The One Definition Rule (ODR) is an important rule of the C++ programming language that prescribes that classes/structs and non-inline functions cannot have more than one definition in the entire program and templates and types cannot have more than one definition by translation unit. It is defined in the ISO C++ Standard (ISO/IEC 14882) 2003, at section 3.2. Some other programming languages have similar but differently defined rules towards the same objective.

In short, the ODR states that:

Some violations of the ODR must be diagnosed by the compiler. Other violations, particularly those that span translation units, are not required to be diagnosed.

In general, a translation unit shall contain no more than one definition of any class type. In this example, two definitions of the class type C occur in the same translation unit. This typically occurs if a header file is included twice by the same source file without appropriate header guards.

In the following, forming a pointer to S or defining a function taking a reference to S are examples of legal constructs, because they do not require the type of S to be complete. Therefore, a definition is not required.

Defining an object of type S, a function taking an argument of type S, or using S in a sizeof expression are examples of contexts where S must be complete, and therefore require a definition.

In certain cases, there can be more than one definition of a type or a template. A program consisting of multiple header files and source files will typically have more than one definition of a type, but not more than one definition per translation unit.

If a program contains more than one definition of a type, then each definition must be equivalent.