I have some confusions regarding static constexpr
member variables in C++11.
template
struct cond_I
{ s
According to the standard 9.4.2/p3 Static data members [class.static.data] (Emphasis Mine):
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 assignment-expression is a constant expression (5.20). 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.
As M.M earlier explained in the comments ostream::operator<<(ostream&, const complex<T>&)
passes by reference so value is considered odr-used in the program. Thus, as the wording above dictates you have to provide a definition.
Now as you’ve already found out fundamental types are passed by value, that it is why no definition required.