Reach problem in C ++

I'm a beginner in C ++ programming and I came across a problem when I tried to run my code in atom , using the gpp-compiler package and MinGW: 'function' was not declared in this scope l.8). I did not understand the reason for the error, can anyone help me?

Here is the main function:

#include <iostream>
#include "header.hpp"

using namespace std;

int main(){
    int* a;
    a = function(50);
    for(int d = 0; d < a.length; d++){
       .
       .
       .
    }
    system("pause");
    return 0;
}

and the header file:

#ifndef FUNCTIONS_H_INCLUDED
#define FUNCTIONS_H_INCLUDED
#include <vector>

int* function(int num){
    std::vector<int> c;
       .
       .
       .     
    int z[(int)c.size()];
    for(int b = 0; b < (int)c.size(); b++){
        z[b] = c.at(b);
    }
    return z;
}

#endif

To be more specific, the error message was as follows:

  

... \ main.cpp: In function 'int main ()':

     

... \ main.cpp: 9: 18: error: 'function' was not declared in this scope
  a = function (50);

     

... \ main.cpp: 10: 23: error: request for member 'length' in 'a', which   is of non-class type 'int *' for (int d = 0; d