Exception in C++ with function pointers: Access violation executing location 0x00000000 - c++

Question

I'm having some trouble with function pointers and passing them as inputs to other functions in C++. I've written some simplified code that sums up the trouble that I'm having. I have two .cpp files as below
functions.cpp
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#define _CRT_SECURE_NO_WARNINGS
typedef double(*real_function)(double);
double one(double x) {
return double(1);
}
void applyfunction(int length, real_function f, double* result) {
int j;
result[0] = 0;
for (j = 1; j < length; j++) {
result[j] = f(result[j - 1]);
}
}
Source.cpp
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#define _CRT_SECURE_NO_WARNINGS
typedef double(*real_function)(double);
real_function one;
void applyfunction(int, real_function, double*);
int main(void) {
double* result;
result = (double*)calloc(10, sizeof(double));
applyfunction(10, one, result);
return(0);
}
When I compile this, I get the following error in Visual Studio 2015
Exception thrown at 0x00000000 in Project3.exe: 0xC0000005: Access violation executing location 0x00000000.
If there is a handler for this exception, the program may be safely continued.
What is going wrong? The funny thing is that if we define those functions one and applyfunction in the source.cpp file (i.e. put all code in a single file), things seem to work. So, I think that it must be something very simple that I am getting wrong. Thank you for any help.

Answer 1

real_function one;
This creates a global variable one which is initialized to a null pointer.
applyfunction(10, one, result);
You then pass that null pointer to applyfunction...
result[j] = f(result[j - 1]);
...wherein you try to call it, generating the null pointer exception.
To fix this, don't create a variable one in Source.cpp. Instead, add a prototype that matches the definition in functions.cpp. In other words, replace real_function one; with
double one(double x);

Answer 2

real_function one;
doesn't declare the function one in the other file; it defines a function pointer called one. Because it's global (and thus has static storage), it's initialized to null. Your program crashes because it's trying to call a null pointer.
One way to fix this is to change
typedef double(*real_function)(double);
to
typedef double real_function(double);
This way real_function actually names a function type (not a pointer).