On 7/2/2011 11:41 AM, John Maddock wrote:
No, but you *may* need an out-of-line definition for the member "value" if the compiler considers the address to be "used": my guess is you're only using it within the context of another metafunction, so probably this should be OK.... probably ;)
Is there a relevant section in the C++ standard about this which you can cite offhand ?
In my quick test of this I did not specify an out-of-line definition but then again I am not "using" the const value in any other way then to test against it.
C++11 says of these:
"If a non-volatile const static data member is of integral or enumeration type, its declaration in the class definition can specify a brace-or-equal-initializer in which every initializer-clause that is an assignmentexpression is a constant expression (5.19). A static data member of literal type can be declared in the class definition with the constexpr specifier; if so, its declaration shall specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression. [ Note: In both these cases, the member may appear in constant expressions. -end note ] The member shall still be defined in a namespace scope if it is odr-used (3.2) in the program and the namespace scope definition shall not contain an initializer."
I couldn't find in 3.2 where it specifies what constitutes odr-used in this case, but I did note that unevaluated-operands are excluded, so if you're using it within a constant-expression it seems you may be OK.
I am using it within a constant expression since TTI is a compile-time library like type traits. But I am sure you already know that.