ENH: Replace magic number by appropriate definition (from PR review)

gdal/frmts/cderived/cderiveddataset.cpp

@@ -56,7 +56,8 @@ GDALDataset * ComplexDerivedDataset::Open(GDALOpenInfo * poOpenInfo)
   CPLString filename(poOpenInfo->pszFilename);
 
   /* DERIVED_SUBDATASET should be first domain */
-  size_t dsds_pos = filename.find("DERIVED_SUBDATASET:");
+  const size_t dsds_pos = filename.find("DERIVED_SUBDATASET:");
+  const size_t nPrefixLen = strlen("DERIVED_SUBDATASET:");
 
   if (dsds_pos == std::string::npos)
     {
@@ -65,14 +66,14 @@ GDALDataset * ComplexDerivedDataset::Open(GDALOpenInfo * poOpenInfo)
     }
 
   /* Next, we need to now which derived dataset to compute */
-  size_t alg_pos = filename.find(":",dsds_pos+20);
+  const size_t alg_pos = filename.find(":",dsds_pos+20);
   if (alg_pos == std::string::npos)
     {
     /* Unable to Open if we do not find the name of the derived dataset */
     return NULL;
     }
 
-  CPLString odDerivedName = filename.substr(dsds_pos+19,alg_pos-dsds_pos-19);
+  CPLString odDerivedName = filename.substr(dsds_pos+nPrefixLen,alg_pos-dsds_pos-nPrefixLen);
 
   CPLDebug("ComplexDerivedDataset::Open","Derived dataset requested: %s",odDerivedName.c_str());