Question

A template class is a multitude of possible classes, so I was asking me: the preprocessor directives

#ifndef MY_CLASS
#define MY_CLASS

template<typename T>
class My_Class
{};

#endif

are necessaries?

Answer 1

The include guards prevent repeated inclusion of the same file. You need them to prevent a translation unit that would contain the following definitions, which are illegal:

template <typename T> class My_Class { };
template <typename T> class My_Class { };

// Error: redefinition of ‘class Foo<T>’

Repeat inclusion is very easy to occur. For example, consider a case where you include A and B, and A already includes B for some undocumented reason. You may not be entitled to omit B from your explicit includes, but neither should A be required to not include it. Only by using include guards (or some equivalent mechanism) can you make such a file structure possible.

In a nutshell: You can have repeated declarations, but only one definition within one translation unit.

_{Counter example: The following is legal:}

template <typename T> class Foo;
template <typename T> class Foo;
template <typename T> class Foo;
template <typename T> class Foo;
template <typename T> class Foo { };
int main() {}