C++: One Definition Rule (ODR)

Every C++ programmer is aware of the One Definition Rule (ODR), but in a vague sense: “Only one definition of something is allowed in something.” I was surprised to learn that the ODR stumps not just us mortals, but the architects of C++ itself. More than a decade after the birth of C++, in a 1995 C++ Standards meeting it took the architects 2 entire days to put down this rule to paper! 😊

Attempt 1

Here is a simple breakdown of the ODR:

• In a program, there can be only one definition of:

  • (Non-inline) Function

  • Variable

• In a translation unit (a .cpp file), there can be only one definition of:

  • Class

Attempt 2

Alternatively, we can flip around the definition of the rule and try to understand it from the point-of-view of the entities:

• Class: There can be only one definition of a class in a translation unit.

  • Note: This means that the same class definition can appear in several translation units. In fact, this is what happens when the header file of a class in included in several source files.

• (Non-inline) Function or (global) Variable: There can be only one definition of such a function or variable in the entire program.

  • When this rule is broken, a redefinition error is produced by the linker.

References

• Standard for C++ Programming Language: The ODR section is arcane and runs to 3 pages!

• Section 9.2.3 in The C++ Programming Language by Stroustrup: Surprisingly, the C++ creator himself only describes the ODR with relation to classes, clearly skipping (or assuming) it for functions and variables!

• Articles Diary of an Observer by Francis Glassborow and The Casting Vote by Sean A. Corfield in Issue 9 (August 1995) of Overload magazine: C++ architects describe in entertaining detail how nailing the ODR definition took them 2 full days at the C++ standards meeting.