Initializing Constant Static Array In Header File

Question

I have just found out that the following is not valid.

//Header File
class test
{
    const static char array[] = { \'1\', \'2\', \'3\' };
};

Answer 1

With constexpr you must define the value on the header even in C++11

If you use constexpr instead of const, then this answer suggests that you not only can, but must, define on header even in C++11:

#include 

struct MyClass {
    static constexpr int is[] = {1, 2, 3};
    static constexpr int i = 1;
};

// TODO is this ever mandatory? Create example that fails on -std=c++11.
// Pretty sure never mandatory in C++17 https://stackoverflow.com/a/40959093/895245
// constexpr int MyClass::is[];

int main (void) {
    assert(MyClass::is[0] == 1);
    assert(&MyClass::is[0] == &MyClass::is[1] - 1);
    assert(MyClass::i == 1);
    assert(&MyClass::i == &MyClass::i);
}

Compile and run with:

g++-10 -ggdb3 -O0 -std=c++11 -Wall -Wextra -pedantic -o main.out main.cpp
./main.out 

If instead you try:

struct MyClass {
    static constexpr int is[];
};

constexpr int MyClass::is[] = {1, 2, 3};

compilation fails with:

main.cpp:4:26: error: ‘constexpr’ static data member ‘is’ must have an initializer

Tested on Ubuntu 20.04.