Question

I’ve defined a util.h file with functions that i want to use throughout several different other files. This header has an include guard, however when i use it in two different files, I get a multiple definition of... error. What am i doing wrong?

I’ve read this but this pertains to variable declaration/definition. This answer seems more relevant but it’s not clear to me how i can fix this.

// util.h
// include lots of standard headers
#include ...

#ifndef UTIL_H
#define UTIL_H

using namespace std;
// multiple definition of `randarr(int, int, int)`
int* randarr(int size, int min, int max) {
    int *ret = new int[size];
    for (int i=0; i<size; i++)
            ret[i] = (int) (((double) rand() / RAND_MAX) * max) + min;
    return ret;
}
// no error
template<typename T> void printarr(T* v, int begin, int end) {
    for (int i=begin; i<end; i++)
    cout << v[i] << " ";
    cout << endl;
}
// multiple definition of `is_prime(int)`
bool is_prime(int n) {
    if (n == 2 || n == 3 || n == 5) return true;
    if (n <= 1 || (n&1) == 0) return false;

    for (int i = 3; i*i <= n; i += 2)
            if (n % i == 0) return false;

    return true;
}
#endif

---

// example.cpp
#include ...// lots of standard includes
#include "util.h"
void f() {
    randarr(...);
    printarr(...);
    is_prime(...);
    ...
}

---

// Main.cpp
#include "util.h"
int main() {

}

Answer 1

The include guards are not the cause of the error; you’re violating the One Definition Rule. Since util.h is being included in 2 source files, the translation units created after preprocessing each source file will contain a definition of each of the functions, thus leading to the multiple definition error.

To get rid of the error, mark the functions inline

inline int* randarr(int size, int min, int max) {
  // ...
}

template<typename T> 
inline void printarr(T* v, int begin, int end) {
  // ...
}

inline bool is_prime(int n) {
  // ...
}