overloaded.h

#pragma once

/** @file
The "overloaded" variant-visit pattern.

The C++ std::visit (typeswitch dispatch) is very useful for std::variant but
doesn't support it natively.  There is a commonly used two-line boilerplate
that bridges this gap; see
https://en.cppreference.com/w/cpp/utility/variant/visit

This file should be included by classes that wish to
use the variant visit pattern, i.e.

@code {.cpp}
  using MyVariant = std::variant<int, std::string>;
  MyVariant v = 5;
  std::string result = std::visit<const char*>(overloaded{
    [](const int arg) { return "found an int"; },
    [](const std::string& arg) { return "found a string"; }
  }, v);
  EXPECT_EQ(result, "found an int");
@endcode

@warning This file must never be included by a header, only by a cc file.
         This is enforced by a lint rule in `tools/lint/drakelint.py`.
*/

// We put this in the anonymous namespace to ensure that it will not generate
// linkable symbols for every specialization in every library that uses it.
namespace {  // NOLINT(build/namespaces_headers)

// Boilerplate for `std::visit`; see
// https://en.cppreference.com/w/cpp/utility/variant/visit
template <class... Ts>
struct overloaded : Ts... {
  using Ts::operator()...;
};

// Even though Drake requires C++20, Clang versions prior to 17 do not implement
// P1816 yet, so we need to write out a deduction guide. We can remove this once
// Drake requires Clang 17 or newer (as well as the matching Apple Clang).
template <class... Ts>
overloaded(Ts...) -> overloaded<Ts...>;

}  // namespace