This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2024-10-26
Is this valid C++? The question is whether a member constant can be specialized. My inclination is to say no.
template <class T> struct A { static const T i = 0; }; template<> const int A<int>::i = 42; int main () { return A<int>::i; }John Spicer: This is ill-formed because 11.4.9.3 [class.static.data] paragraph 4 prohibits an initializer on a definition of a static data member for which an initializer was provided in the class.
The program would be valid if the initializer were removed from the specialization.
Daveed Vandevoorde: Or at least, the specialized member should not be allowed in constant-expressions.
Bill Gibbons: Alternatively, the use of a member constant within the definition could be treated the same as the use of "sizeof(member class)". For example:
template <class T> struct A { static const T i = 1; struct B { char b[100]; }; char x[sizeof(B)]; // specialization can affect array size char y[i]; // specialization can affect array size }; template<> const int A<int>::i = 42; template<> struct A<int>::B { char z[200] }; int main () { A<int> a; return sizeof(a.x) // 200 (unspecialized value is 100) + sizeof(a.y); // 42 (unspecialized value is 1) }For the member template case, the array size "sizeof(B)" cannot be evaluated until the template is instantiated because B might be specialized. Similarly, the array size "i" cannot be evaluated until the template is instantiated.
Rationale (10/99): The Standard is already sufficiently clear on this question.