FAQ 13.04 What is the one-definition rule (ODR)?

The ODR says that C++ constructs must be identically defined in every compilation unit in which they are used.

For example, if source file foo.cpp and source file bar.cpp are in the same program and class Fred appears in both source files, then class Fred has to be identically defined in the two source files. Similarly, the definition of an inline function must be the same in the two source files.

Here's what "identically defined" means. With respect to the ODR, two definitions contained in different source files are said to be identically defined if and only if they are token-for-token identical and the tokens have the same meaning in both source files. The last clause is important: it warns that merely including the same definition in two places isn't good enough to prevent ODR problems, because the tokens (such as typedefs) can have different meanings in the different locations. Note that "identically defined" does not mean character-by-character equivalence. For example, two definitions can have different whitespace or different comments yet still be "identical."