Accessing global const variable from inside class function

[wolfv]

How to Accessing global const variable from inside a class function?

The following example is my failed attempt.
If I remove the two "const" it works as expected.

global_class.h

#include <iostream>

class AClass
{
    public:
        void func()
        {
            extern const int g_ONE;

            std::cout << g_ONE << std::endl; //undefined reference to `g_ONE'
        }
};

global_main.cpp

#include "global_class.h"

AClass anObject;

const int g_ONE=1;

int main()
{
    anObject.func();
}

compiler error:

$ g++ global_main.cpp
/tmp/cclPgitM.o: In function `AClass::func()':
global_main.cpp:(.text._ZN6AClass4funcEv[_ZN6AClass4funcEv]+0xe): undefined reference to `g_ONE'
collect2: error: ld returned 1 exit status

Thanks for taking a look.

[helios]

Those are two different constants. One is a global and the other is a local. I have no idea why the compiler allows declaring an extern local constant, or how one is supposed to define such a symbol.

Possibility 1: g_ONE is a local constant.

class AClass
{
    public:
        void func()
        {
            const int g_ONE = 1;

            std::cout << g_ONE << std::endl;
        }
};

Possibility 2: g_ONE is a static constant member, defined separately.

class AClass
{
    static const int g_ONE;
    public:
        void func()
        {
            std::cout << g_ONE << std::endl;
        }
};

//In source:

const int AClass::g_ONE = 1;

Possibility 3: g_ONE is a static constant member, defined immediately.

class AClass
{
    static const int g_ONE = 1;
    public:
        void func()
        {
            std::cout << g_ONE << std::endl;
        }
};

Possibility 4: g_ONE is a global constant.

extern const int g_ONE;

class AClass
{
    public:
        void func()
        {
            std::cout << g_ONE << std::endl;
        }
};

//In source:

const int g_ONE = 1;

[wolfv]

Thanks helios. I use Possibility 4.