CODEWITHSUNDEEP
c++c++11linkerfunction-pointers
Artem Pelenitsyn
Artem Pelenitsyn

Reputation: 2578

Trying to get plain function pointer from std::function

I'm trying to get plain function pointer from some std::function. The task has been discussed number of times at SO (like here) and was admitted unsolvable. But I tried such a solution (I fixed the function pointer signture and degenerated the usage of a plain function pointer for the sake of simplicity):

#include <functional>

typedef int(*fp)(int);

template<int idx, typename F>
struct wrap_f_struct {
    static F impl;

    static int f(int a) { return impl(a); }    
};

template<int idx, typename F>
fp wrap_f(F f) { 
    wrap_f_struct<idx, F>::impl = f;
    return wrap_f_struct<idx, F>::f;
}

int add(int a, int b) { return a + b; }

int main() {
    using namespace std::placeholders; 
    std::function<int(int)> add2 = std::bind(add, _1, 2);

    (wrap_f<1>(add2))(1);
}

Well, this isn't linking for some reason I can't understand:

/tmp/ccrcFz32.o: In function `int (*wrap_f<1, std::function<int (int)> >(std::function<int (int)>))(int)':
cast_fp_min.cpp:(.text._Z6wrap_fILi1ESt8functionIFiiEEEPS1_T0_[_Z6wrap_fILi1ESt8functionIFiiEEEPS1_T0_]+0x10): undefined reference to `wrap_f_struct<1, std::function<int (int)> >::impl'
/tmp/ccrcFz32.o: In function `wrap_f_struct<1, std::function<int (int)> >::f(int)':
cast_fp_min.cpp:(.text._ZN13wrap_f_structILi1ESt8functionIFiiEEE1fEi[_ZN13wrap_f_structILi1ESt8functionIFiiEEE1fEi]+0x10): undefined reference to `wrap_f_struct<1, std::function<int (int)> >::impl'
collect2: error: ld returned 1 exit status

My question is: can someone explain me the exact reason why this linkage error occurs?

Upvotes: 1

Views: 269

Answers (1)

Some programmer dude
Some programmer dude

Reputation: 409176

Static member variables are only declared in a structure/class. They need to be defined as well, which you do not do.

Add e.g.

template<int idx, typename F>
F wrap_f_struct<idx, F>::impl;

Upvotes: 1

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