Redefinition of custom delete - c++

Question

I am getting an error saying that I have redefined a function custom_delete.
header.h
#include <iostream>
#include <string>
static int unfreed_count = 0;
#define DELETE(O) custom_delete(O,__PRETTY_FUNCTION__, __LINE__)
void custom_delete(void* ptr, const std::string& function_name, unsigned int line_number) {
unfreed_count--;
std::cout << "delete called in " + function_name + ":" << line_number << std::endl;
std::cout << "unfreed_count: = " << unfreed_count << std::endl << std::endl;
free(ptr);
}
main.cpp
#include "header.h"
int main(int argc, char* argv[]) {
int* ptr = new int;
DELETE(ptr);
}
This code results in the following error message upon attempted compilation:
main.cpp: In function 'void custom_delete(void*, const string&, unsigned int)':
main.cpp:5:6: error: redefinition of 'void custom_delete(void*, const string&, unsigned int)'
void custom_delete(void* ptr, const std::string& function_name, unsigned int line_number) {
^
In file included from main.cpp:21:0:
header.h:7:6: note: 'void custom_delete(void*, const string&, unsigned int)' previously defined here
void custom_delete(void* ptr, const std::string& function_name, unsigned int line_number) {
^
I was trying to make a custom version of delete that printed out when it was used. This was in a larger project, so I made a separate project with just these two files and got the same errors.
I tried commenting out all the code inside the custom_delete function. I also tried writing the prototype before the macro. I got the same error each time.
---EDIT---
I found there was another file being compiled:
header.cpp
#include "header.h"
// Some commented out functions
If I remove the #include "header.h" everything works. However, I eventually will need to add the functions to header.cpp. These functions need stuff that will be added to header.h. What should I do?

Answer 1

It looks like you don't have include guard in your headers.
Add #pragma once as the first line of your header.h
or for more portable solution, add
#ifndef HEADER_H
#define HEADER_H
as first two lines, and
#endif
as last line of the header.h.
Also, to avoid link errors, add inline keyword to declaration of your function, like inline void custom_delete(...
or move function implementation to .cpp.