Undefined reference to static constexpr char[]

Question

I want to have a static const char array in my class. GCC complained and told me I should use constexpr, although now it\'s telling me

Answer 1

C++17 introduces inline variables

C++17 fixes this problem for constexpr static member variables requiring an out-of-line definition if it was odr-used. See the second half of this answer for pre-C++17 details.

Proposal P0386 Inline Variables introduces the ability to apply the inline specifier to variables. In particular to this case constexpr implies inline for static member variables. The proposal says:

The inline specifier can be applied to variables as well as to functions. A variable declared inline has the same semantics as a function declared inline: it can be defined, identically, in multiple translation units, must be defined in every translation unit in which it is odr­-used, and the behavior of the program is as if there is exactly one variable.

and modified [basic.def]p2:

A declaration is a definition unless
...

• it declares a static data member outside a class definition and the variable was defined within the class with the constexpr specifier (this usage is deprecated; see [depr.static_constexpr]),

...

and add [depr.static_constexpr]:

For compatibility with prior C++ International Standards, a constexpr static data member may be redundantly redeclared outside the class with no initializer. This usage is deprecated. [ Example:

struct A {
  static constexpr int n = 5;  // definition (declaration in C++ 2014)
};

constexpr int A::n;  // redundant declaration (definition in C++ 2014)

 — end example ]

---

C++14 and earlier

In C++03, we were only allowed to provide in-class initializers for const integrals or const enumeration types, in C++11 using constexpr this was extended to literal types.

In C++11, we do not need to provide a namespace scope definition for a static constexpr member if it is not odr-used, we can see this from the draft C++11 standard section 9.4.2 [class.static.data] which says (emphasis mine going forward):

[...]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.

So then the question becomes, is baz odr-used here:

std::string str(baz); 

and the answer is yes, and so we require a namespace scope definition as well.

So how do we determine if a variable is odr-used? The original C++11 wording in section 3.2 [basic.def.odr] says:

An expression is potentially evaluated unless it is an unevaluated operand (Clause 5) or a subexpression thereof. A variable whose name appears as a potentially-evaluated expression is odr-used unless it is an object that satisfies the requirements for appearing in a constant expression (5.19) and the lvalue-to-rvalue conversion (4.1) is immediately applied.

So baz does yield a constant expression but the lvalue-to-rvalue conversion is not immediately applied since it is not applicable due to baz being an array. This is covered in section 4.1 [conv.lval] which says :

A glvalue (3.10) of a non-function, non-array type T can be converted to a prvalue.53 [...]

What is applied in the array-to-pointer conversion.

This wording of [basic.def.odr] was changed due to Defect Report 712 since some cases were not covered by this wording but these changes do not change the results for this case.