Template Specialization

There may be performance advantages to treating some combinations of template arguments as a special case, as in the following examples for twice. Alternatively, a template description might fail to work for a set of its possible arguments, as in the following examples for sort. Template specialization allows you to define alternative implementations for a given combination of actual template arguments. The template specialization overrides the default instantiation.

Template Specialization Declaration

You must declare a specialization before any use of that combination of template arguments. The following examples declare specialized implementations of twice and sort.

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template <> unsigned twice<unsigned>( unsigned original );

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template <> sort<char*>( Array<char*> store );

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You can omit the template arguments if the compiler can unambiguously determine them. For example:

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template <> unsigned twice( unsigned original );

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template <> sort( Array<char*> store );

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Template Specialization Definition

You must define all template specializations that you declare. The following examples define the functions declared in the preceding section.

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template <> unsigned twice<unsigned>( unsigned original )
    { return original << 1; }

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#include <string.h>
template <> void sort<char*>( Array<char*> store )
    { int num_elems = store.GetSize( );
      for ( int i = 0;  i < num_elems-1;  i++ )
          for ( int j = i+1;  j < num_elems;  j++ )
              if ( strcmp( store[j-1], store[j] ) > 0 )
                  { char *temp = store[j];
                    store[j] = store[j-1];
                    store[j-1] = temp; } }

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Template Specialization Use and Instantiation

A specialization is used and instantiated just as any other template, except that the definition of a completely specialized template is also an instantiation.