An exception to the "only one implementation" rule?

Question

While I was reading the accepted answer of this question, I had the following question:

Typically, methods are defined in header files (.hpp or whatever), and implementation in source files (.cpp or whatever).

One of the main reasons it is bad practice to ever include a "source file" (#include <source_file.cpp>) is that its methods implementation would then be duplicated, resulting in linking errors.

When one writes:

#ifndef BRITNEYSPEARS_HPP
#define BRITNEYSPEARS_HPP

class BritneySpears
{
  public:

    BritneySpears() {}; // Here the constructor has implementation.
};

#endif /* BRITNEYSPEARS_HPP */

He is giving the implementation of the constructor (here an "empty" implementation, but still).

But why then including this header file multiple times (aka. on different source files) will not generate a "duplicate definition" error at link time ?

Answer 1

Inline functions are exceptions to the "one definition rule": you are allowed to have identical implementations of them in more than one compilation unit. Functions are inline if they are declared inline or implemented inside a class definition.