2.1.2 Using the Template

Thus, templates are compiled twice:

Without instantiation, the template code itself is checked for correct syntax. Syntax errors are discovered, such as missing semicolons.

At the time of instantiation, the template code is checked to ensure that all calls are valid. Invalid calls are discovered, such as unsupported function calls.

This leads to an important problem in the handling of templates in practice: When a function template is used in a way that triggers its instantiation, a compiler will (at some point) need to see that template's definition. This breaks the usual compile and link distinction for ordinary functions, when the declaration of a function is sufficient to compile its use. Methods of handling this problem are discussed in Chapter 6. For the moment, let's take the simplest approach: Each template is implemented inside a header file by using inline functions.