Source Files and Programs(Chapter 9 of The C++ Programming

　　Consequently, the rule in the standard that says that there must be a unique definition of a class, template, etc., is phrased in a somewhat more complicated and subtle manner. This rule is commonly referred to as ‘‘the one definition rule,’’ the ODR. That is, two definitions of a class, template, or inline function are accepted as examples of the same unique definition if and only if

　　[1] they appear in different translation units, and

　　[2] they are token-for-token identical, and

　　[3] the meanings of those tokens are the same in both translation units.