Instantiations and Specializations

To extend your understanding of templates, let’s investigate the terms instantiation and specialization. Keep in mind that including a function template in your code does not in itself generate a function definition. It’s merely a plan for generating a function definition. When the compiler uses the template to generate a function definition for a particular type, the result is termed an instantiation of the template. For example, in Listing 8.13, the function call Swap(i,j) causes the compiler to generate an instantiation of Swap(), using int as the type. The template is not a function definition, but the specific instantiation using int is a function definition. This type of instantiation is termed implicit instantiation because the compiler deduces the necessity for making the definition by noting that the program uses a Swap() function with int parameters.

Originally, using implicit instantiation was the only way the compiler generated function definitions from templates, but now C++ allows for explicit instantiation. That means you can instruct the compiler to create a particular instantiation—for example, Swap<int>()—directly. The syntax is to declare the particular variety you want, using the <> notation to indicate the type and prefixing the declaration with the keyword template:

template void Swap<int>(int, int);  // explicit instantiation

A compiler that implements this feature will, upon seeing this declaration, use the Swap() template to generate an instantiation, using the int type. That is, this declaration means “Use the Swap() template to generate a function definition for the int type.” Contrast the explicit instantiation with the explicit specialization, which uses one or the other of these equivalent declarations:

template <> void Swap<int>(int &, int &);  // explicit specialization
template <> void Swap(int &, int &);       // explicit specialization

The difference is that these last two declarations mean “Don’t use the Swap() template to generate a function definition. Instead, use a separate, specialized function definition explicitly defined for the int type.” These prototypes have to be coupled with their own function definitions. The explicit specialization declaration has <> after the keyword template, whereas the explicit instantiation omits the <>.

Explicit instantiations also can be created by using the function in a program. For instance, consider the following:

template <class T>
T Add(T a, T b)    // pass by value
{
    return a + b;
}
...
int m = 6;
double x = 10.2;
cout << Add<double>(x, m) << endl;  // explicit instantiation

The template would fail to match the function call Add(x, m) because the template expects both function arguments to be of the same type. But using Add<double>(x, m) forces the type double instantiation, and the argument m is type cast to type double to match the second parameter of the Add<double>(double, double) function.

What if you do something similar with Swap()?

int m = 5;
double x  = 14.3;
Swap<double>(m, x);  // almost works

This generates an explicit instantiation for type double. Unfortunately, in this case, the code won’t work because the first formal parameter, being type double &, can’t refer to the type int variable m.

Implicit instantiations, explicit instantiations, and explicit specializations collectively are termed specializations. What they all have in common is that they represent a function definition that uses specific types rather than one that is a generic description.

The addition of the explicit instantiation led to the new syntax of using template and template <> prefixes in declarations to distinguish between the explicit instantiation and the explicit specialization. As in many other cases, the cost of doing more is more syntax rules. The following fragment summarizes these concepts:

...
template <class T>
void Swap (T &, T &);  // template prototype

template <> void Swap<job>(job &, job &);   // explicit specialization for job
int main(void)
{
  template void Swap<char>(char &, char &); // explicit instantiation for char
  short a, b;
  ...
  Swap(a,b);    // implicit template instantiation for short
  job n, m;
  ...
  Swap(n, m);   // use explicit specialization for job
  char g, h;
  ...
  Swap(g, h);  // use explicit template instantiation for char
  ...
}

When the compiler reaches the explicit instantiation for char, it uses the template definition to generate a char version of Swap(). For the remaining uses of Swap(), the compiler matches a template to the actual arguments used in the function call. For example, when the compiler reaches the function call Swap(a,b), it generates a short version of Swap() because the two arguments are type short. When the compiler reaches Swap(n,m), it uses the separate definition (the explicit specialization) provided for the job type. When the compiler reaches Swap(g,h), it uses the template specialization it already generated when it processed the explicit instantiation.