The compiler cannot generate code for a template until you:

• declare the template class
• provide a template definition
• specify the data type(s) for the template

Specifying the data type(s) and other arguments for a template is called instantiating the template. CodeWarrior C++ gives you two ways to instantiate a template. You can let the compiler instantiate it automatically when you first use it, or you can explicitly create all the instantiations you'll need.

To instantiate templates automatically, include the template definition file in all the source files that use the template, and just use the template members as you would any other type or function. The compiler automatically generates code for a template instantiation whenever it sees a new one. Listing 4.11 shows how to automatically instantiate the templates in Listing 4.8 and Listing 4.9, class Templ and class Max.

#include <iostreams.h>
#include "templ.cp" // includes templ.h as well

void main(void) {
  Templ<long> a = 1, b = 2;
    // The compiler instantiates Templ<long> here.
  cout << Max(a.Get(), b.Get());
    // The compiler instantiates Max<long>() here.
}

If you use automatic instantiation, the compiler may take longer to compile your program because it has to determine on its own which instantiations you'll need. Also, the object code for the template instantiations will be scattered throughout your program.

To instantiate templates explicitly, include the template definition file in a source file, and write a template instantiation statement for every instantiation. The syntax for a class template instantiation is

template class class-name<templ-specs>;

The syntax for a function template instantiation is

template return-type func-name<templ-specs>(arg-specs)

Listing 4.12 shows how to explicitly instantiate the templates in Listing 4.8 and Listing 4.9.

#include "templ.cp"

template class Templ<long>; // class instantiation
template long Max<long>(long,long); // function instantiation

When you're explicitly instantiating a function, you do not need to include in templ-specs any arguments that the compiler can deduce from arg-specs. For example, in Listing 4.12 you can instantiate Max<long>() like this:

template long Max<>(long, long);
  // The compiler can tell from the arguments
  // that you're instantiating Max<long>()

If you use explicit instantiation, the compiler compiles your program more quickly. Because the instantiations can be in one file with no other code, you can even choose to put them in a separate library.