One Definition Rule - Definitions of Static Const Data Members

Definitions of Static Const Data Members

In pre-standard C++, all static data members required a definition outside of their class. However, during the C++ standardization process it was decided to lift this requirement for static const integral members. The intent was to allow uses such as:

struct C { static const int N = 10; }; char data; // N "used" without out-of-class definition

without a namespace scope definition for N.

Nevertheless, the wording of the 1998 C++ standard still required a definition if the member was used in the program. This included the member appearing anywhere except as the operand to sizeof or typeid, effectively making the above ill-formed.

This was identified as a defect, and the wording was adjusted to allow such a member to appear anywhere a constant expression is required, without requiring an out-of-class definition. This includes array bounds, case expressions, static member initializers, and nontype template arguments.

struct C { static const int N = 10; static const int U = N; // Legal per C++03 }; char data; // Legal per C++03 template struct D; template<> struct D {}; // Legal per C++03

However, using a static const integral member anywhere except where an integral constant-expression is required requires a definition:

struct C { static const int N = 10; }; int main { int i = C::N; // ill-formed, definition of C::N required }

This requirement has been relaxed in the current C++ standard, C++11.

Read more about this topic:  One Definition Rule

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