Const Static Function Pointer in Class ~ How to initialize it?

Question

I have this small piece of code:

int add(int x, int y)
{
    return x + y;
}

class A
{
public:
    const static int (*FP)(int, int) = &add;
};

int main()
{
    int x = 3;
    int y = 2;
    int z = A::FP(x, y);
    return 0;
}

Under VS2012 this generates the following error: error C2864: 'A::FP' : only static const integral data members can be initialized within a class.

Is there something I am not seeing? Or is it plainly not possible for some reason?

Christian

Answer 1

Initialize outside of the class definition, using a typedef to make the constness possible:

typedef int (*func_t)(int, int);

class A
{
public:
    const static func_t FP;
};

const func_t A::FP = &add;

Without the typedef the declaration:

const static int (*FP)(int, int) = &add;

is a static function pointer named FP with a return type of const int, not a const function pointer. When compiled with warnling level /W4 the following diagnostic is emitted:

warning C4180: qualifier applied to function type has no meaning; ignored

this was not immediately apparent due to the ordering of the declaration, const static int instead of static const int.

Answer 2

In C++03. In class initialization of non-intgeral or enum data-types is not allowed.

class A
{
public:
    typedef int (*FP_ptr)(int, int);
    const static FP_ptr FP;
};

const A::FP_ptr 
A::FP = &add;

---

C++11

class A
{
public:
    constexpr static int (*FP)(int, int) = &add;
};