c++ - Undefined reference to static constexpr char[]

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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
it's an undefined reference. If I make the array a non-member then it compiles. What is
going on?

//
            .hpp

struct foo {
 void bar();
 static
            constexpr char baz[] = "quz";
};

// .cpp
void
            foo::bar() {
 std::string str(baz); // undefined reference to
            baz
}

Answer

Add to your cpp
file:

constexpr char
            foo::baz[];

Reason:
You have to provide the definition of the static member as well as
the declaration. The declaration and the initializer go inside the class definition, but
the member definition has to be separate.