initialize map with static member variable - c++

Question

I do not understand why I cannot use a public const static member of a class in the initializer list of a map (probably any container). As I understand it "MyClass::A" is an rvalue, it seems like it should be the exact same as the case where I am using "THING" which is also a static const just outside of a class.
Here is the error:
Undefined symbols for architecture x86_64:
"MyClass::A", referenced from:
_main in map-380caf.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
And here is the code:
#include <iostream>
#include <map>
#include <string>
static const int THING = 1;
class MyClass {
public:
static const int A = 1;
};
int
main()
{
int a;
typedef std::map<int, std::string> MyMap;
// compiles and works fine
a = MyClass::A;
std::cout << a << std::endl;
// compiles and works fine
MyMap other_map = { {THING, "foo"} };
std::cout << other_map.size() << std::endl;
// Does not compile
MyMap my_map = { {MyClass::A, "foo"} };
std::cout << my_map.size() << std::endl;
return 0;
}
UPDATE 1:
Using clang on OS X:
Apple LLVM version 7.0.0 (clang-700.0.72)
Target: x86_64-apple-darwin14.5.0
Thread model: posix
compiler flags:
clang++ map.cc -std=c++1y

Answer 1

Something in the map code probably tried to take the address of a reference to your int.
The class definition here:
class MyClass {
public:
static const int A = 1;
};
does not actually create any memory for A. In order to do that you have to do in the header file:
class MyClass {
public:
static const int A;
};
and in a CPP file:
const int MyClass::A = 1;
Or I guess with the newest C++ versions you can leave the = 1 in the header and just declare the storage in the CPP file with:
const int MyClass::A;