!</p></li>
<li><p>Un errore molto diffuso è quello di implementare il calcolo della
varianza così:</p>
<div class="sourceCode" id="cb1"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="cf">for</span><span class="op">(</span><span class="dt">int</span> i <span class="op">=</span> <span class="dv">0</span><span class="op">;</span> i <span class="op"><</span> N<span class="op">;</span> <span class="op">++</span>i<span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a> accum <span class="op">+=</span> pow<span class="op">(</span>v<span class="op">[</span>i<span class="op">]</span> <span class="op">-</span> calcola_media<span class="op">(</span>v<span class="op">,</span> N<span class="op">),</span> <span class="dv">2</span><span class="op">);</span></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div></li>
<li><p>Il codice invoca <code>calcola_media</code> per <code>N</code>
volte! Molto meglio scrivere così:</p>
<div class="sourceCode" id="cb2"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="dt">double</span> media <span class="op">=</span> calcola_media<span class="op">(</span>v<span class="op">,</span> N<span class="op">);</span></span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a><span class="cf">for</span><span class="op">(</span><span class="dt">int</span> i <span class="op">=</span> <span class="dv">0</span><span class="op">;</span> i <span class="op"><</span> N<span class="op">;</span> <span class="op">++</span>i<span class="op">)</span> <span class="op">{</span></span>
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a> accum <span class="op">+=</span> pow<span class="op">(</span>v<span class="op">[</span>i<span class="op">]</span> <span class="op">-</span> media<span class="op">,</span> <span class="dv">2</span><span class="op">);</span></span>
<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div></li>
</ul>
</section>
<section id="scrittura-di-test" class="slide level1">
<h1>Scrittura di test</h1>
</section>
<section id="verificate-il-vostro-codice" class="slide level1">
<h1>Verificate il vostro codice!</h1>
<pre><code>N = 365:
- Mean : -1.0813335533916488
- Variance : 6.67466568793561 (corrected: 6.693002681583785)
- Standard deviation : 2.5835374369138933 (corrected: 2.587083818043742)
- Median : -0.9156626506024095
N = 10:
- Mean : -1.1889156626506023
- Variance : 4.270508578893889 (corrected: 4.745009532104321)
- Standard deviation : 2.0665208876016448 (corrected: 2.1783042790446703)
- Median : -1.3186746987951807
N = 9:
- Mean : -0.9204819277108434
- Variance : 4.024442831567232 (corrected: 4.527498185513136)
- Standard deviation : 2.0061014011179075 (corrected: 2.12779185671746)
- Median : -0.9</code></pre>
</section>
<section id="verifica-della-correttezza-dei-codici"
class="slide level1">
<h1>Verifica della correttezza dei codici</h1>
<ul>
<li><p>Quando si scrive un programma, è indispensabile verificare che
funzioni.</p></li>
<li><p>La scorsa settimana vi avevo fornito i risultati attesi, che
avete (<strong>spero!</strong>) confrontato con l’output dei vostri
programmi.</p></li>
<li><p>Ma se nelle prossime settimane deciderete di mettere mano ai
vecchi esercizi per migliorarli, dovrete ricontrollare i numeri dopo
ogni modifica del codice!</p></li>
<li><p>Non sarebbe meglio se fosse il computer a fare questi controlli
per voi?</p></li>
</ul>
</section>
<section id="un-semplice-esempio" class="slide level1">
<h1>Un semplice esempio</h1>
<div class="sourceCode" id="cb4"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a><span class="pp">#include </span><span class="im"><iostream></span></span>
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span> calc<span class="op">(</span><span class="dt">int</span> a<span class="op">,</span> <span class="dt">int</span> b<span class="op">)</span> <span class="op">{</span> <span class="cf">return</span> a <span class="op">+</span> b<span class="op">;</span> <span class="op">}</span></span>
<span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb4-5"><a href="#cb4-5" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span> main<span class="op">()</span> <span class="op">{</span></span>
<span id="cb4-6"><a href="#cb4-6" aria-hidden="true" tabindex="-1"></a> <span class="bu">std::</span>cout <span class="op"><<</span> <span class="st">"Inserisci due numeri: "</span><span class="op">;</span></span>
<span id="cb4-7"><a href="#cb4-7" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb4-8"><a href="#cb4-8" aria-hidden="true" tabindex="-1"></a> <span class="dt">int</span> a<span class="op">,</span> b<span class="op">;</span></span>
<span id="cb4-9"><a href="#cb4-9" aria-hidden="true" tabindex="-1"></a> <span class="bu">std::</span>cin <span class="op">>></span> a <span class="op">>></span> b<span class="op">;</span></span>
<span id="cb4-10"><a href="#cb4-10" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb4-11"><a href="#cb4-11" aria-hidden="true" tabindex="-1"></a> <span class="bu">std::</span>cout <span class="op"><<</span> <span class="st">"Il risultato è "</span> <span class="op"><<</span> calc<span class="op">(</span>a<span class="op">,</span> b<span class="op">)</span> <span class="op"><<</span> <span class="st">"</span><span class="sc">\n</span><span class="st">"</span><span class="op">;</span></span>
<span id="cb4-12"><a href="#cb4-12" aria-hidden="true" tabindex="-1"></a> <span class="cf">return</span> <span class="dv">0</span><span class="op">;</span></span>
<span id="cb4-13"><a href="#cb4-13" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
</section>
<section id="un-semplice-esempio-1" class="slide level1">
<h1>Un semplice esempio</h1>
<ul>
<li><p>Per verificare la correttezza del codice, si può eseguire alcune
volte il programma.</p>
<pre class="text"><code>$ g++ test1.cpp -o test1
$ ./test1
Insert two numbers: 4 6
The result is 10
$ ./test1
Insert two numbers: -1 3
The result is 2
$</code></pre></li>
<li><p>Il conto però, come abbiamo visto, va verificato a mano ogni
volta.</p></li>
</ul>
</section>
<section id="test-automatici" class="slide level1">
<h1>Test automatici</h1>
<ul>
<li><p>Si può verificare automaticamente la correttezza del codice con
<code>assert</code>:</p>
<div class="sourceCode" id="cb6"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a><span class="pp">#include </span><span class="im"><cassert></span></span>
<span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb6-3"><a href="#cb6-3" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span> test_sum<span class="op">()</span> <span class="op">{</span></span>
<span id="cb6-4"><a href="#cb6-4" aria-hidden="true" tabindex="-1"></a> <span class="co">// No need to write std::assert, as it is a macro</span></span>
<span id="cb6-5"><a href="#cb6-5" aria-hidden="true" tabindex="-1"></a> <span class="ot">assert</span><span class="op">(</span>sum<span class="op">(</span><span class="dv">4</span><span class="op">,</span> <span class="dv">6</span><span class="op">)</span> <span class="op">==</span> <span class="dv">10</span><span class="op">);</span></span>
<span id="cb6-6"><a href="#cb6-6" aria-hidden="true" tabindex="-1"></a> <span class="ot">assert</span><span class="op">(</span>sum<span class="op">(-</span><span class="dv">1</span><span class="op">,</span> <span class="dv">3</span><span class="op">)</span> <span class="op">==</span> <span class="dv">2</span><span class="op">);</span></span>
<span id="cb6-7"><a href="#cb6-7" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div></li>
<li><p>Una chiamata ad <code>assert</code> viene tradotta più o meno
così:</p>
<div class="sourceCode" id="cb7"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true" tabindex="-1"></a><span class="co">// assert(sum(4, 6) == 10);</span></span>
<span id="cb7-2"><a href="#cb7-2" aria-hidden="true" tabindex="-1"></a><span class="cf">if</span> <span class="op">(!</span> <span class="op">(</span>sum<span class="op">(</span><span class="dv">4</span><span class="op">,</span> <span class="dv">6</span><span class="op">)</span> <span class="op">==</span> <span class="dv">10</span><span class="op">))</span> <span class="op">{</span></span>
<span id="cb7-3"><a href="#cb7-3" aria-hidden="true" tabindex="-1"></a> make_the_program_crash<span class="op">();</span></span>
<span id="cb7-4"><a href="#cb7-4" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div></li>
</ul>
</section>
<section id="test-automatici-1" class="slide level1">
<h1>Test automatici</h1>
<ul>
<li><p>Il resto del codice resta uguale, ma nel <code>main</code> si
deve invocare <code>test_sum()</code> prima di ogni altra cosa:</p>
<div class="sourceCode" id="cb8"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span> main<span class="op">(</span><span class="dt">int</span> argc<span class="op">,</span> <span class="dt">char</span> <span class="op">*</span>argv<span class="op">[])</span> <span class="op">{</span></span>
<span id="cb8-2"><a href="#cb8-2" aria-hidden="true" tabindex="-1"></a> <span class="co">// This must be the very first thing!</span></span>
<span id="cb8-3"><a href="#cb8-3" aria-hidden="true" tabindex="-1"></a> test_sum<span class="op">();</span></span>
<span id="cb8-4"><a href="#cb8-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb8-5"><a href="#cb8-5" aria-hidden="true" tabindex="-1"></a> <span class="co">// Now implement the exercise as requested</span></span>
<span id="cb8-6"><a href="#cb8-6" aria-hidden="true" tabindex="-1"></a> …</span>
<span id="cb8-7"><a href="#cb8-7" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div></li>
<li><p>Potete implementare più funzioni <code>test_*()</code>, che poi
chiamerete una di seguito all’altra nel <code>main</code>.</p></li>
</ul>
</section>
<section id="test-automatici-2" class="slide level1">
<h1>Test automatici</h1>
<ul>
<li><p>Se c’è un errore, il programma si blocca con un messaggio:</p>
<pre class="text"><code>$ ./test2
test2.cpp:4:void test_sum():
Assertion `sum(-1,3) == 2' failed
Aborted (core dumped)
$</code></pre></li>
<li><p><strong>Avvertenza</strong>: questo output si ottiene solo se
avete implementato il <a
href="https://ziotom78.github.io/tnds-tomasi-notebooks/tomasi-lezione-01.html#/flag-del-compilatore">suggerimento
della scorsa lezione</a> e usate il flag <code>-g3</code> nel
<code>Makefile</code>.</p></li>
</ul>
</section>
<section id="test-automatici-3" class="slide level1">
<h1>Test automatici</h1>
<ul>
<li><p>Inserite sempre all’inizio del <code>main</code> una serie di
test.</p></li>
<li><p>Il <code>main</code> dei vostri prossimi esercizi sembrerà
questo:</p>
<div class="sourceCode" id="cb10"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span> main<span class="op">(</span><span class="dt">int</span> argc<span class="op">,</span> <span class="dt">char</span> <span class="op">*</span>argv<span class="op">[])</span> <span class="op">{</span></span>
<span id="cb10-2"><a href="#cb10-2" aria-hidden="true" tabindex="-1"></a> <span class="co">// Put the tests of the functions you're going to use</span></span>
<span id="cb10-3"><a href="#cb10-3" aria-hidden="true" tabindex="-1"></a> test_bisection<span class="op">();</span></span>
<span id="cb10-4"><a href="#cb10-4" aria-hidden="true" tabindex="-1"></a> test_simpson_integral<span class="op">();</span></span>
<span id="cb10-5"><a href="#cb10-5" aria-hidden="true" tabindex="-1"></a> test_random_generator<span class="op">();</span></span>
<span id="cb10-6"><a href="#cb10-6" aria-hidden="true" tabindex="-1"></a> test_hit_or_miss<span class="op">();</span></span>
<span id="cb10-7"><a href="#cb10-7" aria-hidden="true" tabindex="-1"></a> …</span>
<span id="cb10-8"><a href="#cb10-8" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div></li>
<li><p>In questa e nelle lezioni successive vi fornirò una serie di
comandi <code>assert</code> da inserire nei vostri codici: vi aiuteranno
a verificare che l’esercizio sia corretto.</p></li>
</ul>
</section>
<section id="test-sui-floating-point" class="slide level1">
<h1>Test sui floating-point</h1>
<ul>
<li><p>Se provate a scrivere dei test per gli esercizi di queste prime
lezioni, vi imbatterete però in un problema legato ai numeri
<em>floating-point</em>.</p></li>
<li><p>Si può vedere facilmente che i numeri <em>floating-point</em>
sono solo un’approssimazione dei numeri reali. Ad esempio, il numero 0.1
non è rappresentabile esattamente con un <code>float</code> o un
<code>double</code>.</p></li>
<li><p>Occorre fissare una tolleranza <span
class="math inline">\epsilon</span> e verificare che il risultato del
calcolo <span class="math inline">x_\text{calc}</span> differisca dal
valore atteso <span class="math inline">x_\text{exp}</span> per meno di
<span class="math inline">\epsilon</span>, ossia</p>
<p><span class="math display">\left|x_\text{calc} - x_\text{exp}\right|
< \epsilon.</span></p></li>
</ul>
</section>
<section id="esercizio-1.1-assert" class="slide level1">
<h1>Esercizio 1.1: assert</h1>
<div class="sourceCode" id="cb11"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb11-1"><a href="#cb11-1" aria-hidden="true" tabindex="-1"></a><span class="co">// Return true if `calculated` and `expected` differ by less than `epsilon`</span></span>
<span id="cb11-2"><a href="#cb11-2" aria-hidden="true" tabindex="-1"></a><span class="dt">bool</span> are_close<span class="op">(</span><span class="dt">double</span> calculated<span class="op">,</span> <span class="dt">double</span> expected<span class="op">,</span> <span class="dt">double</span> epsilon <span class="op">=</span> <span class="fl">1e-7</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb11-3"><a href="#cb11-3" aria-hidden="true" tabindex="-1"></a> <span class="cf">return</span> fabs<span class="op">(</span>calculated <span class="op">-</span> expected<span class="op">)</span> <span class="op"><</span> epsilon<span class="op">;</span></span>
<span id="cb11-4"><a href="#cb11-4" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span>
<span id="cb11-5"><a href="#cb11-5" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb11-6"><a href="#cb11-6" aria-hidden="true" tabindex="-1"></a><span class="dt">void</span> test_statistical_functions<span class="op">(</span><span class="dt">void</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb11-7"><a href="#cb11-7" aria-hidden="true" tabindex="-1"></a> <span class="dt">double</span> mydata<span class="op">[]</span> <span class="op">=</span> <span class="op">{</span><span class="dv">1</span><span class="op">,</span> <span class="dv">2</span><span class="op">,</span> <span class="dv">3</span><span class="op">,</span> <span class="dv">4</span><span class="op">};</span> <span class="co">// Use these instead of data.dat</span></span>
<span id="cb11-8"><a href="#cb11-8" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb11-9"><a href="#cb11-9" aria-hidden="true" tabindex="-1"></a> <span class="ot">assert</span><span class="op">(</span>are_close<span class="op">(</span>CalcolaMedia<span class="op">(</span>mydata<span class="op">,</span> <span class="dv">4</span><span class="op">),</span> <span class="fl">2.5</span><span class="op">));</span></span>
<span id="cb11-10"><a href="#cb11-10" aria-hidden="true" tabindex="-1"></a> <span class="ot">assert</span><span class="op">(</span>are_close<span class="op">(</span>CalcolaVarianza<span class="op">(</span>mydata<span class="op">,</span> <span class="dv">4</span><span class="op">),</span> <span class="fl">1.25</span><span class="op">));</span></span>
<span id="cb11-11"><a href="#cb11-11" aria-hidden="true" tabindex="-1"></a> <span class="ot">assert</span><span class="op">(</span>are_close<span class="op">(</span>CalcolaMediana<span class="op">(</span>mydata<span class="op">,</span> <span class="dv">4</span><span class="op">),</span> <span class="fl">2.5</span><span class="op">));</span> <span class="co">// Even</span></span>
<span id="cb11-12"><a href="#cb11-12" aria-hidden="true" tabindex="-1"></a> <span class="ot">assert</span><span class="op">(</span>are_close<span class="op">(</span>CalcolaMediana<span class="op">(</span>mydata<span class="op">,</span> <span class="dv">3</span><span class="op">),</span> <span class="dv">2</span><span class="op">));</span> <span class="co">// Odd</span></span>
<span id="cb11-13"><a href="#cb11-13" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb11-14"><a href="#cb11-14" aria-hidden="true" tabindex="-1"></a> <span class="co">// Continue from here …</span></span>
<span id="cb11-15"><a href="#cb11-15" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb11-16"><a href="#cb11-16" aria-hidden="true" tabindex="-1"></a> <span class="co">// At the end, be sure to print a message stating that everything was ok</span></span>
<span id="cb11-17"><a href="#cb11-17" aria-hidden="true" tabindex="-1"></a> cerr <span class="op"><<</span> <span class="st">"All the statistical tests have passed! 🥳</span><span class="sc">\n</span><span class="st">"</span><span class="op">;</span></span>
<span id="cb11-18"><a href="#cb11-18" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p>Questi <code>assert</code> vanno bene anche per gli esercizi di oggi,
con opportuni aggiustamenti (es., usare <code>Vettore</code> anziché
<code>double *</code>).</p>
</section>
<section id="messaggio-alla-fine-dei-test" class="slide level1">
<h1>Messaggio alla fine dei test</h1>
<ul>
<li><p>Se gli <code>assert()</code> ricevono un valore
<code>true</code>, non stampano nulla.</p></li>
<li><p>È meglio però avere un feedback a video, altrimenti potreste non
essere sicuri che i test siano effettivamente stati eseguiti. Ad
esempio, potreste dimenticarvi di chiamare
<code>test_statistical_functions()</code> nel
<code>main()</code>!</p></li>
<li><p>Il codice della slide precedente produce questo messaggio:</p>
<pre><code>All the statistical tests have passed! 🥳</code></pre>
<p>Abituatevi ad aspettarvi questo genere di messaggio in cima ad ogni
esercizio che scriverete d’ora in poi.</p></li>
</ul>
</section>
<section id="esempio-di-output" class="slide level1">
<h1>Esempio di output</h1>
<ul>
<li><p>Se avete sbagliato ad implementare una delle funzioni, questo è
quello che accade quando eseguite il programma:</p>
<pre class="text"><code>$ make
esercizio01.1: esercizio01.1.cpp:53: int main(): ↲
Assertion `are_close(CalcolaMediana(mydata, num), 2.5)' failed.
Aborted (core dumped)
$</code></pre></li>
<li><p>Anche quando avete verificato che gli <code>assert</code> passano
con successo, lasciateli al loro posto: nel caso in cui in futuro
dobbiate modificare l’implementazione delle funzioni (ad esempio per
renderla più veloce), continueranno a fungere da controllo. (Ed è
appagante vedere la faccina che festeggia 🥳!)</p></li>
</ul>
</section>
<section id="assert-negli-esercizi" class="slide level1">
<h1>Assert negli esercizi</h1>
<ul>
<li><p>Le liste di <code>assert</code> che vi fornisco sono state
costruite anno dopo anno, alla luce degli errori che solitamente hanno
fatto i vostri precedenti colleghi nei loro esercizi.</p></li>
<li><p>Non presentatevi all’esame finché non riuscite a far passare
<strong>tutti</strong> gli <code>assert</code> di <strong>tutti</strong>
gli esercizi!</p></li>
<li><p>Molte volte degli studenti hanno presentato uno scritto in cui
avevano usato librerie con errori! E quasi sempre ritrovavo commenti del
genere negli esercizi che consegnavano:</p>
<div class="sourceCode" id="cb14"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb14-1"><a href="#cb14-1" aria-hidden="true" tabindex="-1"></a><span class="co">// Siccome non riuscivo a far passare i test, ho commentato gli assert.</span></span></code></pre></div></li>
</ul>
</section>
<section id="implementazione-di-vettore" class="slide level1">
<h1>Implementazione di <code>Vettore</code></h1>
</section>
<section id="operator-array" class="slide level1">
<h1>Accesso ai dati</h1>
<ul>
<li><p>Il testo dell’esercizio richiede di implementare i metodi
<code>GetComponent</code> e <code>SetComponent</code> per leggere e
scrivere valori nell’array:</p>
<div class="sourceCode" id="cb15"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb15-1"><a href="#cb15-1" aria-hidden="true" tabindex="-1"></a>Vettore v<span class="op">(</span><span class="dv">2</span><span class="op">);</span></span>
<span id="cb15-2"><a href="#cb15-2" aria-hidden="true" tabindex="-1"></a>v<span class="op">.</span>SetComponent<span class="op">(</span><span class="dv">0</span><span class="op">,</span> <span class="fl">162.3</span><span class="op">);</span></span>
<span id="cb15-3"><a href="#cb15-3" aria-hidden="true" tabindex="-1"></a>v<span class="op">.</span>SetComponent<span class="op">(</span><span class="dv">1</span><span class="op">,</span> <span class="fl">431.7</span><span class="op">);</span></span>
<span id="cb15-4"><a href="#cb15-4" aria-hidden="true" tabindex="-1"></a><span class="bu">std::</span>cout <span class="op"><<</span> v<span class="op">.</span>GetComponent<span class="op">(</span><span class="dv">1</span><span class="op">)</span> <span class="op"><<</span> endl<span class="op">;</span> <span class="co">// Print 431.7</span></span></code></pre></div></li>
<li><p>Questo è però più scomodo rispetto ai semplici array:</p>
<div class="sourceCode" id="cb16"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb16-1"><a href="#cb16-1" aria-hidden="true" tabindex="-1"></a><span class="dt">double</span> v<span class="op">[</span><span class="dv">2</span><span class="op">];</span></span>
<span id="cb16-2"><a href="#cb16-2" aria-hidden="true" tabindex="-1"></a>v<span class="op">[</span><span class="dv">0</span><span class="op">]</span> <span class="op">=</span> <span class="fl">162.3</span><span class="op">;</span></span>
<span id="cb16-3"><a href="#cb16-3" aria-hidden="true" tabindex="-1"></a>v<span class="op">[</span><span class="dv">1</span><span class="op">]</span> <span class="op">=</span> <span class="fl">431.7</span><span class="op">;</span></span>
<span id="cb16-4"><a href="#cb16-4" aria-hidden="true" tabindex="-1"></a><span class="bu">std::</span>cout <span class="op"><<</span> v<span class="op">[</span><span class="dv">1</span><span class="op">]</span> <span class="op"><<</span> endl<span class="op">;</span> <span class="co">// Print 431.7</span></span></code></pre></div></li>
</ul>
</section>
<section id="uso-di-operator" class="slide level1">
<h1>Uso di <code>operator[]</code></h1>
<ul>
<li><p>Si può rendere valida la scrittura <code>miovett[3]</code> anche
con oggetti di tipo <code>Vettore</code> implementando il metodo
<code>operator[]</code>:</p>
<div class="sourceCode" id="cb17"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb17-1"><a href="#cb17-1" aria-hidden="true" tabindex="-1"></a><span class="dt">double</span> Vettore<span class="op">::</span><span class="kw">operator</span><span class="op">[](</span><span class="dt">int</span> index<span class="op">)</span> <span class="op">{</span></span>
<span id="cb17-2"><a href="#cb17-2" aria-hidden="true" tabindex="-1"></a> <span class="ot">assert</span><span class="op">(</span>index <span class="op">>=</span> <span class="dv">0</span> <span class="op">&&</span> index <span class="op"><</span> <span class="va">m_size</span><span class="op">);</span></span>
<span id="cb17-3"><a href="#cb17-3" aria-hidden="true" tabindex="-1"></a> <span class="cf">return</span> <span class="va">m_arr</span><span class="op">[</span>index<span class="op">];</span></span>
<span id="cb17-4"><a href="#cb17-4" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div></li>
<li><p>In questo modo la linea di codice
<code>std::cout << miovett[5] << "\n"</code> sarà
equivalente a</p>
<div class="sourceCode" id="cb18"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb18-1"><a href="#cb18-1" aria-hidden="true" tabindex="-1"></a><span class="ot">assert</span><span class="op">(</span><span class="dv">5</span> <span class="op">>=</span> <span class="dv">0</span> <span class="op">&&</span> <span class="dv">5</span> <span class="op"><</span> miovett<span class="op">.</span><span class="va">m_size</span><span class="op">);</span></span>
<span id="cb18-2"><a href="#cb18-2" aria-hidden="true" tabindex="-1"></a><span class="bu">std::</span>cout <span class="op"><<</span> miovett<span class="op">.</span><span class="va">m_arr</span><span class="op">[</span><span class="dv">5</span><span class="op">]</span> <span class="op"><<</span> endl<span class="op">;</span></span></code></pre></div></li>
</ul>
</section>
<section id="uso-di-operator-1" class="slide level1">
<h1>Uso di <code>operator[]</code></h1>
<ul>
<li><p>Se si ritorna un <em>reference</em>, è possibile anche fare
assegnamenti:</p>
<div class="sourceCode" id="cb19"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb19-1"><a href="#cb19-1" aria-hidden="true" tabindex="-1"></a><span class="co">// !</span></span>
<span id="cb19-2"><a href="#cb19-2" aria-hidden="true" tabindex="-1"></a><span class="dt">double</span> <span class="op">&</span> Vettore<span class="op">::</span><span class="kw">operator</span><span class="op">[](</span><span class="dt">int</span> index<span class="op">)</span> <span class="op">{</span></span>
<span id="cb19-3"><a href="#cb19-3" aria-hidden="true" tabindex="-1"></a> <span class="ot">assert</span><span class="op">(</span>index <span class="op">>=</span> <span class="dv">0</span> <span class="op">&&</span> index <span class="op"><</span> <span class="va">m_size</span><span class="op">);</span></span>
<span id="cb19-4"><a href="#cb19-4" aria-hidden="true" tabindex="-1"></a> <span class="cf">return</span> <span class="va">m_arr</span><span class="op">[</span>index<span class="op">];</span></span>
<span id="cb19-5"><a href="#cb19-5" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div></li>
<li><p>Così il programma seguente diventa legale:</p>
<div class="sourceCode" id="cb20"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb20-1"><a href="#cb20-1" aria-hidden="true" tabindex="-1"></a>Vettore v<span class="op">(</span><span class="dv">2</span><span class="op">);</span></span>
<span id="cb20-2"><a href="#cb20-2" aria-hidden="true" tabindex="-1"></a>v<span class="op">[</span><span class="dv">0</span><span class="op">]</span> <span class="op">=</span> <span class="fl">162.3</span><span class="op">;</span> <span class="co">// Assignment, works thanks to the reference</span></span>
<span id="cb20-3"><a href="#cb20-3" aria-hidden="true" tabindex="-1"></a>v<span class="op">[</span><span class="dv">1</span><span class="op">]</span> <span class="op">=</span> <span class="fl">431.7</span><span class="op">;</span> <span class="co">// Ditto</span></span>
<span id="cb20-4"><a href="#cb20-4" aria-hidden="true" tabindex="-1"></a><span class="bu">std::</span>cout <span class="op"><<</span> v<span class="op">[</span><span class="dv">1</span><span class="op">]</span> <span class="op"><<</span> endl<span class="op">;</span> <span class="co">// Print 431.7</span></span></code></pre></div></li>
</ul>
</section>
<section id="uso-di-header-files" class="slide level1">
<h1>Uso di header files</h1>
</section>
<section id="uso-di-header-files-1" class="slide level1">
<h1>Uso di header files</h1>
<ul>
<li><p>Avete visto a lezione l’utilità degli <em>header files</em>,
ossia dei file con estensione <code>.h</code>, <code>.hh</code> o
<code>.hpp</code>.</p></li>
<li><p>Capita spesso che un file sia incluso più volte nel corso di una
stessa compilazione.</p></li>
<li><p>Consideriamo questo esempio:</p>
<div class="sourceCode" id="cb21"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb21-1"><a href="#cb21-1" aria-hidden="true" tabindex="-1"></a><span class="co">// File main.cpp</span></span>
<span id="cb21-2"><a href="#cb21-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb21-3"><a href="#cb21-3" aria-hidden="true" tabindex="-1"></a><span class="pp">#include </span><span class="im">"vettore.h"</span></span>
<span id="cb21-4"><a href="#cb21-4" aria-hidden="true" tabindex="-1"></a><span class="pp">#include </span><span class="im">"statistiche.h"</span></span>
<span id="cb21-5"><a href="#cb21-5" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb21-6"><a href="#cb21-6" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span> main<span class="op">()</span> <span class="op">{</span></span>
<span id="cb21-7"><a href="#cb21-7" aria-hidden="true" tabindex="-1"></a> <span class="co">// ...</span></span>
<span id="cb21-8"><a href="#cb21-8" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div></li>
<li><p>Nel <code>main</code> si usano sia le funzioni dichiarate in
<code>vettore.h</code> che quelle dichiarate in
<code>statistiche.h</code>, quindi ci vogliono entrambi gli
<code>#include</code>.</p></li>
</ul>
</section>
<section id="uso-di-header-files-2" class="slide level1">
<h1>Uso di header files</h1>
<ul>
<li><p>Supponiamo che questo sia il contenuto di
<code>vettore.h</code>:</p>
<div class="sourceCode" id="cb22"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb22-1"><a href="#cb22-1" aria-hidden="true" tabindex="-1"></a><span class="co">// File vettore.h</span></span>
<span id="cb22-2"><a href="#cb22-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb22-3"><a href="#cb22-3" aria-hidden="true" tabindex="-1"></a><span class="kw">class</span> Vettore <span class="op">{</span></span>
<span id="cb22-4"><a href="#cb22-4" aria-hidden="true" tabindex="-1"></a> <span class="co">// ...</span></span>
<span id="cb22-5"><a href="#cb22-5" aria-hidden="true" tabindex="-1"></a><span class="op">};</span></span></code></pre></div>
<p>e questo sia il contenuto di <code>statistiche.h</code>:</p>
<div class="sourceCode" id="cb23"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb23-1"><a href="#cb23-1" aria-hidden="true" tabindex="-1"></a><span class="co">// File statistiche.h</span></span>
<span id="cb23-2"><a href="#cb23-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb23-3"><a href="#cb23-3" aria-hidden="true" tabindex="-1"></a><span class="pp">#include </span><span class="im">"vettore.h"</span></span>
<span id="cb23-4"><a href="#cb23-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb23-5"><a href="#cb23-5" aria-hidden="true" tabindex="-1"></a><span class="dt">double</span> CalcolaMedia<span class="op">(</span><span class="at">const</span> Vettore <span class="op">&</span> vett<span class="op">);</span></span></code></pre></div></li>
</ul>
</section>
<section id="uso-di-header-files-3" class="slide level1">
<h1>Uso di header files</h1>
<p>Compilare il programma <code>main.cpp</code> provocherebbe un errore
di compilazione:</p>
<ol type="1">
<li>Il primo <code>#include</code> definisce la classe
<code>Vettore</code>;</li>
<li>Il secondo <code>#include</code> carica
<code>statistiche.h</code>…</li>
<li>…che a sua volta definisce di nuovo la classe <code>Vettore</code>:
ma il C++ non ammette di definire due classi con lo stesso nome (neppure
se sono identiche!).</li>
</ol>
<div class="sourceCode" id="cb24"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb24-1"><a href="#cb24-1" aria-hidden="true" tabindex="-1"></a><span class="co">// File main.cpp</span></span>
<span id="cb24-2"><a href="#cb24-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb24-3"><a href="#cb24-3" aria-hidden="true" tabindex="-1"></a><span class="pp">#include </span><span class="im">"vettore.h"</span></span>
<span id="cb24-4"><a href="#cb24-4" aria-hidden="true" tabindex="-1"></a><span class="pp">#include </span><span class="im">"statistiche.h"</span></span>
<span id="cb24-5"><a href="#cb24-5" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb24-6"><a href="#cb24-6" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span> main<span class="op">()</span> <span class="op">{</span></span>
<span id="cb24-7"><a href="#cb24-7" aria-hidden="true" tabindex="-1"></a> <span class="co">// ...</span></span>
<span id="cb24-8"><a href="#cb24-8" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
</section>
<section id="uso-di-header-files-4" class="slide level1">
<h1>Uso di header files</h1>
<p>In pratica, <code>g++</code> è come se vedesse questo codice:</p>
<div class="sourceCode" id="cb25"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb25-1"><a href="#cb25-1" aria-hidden="true" tabindex="-1"></a><span class="co">// Questo viene da #include "vettore.h"</span></span>
<span id="cb25-2"><a href="#cb25-2" aria-hidden="true" tabindex="-1"></a><span class="kw">class</span> Vettore <span class="op">{</span></span>
<span id="cb25-3"><a href="#cb25-3" aria-hidden="true" tabindex="-1"></a> <span class="co">// ...</span></span>
<span id="cb25-4"><a href="#cb25-4" aria-hidden="true" tabindex="-1"></a><span class="op">};</span></span>
<span id="cb25-5"><a href="#cb25-5" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb25-6"><a href="#cb25-6" aria-hidden="true" tabindex="-1"></a><span class="co">// Questo viene dal secondo #include</span></span>
<span id="cb25-7"><a href="#cb25-7" aria-hidden="true" tabindex="-1"></a><span class="kw">class</span> Vettore <span class="op">{</span></span>
<span id="cb25-8"><a href="#cb25-8" aria-hidden="true" tabindex="-1"></a> <span class="co">// ...</span></span>
<span id="cb25-9"><a href="#cb25-9" aria-hidden="true" tabindex="-1"></a><span class="op">};</span></span>
<span id="cb25-10"><a href="#cb25-10" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb25-11"><a href="#cb25-11" aria-hidden="true" tabindex="-1"></a><span class="dt">double</span> CalcolaMedia<span class="op">(</span><span class="at">const</span> Vettore <span class="op">&</span> vett<span class="op">);</span></span>
<span id="cb25-12"><a href="#cb25-12" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb25-13"><a href="#cb25-13" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span> main<span class="op">()</span> <span class="op">{</span> <span class="co">/* ... */</span> <span class="op">}</span></span></code></pre></div>
</section>
<section id="header-guards" class="slide level1">
<h1><em>Header guards</em></h1>
<ul>
<li><p>Questo è un problema che risale ai primordi del linguaggio C
(fine anni ’60), ed è stato storicamente risolto con l’uso di <em>header
guards</em>:</p>
<div class="sourceCode" id="cb26"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb26-1"><a href="#cb26-1" aria-hidden="true" tabindex="-1"></a><span class="co">// File vettore.h</span></span>
<span id="cb26-2"><a href="#cb26-2" aria-hidden="true" tabindex="-1"></a><span class="pp">#ifndef __VETTORE_H__</span></span>
<span id="cb26-3"><a href="#cb26-3" aria-hidden="true" tabindex="-1"></a><span class="pp">#define __VETTORE_H__</span></span>
<span id="cb26-4"><a href="#cb26-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb26-5"><a href="#cb26-5" aria-hidden="true" tabindex="-1"></a><span class="kw">class</span> Vettore <span class="op">{</span></span>
<span id="cb26-6"><a href="#cb26-6" aria-hidden="true" tabindex="-1"></a> <span class="co">// ...</span></span>
<span id="cb26-7"><a href="#cb26-7" aria-hidden="true" tabindex="-1"></a><span class="op">};</span></span>
<span id="cb26-8"><a href="#cb26-8" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb26-9"><a href="#cb26-9" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span></code></pre></div>
<p>In questo modo, la seconda volta che il file viene incluso verrà
saltato. L’identificatore <code>__VETTORE_H__</code> è
arbitrario.</p></li>
</ul>
</section>
<section id="pragma-once" class="slide level1">
<h1><code>#pragma once</code></h1>
<ul>
<li><p>I recenti compilatori C++, incluso il <code>g++</code>,
permettono un’alternativa più semplice alle <em>header
guards</em>.</p></li>
<li><p>La seguente scrittura è più agile e mette al riparo da
errori:</p>
<div class="sourceCode" id="cb27"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb27-1"><a href="#cb27-1" aria-hidden="true" tabindex="-1"></a><span class="pp">#pragma once </span><span class="co">// Questo file sarà incluso una volta sola</span></span>
<span id="cb27-2"><a href="#cb27-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb27-3"><a href="#cb27-3" aria-hidden="true" tabindex="-1"></a><span class="kw">class</span> Vettore <span class="op">{</span></span>
<span id="cb27-4"><a href="#cb27-4" aria-hidden="true" tabindex="-1"></a> <span class="co">// ...</span></span>
<span id="cb27-5"><a href="#cb27-5" aria-hidden="true" tabindex="-1"></a><span class="op">};</span></span></code></pre></div>
<p>È più comoda perché si deve aggiungere una sola riga senza dover
inventare un identificatore (<code>__VETTORE_H__</code>).</p></li>
</ul>
</section>
<section id="possibili-errori" class="slide level1">
<h1>Possibili errori</h1>
<p>L’uso di <code>#pragma once</code> mette al riparo anche da una serie
di errori che gli studenti di questo corso fanno spesso.</p>
<div class="sourceCode" id="cb28"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb28-1"><a href="#cb28-1" aria-hidden="true" tabindex="-1"></a><span class="co">// Primo esempio</span></span>
<span id="cb28-2"><a href="#cb28-2" aria-hidden="true" tabindex="-1"></a><span class="pp">#ifndef __vettore_h__</span></span>
<span id="cb28-3"><a href="#cb28-3" aria-hidden="true" tabindex="-1"></a><span class="pp">#define __vettore_h_</span></span>
<span id="cb28-4"><a href="#cb28-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb28-5"><a href="#cb28-5" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span>
<span id="cb28-6"><a href="#cb28-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb28-7"><a href="#cb28-7" aria-hidden="true" tabindex="-1"></a><span class="co">// Secondo esempio</span></span>
<span id="cb28-8"><a href="#cb28-8" aria-hidden="true" tabindex="-1"></a><span class="pp">#ifdef __vettore_h__</span></span>
<span id="cb28-9"><a href="#cb28-9" aria-hidden="true" tabindex="-1"></a><span class="pp">#define __vettore_h__</span></span>
<span id="cb28-10"><a href="#cb28-10" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb28-11"><a href="#cb28-11" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span>
<span id="cb28-12"><a href="#cb28-12" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb28-13"><a href="#cb28-13" aria-hidden="true" tabindex="-1"></a><span class="co">// Terzo esempio</span></span>
<span id="cb28-14"><a href="#cb28-14" aria-hidden="true" tabindex="-1"></a><span class="pp">#ifdef __vettore_h__</span></span>
<span id="cb28-15"><a href="#cb28-15" aria-hidden="true" tabindex="-1"></a><span class="pp">#define __Vettore_h__</span></span>
<span id="cb28-16"><a href="#cb28-16" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb28-17"><a href="#cb28-17" aria-hidden="true" tabindex="-1"></a><span class="pp">#endif</span></span></code></pre></div>
</section>
<section id="move-semantics" class="slide level1">
<h1>Costruttori, move semantics, etc.</h1>
</section>
<section id="costruzione-di-una-variabile" class="slide level1">
<h1>«Costruzione» di una variabile</h1>
<ul>
<li><p>Ci sono due passaggi che il compilatore C++ compie quando si
introduce una variabile nel codice:</p>
<ol type="1">
<li>Allocazione della memoria necessaria;</li>
<li>Inizializzazione della memoria.</li>
</ol></li>
<li><p>Esempio:</p>
<div class="sourceCode" id="cb29"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb29-1"><a href="#cb29-1" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span> x<span class="op">;</span> <span class="co">// Allocation</span></span>
<span id="cb29-2"><a href="#cb29-2" aria-hidden="true" tabindex="-1"></a>x <span class="op">=</span> <span class="dv">15</span><span class="op">;</span> <span class="co">// Initialization</span></span>
<span id="cb29-3"><a href="#cb29-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb29-4"><a href="#cb29-4" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span> y <span class="op">=</span> <span class="dv">30</span><span class="op">;</span> <span class="co">// Shortcut, but there are still *two* operations here.</span></span></code></pre></div></li>
</ul>
</section>
<section id="costruzione-di-vettore" class="slide level1">
<h1>«Costruzione» di <code>Vettore</code></h1>
<ul>
<li><p>Le classi, a differenza di <code>int</code>, richiedono
l’invocazione di un costruttore.</p></li>
<li><p>Il costruttore va invocato quando si dichiara la variabile: non è
possibile «differire» l’inizializzazione:</p>
<div class="sourceCode" id="cb30"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb30-1"><a href="#cb30-1" aria-hidden="true" tabindex="-1"></a><span class="dt">int</span> x<span class="op">;</span></span>
<span id="cb30-2"><a href="#cb30-2" aria-hidden="true" tabindex="-1"></a>x <span class="op">=</span> <span class="dv">15</span><span class="op">;</span> <span class="co">// Ok: first allocate, then initialize</span></span>
<span id="cb30-3"><a href="#cb30-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb30-4"><a href="#cb30-4" aria-hidden="true" tabindex="-1"></a>Vettore v<span class="op">;</span></span>
<span id="cb30-5"><a href="#cb30-5" aria-hidden="true" tabindex="-1"></a>v<span class="op">(</span><span class="dv">10</span><span class="op">);</span> <span class="co">// Error, you cannot call the constructor here!</span></span>
<span id="cb30-6"><a href="#cb30-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb30-7"><a href="#cb30-7" aria-hidden="true" tabindex="-1"></a>Vettore w<span class="op">(</span><span class="dv">10</span><span class="op">);</span> <span class="co">// Ok: allocate, then initialize</span></span></code></pre></div></li>
<li><p>I costruttori possono richiedere molto tempo per essere eseguiti,
ad esempio se al loro interno invocano <code>new</code> (com’è il caso
di <code>Vettore</code>).</p></li>
</ul>
</section>
<section id="analogia" class="slide level1">
<h1>Analogia</h1>
<p><img data-src="images/empty_room.jpg" style="width:20.0%" /> <img
data-src="images/furnished_room.jpg" style="width:20.0%" /></p>
<p>Immaginiamo che una variabile sia come un appartamento. Una volta
allocata, è come quando gli imbianchini e i piastrellisti hanno appena
terminato il lavoro. Quando poi viene chiamato il costruttore della
variabile, è come se la casa venisse arredata.</p>
</section>
<section id="costruttori" class="slide level1">
<h1>Costruttori</h1>
<p><img data-src="images/empty_room.jpg" style="width:20.0%" /> <img
data-src="images/furnished_room.jpg" style="width:20.0%" /></p>
<div class="sourceCode" id="cb31"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb31-1"><a href="#cb31-1" aria-hidden="true" tabindex="-1"></a>Vettore<span class="op">::</span>Vettore<span class="op">(</span><span class="dt">int</span> n<span class="op">)</span> <span class="op">:</span> <span class="va">m_N</span><span class="op">(</span>n<span class="op">)</span> <span class="op">{</span></span>
<span id="cb31-2"><a href="#cb31-2" aria-hidden="true" tabindex="-1"></a> <span class="va">m_v</span> <span class="op">=</span> <span class="kw">new</span> <span class="dt">double</span><span class="op">[</span><span class="va">m_N</span><span class="op">];</span> <span class="co">// Allocate (build up the building)</span></span>
<span id="cb31-3"><a href="#cb31-3" aria-hidden="true" tabindex="-1"></a> <span class="cf">for</span><span class="op">(</span><span class="dt">int</span> i <span class="op">=</span> <span class="dv">0</span><span class="op">;</span> i <span class="op"><</span> <span class="va">m_N</span><span class="op">;</span> <span class="op">++</span>i<span class="op">)</span> <span class="co">// Initialize (add the furniture)</span></span>
<span id="cb31-4"><a href="#cb31-4" aria-hidden="true" tabindex="-1"></a> <span class="va">m_v</span><span class="op">[</span>i<span class="op">]</span> <span class="op">=</span> <span class="fl">0.0</span><span class="op">;</span></span>
<span id="cb31-5"><a href="#cb31-5" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
</section>
<section id="costruttori-di-copia" class="slide level1">
<h1>Costruttori di copia</h1>
<p><img data-src="images/empty_room.jpg" style="width:20.0%" /> <img
data-src="images/furnished_room.jpg" style="width:20.0%" /></p>
<p>Un <em>copy constructor</em> corrisponde all’azione di costruire una
stanza vuota identica alla prima (ossia, di tipo <code>Vettore</code>),
e riempirla con gli stessi identici mobili (ossia, assegnando lo stesso
valore a <code>m_N</code> e agli elementi di <code>m_v</code>): è come
se volessi arredare due camere di un albergo in modo che siano
identiche.</p>
</section>
<section id="costruttori-di-copia-1" class="slide level1">
<h1>Costruttori di copia</h1>
<p><img data-src="images/empty_room.jpg" style="width:20.0%" /> <img
data-src="images/furnished_room.jpg" style="width:20.0%" /></p>
<div class="sourceCode" id="cb32"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb32-1"><a href="#cb32-1" aria-hidden="true" tabindex="-1"></a>Vettore<span class="op">::</span>Vettore<span class="op">(</span>const Vettore <span class="op">&</span>vett<span class="op">)</span> <span class="op">:</span> <span class="va">m_N</span><span class="op">(</span>vett<span class="op">.</span><span class="va">m_N</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb32-2"><a href="#cb32-2" aria-hidden="true" tabindex="-1"></a> <span class="va">m_v</span> <span class="op">=</span> new double<span class="op">[</span><span class="va">m_N</span><span class="op">];</span> <span class="co">// Allocate (build up the building)</span></span>
<span id="cb32-3"><a href="#cb32-3" aria-hidden="true" tabindex="-1"></a> <span class="cf">for</span><span class="op">(</span>int i <span class="op">=</span> <span class="dv">0</span><span class="op">;</span> i <span class="op"><</span> <span class="va">m_N</span><span class="op">;</span> <span class="op">++</span>i<span class="op">)</span> <span class="co">// Initialize (add a copy of the furniture</span></span>
<span id="cb32-4"><a href="#cb32-4" aria-hidden="true" tabindex="-1"></a> <span class="va">m_v</span><span class="op">[</span>i<span class="op">]</span> <span class="op">=</span> vett<span class="op">.</span><span class="va">m_v</span><span class="op">[</span>i<span class="op">];</span> <span class="co">// that was used in the old building)</span></span>
<span id="cb32-5"><a href="#cb32-5" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
</section>
<section id="operazione-di-assegnamento" class="slide level1">
<h1>Operazione di assegnamento</h1>
<p><img data-src="images/empty_room.jpg" style="width:20.0%" /> <img
data-src="images/furnished_room.jpg" style="width:20.0%" /></p>
<p>Con una operazione di assegnamento (<code>operator=</code>), abbiamo
due stanze già arredate (variabili già inizializzate), che vogliamo
rendere identiche. Dobbiamo quindi prima <em>svuotare</em> la stanza di
destinazione perché è piena, e solo dopo arredarla allo stesso modo
dell’altra.</p>
</section>
<section id="operazione-di-assegnamento-1" class="slide level1">
<h1>Operazione di assegnamento</h1>
<p><img data-src="images/empty_room.jpg" style="width:20.0%" /> <img
data-src="images/furnished_room.jpg" style="width:20.0%" /></p>
<div class="sourceCode" id="cb33"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb33-1"><a href="#cb33-1" aria-hidden="true" tabindex="-1"></a>Vettore <span class="op">&</span> Vettore<span class="op">::</span><span class="kw">operator</span><span class="op">=(</span>const Vettore <span class="op">&</span>vett<span class="op">)</span> <span class="op">{</span></span>
<span id="cb33-2"><a href="#cb33-2" aria-hidden="true" tabindex="-1"></a> <span class="va">m_N</span> <span class="op">=</span> vett<span class="op">.</span><span class="va">m_N</span><span class="op">;</span></span>
<span id="cb33-3"><a href="#cb33-3" aria-hidden="true" tabindex="-1"></a> <span class="cf">if</span><span class="op">(</span><span class="va">m_v</span><span class="op">)</span> <span class="kw">delete</span><span class="op">[]</span> <span class="va">m_v</span><span class="op">;</span> <span class="co">// Put the old building in the garbage!</span></span>
<span id="cb33-4"><a href="#cb33-4" aria-hidden="true" tabindex="-1"></a> <span class="va">m_v</span> <span class="op">=</span> <span class="kw">new</span> <span class="dt">double</span><span class="op">[</span><span class="va">m_N</span><span class="op">];</span> <span class="co">// Create a new building</span></span>
<span id="cb33-5"><a href="#cb33-5" aria-hidden="true" tabindex="-1"></a> <span class="cf">for</span><span class="op">(</span><span class="dt">int</span> i <span class="op">=</span> <span class="dv">0</span><span class="op">;</span> i <span class="op"><</span> <span class="va">m_N</span><span class="op">;</span> <span class="op">++</span>i<span class="op">)</span> <span class="op">{</span> <span class="co">// Fill the rooms with a copy of the</span></span>
<span id="cb33-6"><a href="#cb33-6" aria-hidden="true" tabindex="-1"></a> <span class="va">m_v</span><span class="op">[</span>i<span class="op">]</span> <span class="op">=</span> vett<span class="op">.</span><span class="va">m_v</span><span class="op">[</span>i<span class="op">];</span> <span class="co">// old furniture</span></span>
<span id="cb33-7"><a href="#cb33-7" aria-hidden="true" tabindex="-1"></a> <span class="op">}</span></span>
<span id="cb33-8"><a href="#cb33-8" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
</section>
<section id="operazione-di-assegnamento-2" class="slide level1">
<h1>Operazione di assegnamento</h1>
<p><img data-src="images/empty_room.jpg" style="width:20.0%" /> <img
data-src="images/furnished_room.jpg" style="width:20.0%" /></p>
<div class="sourceCode" id="cb34"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb34-1"><a href="#cb34-1" aria-hidden="true" tabindex="-1"></a>Vettore v1<span class="op">(</span><span class="dv">10</span><span class="op">),</span> v2<span class="op">(</span><span class="dv">30</span><span class="op">);</span></span>
<span id="cb34-2"><a href="#cb34-2" aria-hidden="true" tabindex="-1"></a><span class="co">// …here I initialize v1 and v2, and I use them for some time.</span></span>
<span id="cb34-3"><a href="#cb34-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb34-4"><a href="#cb34-4" aria-hidden="true" tabindex="-1"></a>v1 <span class="op">=</span> v2<span class="op">;</span> <span class="co">// Assignment through Vettore::operator=</span></span></code></pre></div>
</section>
<section id="move-constructor" class="slide level1">
<h1>Move constructor</h1>
<p><img data-src="images/furnished_room.jpg" style="width:20.0%" /> <img
data-src="images/empty_room.jpg" style="width:20.0%" /></p>
<p>Il <em>move constructor</em> è stato introdotto nel C++11, e
corrisponde a un <strong>trasloco</strong>: ho una stanza già arredata e
una vuota, e voglio spostare i mobili dalla prima alla seconda, senza
comprarne di nuovi: così non spreco nulla!</p>
</section>
<section id="move-constructor-1" class="slide level1">
<h1>Move constructor</h1>
<p><img data-src="images/empty_room.jpg" style="width:20.0%" /> <img
data-src="images/furnished_room.jpg" style="width:20.0%" /></p>
<div class="sourceCode" id="cb35"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb35-1"><a href="#cb35-1" aria-hidden="true" tabindex="-1"></a>Vettore<span class="op">::</span>Vettore<span class="op">(</span>Vettore <span class="op">&&</span> vett<span class="op">)</span> <span class="op">:</span> <span class="va">m_n</span><span class="op">(</span>vett<span class="op">.</span><span class="va">m_N</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb35-2"><a href="#cb35-2" aria-hidden="true" tabindex="-1"></a> <span class="co">// No need for "delete vett.m_v": I want to keep the old furniture!</span></span>
<span id="cb35-3"><a href="#cb35-3" aria-hidden="true" tabindex="-1"></a> <span class="va">m_v</span> <span class="op">=</span> vett<span class="op">.</span><span class="va">m_v</span><span class="op">;</span> <span class="co">// No need to call "new": very fast!</span></span>
<span id="cb35-4"><a href="#cb35-4" aria-hidden="true" tabindex="-1"></a> <span class="co">// No "for" loop to copy the elements: very fast!</span></span>
<span id="cb35-5"><a href="#cb35-5" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
</section>
<section id="move-constructor-2" class="slide level1">
<h1>Move constructor</h1>
<p><img data-src="images/empty_room.jpg" style="width:20.0%" /> <img
data-src="images/furnished_room.jpg" style="width:20.0%" /></p>
<div class="sourceCode" id="cb36"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb36-1"><a href="#cb36-1" aria-hidden="true" tabindex="-1"></a><span class="co">// Function "Read" creates a Vector and fill it with data read from file</span></span>
<span id="cb36-2"><a href="#cb36-2" aria-hidden="true" tabindex="-1"></a>Vettore Read<span class="op">(</span><span class="dt">int</span> ndata<span class="op">,</span> <span class="at">const</span> <span class="dt">char</span> <span class="op">*</span> filename<span class="op">)</span> <span class="op">{</span></span>
<span id="cb36-3"><a href="#cb36-3" aria-hidden="true" tabindex="-1"></a> Vettore result<span class="op">(</span>ndata<span class="op">);</span></span>
<span id="cb36-4"><a href="#cb36-4" aria-hidden="true" tabindex="-1"></a> <span class="co">// …</span></span>
<span id="cb36-5"><a href="#cb36-5" aria-hidden="true" tabindex="-1"></a> <span class="cf">return</span> result<span class="op">;</span></span>
<span id="cb36-6"><a href="#cb36-6" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span>
<span id="cb36-7"><a href="#cb36-7" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb36-8"><a href="#cb36-8" aria-hidden="true" tabindex="-1"></a>Vettore v <span class="op">=</span> Read<span class="op">(</span>ndata<span class="op">,</span> filename<span class="op">);</span></span></code></pre></div>
</section>
<section id="distinguere-tra-i-casi" class="slide level1">
<h1>Distinguere tra i casi</h1>
<div class="sourceCode" id="cb37"><pre
class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb37-1"><a href="#cb37-1" aria-hidden="true" tabindex="-1"></a>Vettore v1<span class="op">(</span><span class="dv">10</span><span class="op">);</span> <span class="co">// Constructor</span></span>
<span id="cb37-2"><a href="#cb37-2" aria-hidden="true" tabindex="-1"></a>Vettore v2<span class="op">(</span>v1<span class="op">);</span> <span class="co">// Copy constructor</span></span>
<span id="cb37-3"><a href="#cb37-3" aria-hidden="true" tabindex="-1"></a>Vettore v3 <span class="op">=</span> v1<span class="op">;</span> <span class="co">// Copy constructor</span></span>
<span id="cb37-4"><a href="#cb37-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb37-5"><a href="#cb37-5" aria-hidden="true" tabindex="-1"></a>v3 <span class="op">=</span> v1<span class="op">;</span> <span class="co">// Assignment</span></span>
<span id="cb37-6"><a href="#cb37-6" aria-hidden="true" tabindex="-1"></a>Vettore v4 <span class="op">=</span> Read<span class="op">(</span><span class="dv">10</span><span class="op">,</span> <span class="st">"data.dat"</span><span class="op">);</span> <span class="co">// Move constructor</span></span>
<span id="cb37-7"><a href="#cb37-7" aria-hidden="true" tabindex="-1"></a> <span class="co">// (at the end of the call to "Read")</span></span></code></pre></div>
</section>
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