Error of incomplete type is not allowed, declaring struct in another cpp file

I declared a struct within the same .cpp file of the main() function. Then no error occurs.

But the error of "incomplete type is not allowed" occurs if I put the struct in another .cpp file.

file2.cpp:

struct a_point {
    float x, y;
};

file2.h :

struct a_point;

file1.cpp :

#include "file2.h"

int main(){

    a_point  pnt_arr[5]; // error occurs, in visual studio 2017

    return 0;
}