CODEWITHSUNDEEP
c++function-pointerspointer-to-member
arayq2
arayq2

Reputation: 2554

What is the type of a function pointer data member?

In the following code, I have a function member and a data member which is a function pointer. For use in a declaration or as a template parameter, the type of a pointer to the function member f is int (Test::*)(int); but what is the analogous type of a pointer to the function pointer data member pf? 

struct Test
{
    int f( int ) { return 0; };
    int (*pf)( int );
};

int main()
{
    using Fmem = int (Test::*)( int );
    Fmem   x = &Test::f;

    // using PFmem = ??;
    // PFmem  y = &Test::pf;
}

Upvotes: 0

Views: 144

Answers (2)

3CxEZiVlQ
3CxEZiVlQ

Reputation: 38538

struct Test
{
    int f( int ) { return 0; };
    int (*pf)( int );
};

int main()
{
    using Fmem = int (Test::*)( int );
    Fmem   x = &Test::f;

    using PFmem = int (* Test::*)( int );
    PFmem y = &Test::pf;
}

If you have doubts you could deduce the type with help of the compiler error message:

using PFmem = int; // any type
PFmem y = &Test::pf;

The code above causes the error message below:

prog.cpp:13:30: error: cannot convert ‘int (* Test::*)(int)’ to ‘int’ in assignment
     PFmem y = &Test::pf;

From this error message we now know the required type is int (* Test::*)(int).

Upvotes: 2

melpomene
melpomene

Reputation: 85767

using PFmem = int (*Test::*)( int );
//                        ^ a pointer ...
//                  ^^^^^^ to a member of Test ...
//                 ^ which is a pointer ...
//                          ^     ^ to a function ...
//                            ^^^ taking an int ...
//            ^^^ returning an int

You can also try to untangle the syntax with template typedefs (I'm not sure if it's worth it):

struct Test
{
    int f( int ) { return 0; };
    int (*pf)( int );
};

template<typename T>                using pointer_to           = T *;
template<class C, typename T>       using pointer_to_member_of = T C::*;
template<typename R, typename ...A> using function             = R (A ...);

int main()
{
    using Fmem = int (Test::*)( int );
    Fmem   x = &Test::f;

    using PFmem = int (*Test::*)( int );
    PFmem  y = &Test::pf;

    pointer_to_member_of<Test, function<int, int>>
        x1 = &Test::f;
    pointer_to_member_of<Test, pointer_to<function<int, int>>>
        y2 = &Test::pf;
}

Upvotes: 3

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