Question

I’m trying to use a lambda as a static member, like this:

struct A
{
    static constexpr auto F = [](){};
};


int main()
{
    A::F();
    return 0;
}

Is this even correct C++11 code? On clang, I get this error:

error: constexpr variable 'F' must be initialized by a constant
      expression
    static constexpr auto F = [](){};
                              ^~~~~~

It seems in clang, lambdas aren’t considered a constant expression. Is this correct? Perhaps they haven’t fully implemented lambdas yet in clang because gcc 4.7 seems to allow it as a constexpr, but it give another error:

error: ‘constexpr const<lambda()> A::F’, declared using local type ‘const<lambda()>’, is used but never defined

I’m not sure, I understand what that means. It seems to correctly deduce the type of the lambda, but it only declares it and not define it. How would I go about defining it?

Answer 1

This code is ill-formed. A constexpr variable is required to be initialized by a constant expression, and [expr.const]p2 says:

A conditional-expression is a core constant expression unless it involves one of the following as a potentially evaluated subexpression […]:

• a lambda-expression

GCC is therefore incorrect to accept this code.

Here’s one way to give a class a static data member of lambda type:

auto a = []{};
struct S {
  static decltype(a) b;
};
decltype(a) S::b = a;