5.2.1 Template Definitions Included

When you put the declarations and definitions for a template within the file that uses the template, the organization is definitions-included. For example:

main.cc

template <class Number> Number twice(Number original);
template <class Number> Number twice(Number original )
    { return original + original; }
int main()
    { return twice<int>(-3); }

When a file using a template includes a file that contains both the template’s declaration and the template’s definition, the file that uses the template also has the definitions-included organization. For example:

twice.h

#ifndef TWICE_H
#define TWICE_H
template <class Number>
Number twice(Number original);
template <class Number> Number twice( Number original )
    { return original + original; }
#endif

main.cc

#include “twice.h”
int main()
    { return twice(-3); }

It is very important to make your template headers idempotent. (See 5.1.2 Idempotent Header Files.)