include guards not working - c++

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) {
// ...
}

Answer 2

You are getting a linker error, not a compiler error. You have implemented the function randarr() in your util.h file, which means the compiler sees a copy of randarr() in each of example.cpp and Main.cpp. When the linker goes to link these together, it complains because you're not permitted to have more than one definition of the same function.
You have two choices:
declare randarr() as inline in the header file
move the definition for randarr() to a util.cpp file
Apply the same fix to is_prime().

Answer 3

You defined functions in a header file. This means, the code for these functions are included in both example.cpp and in Main.cpp. And it also means the code will be generated twice. This is the reason for the "multiple definition" error.
When you define the functions randarr() and is_prime() only once in a separate util.cpp, the errors will be gone.

Answer 4

Your header should only contain the prototype of your function. A prototype describes your function to other files, but do not implement it. The only exception is the template, because each template specialisation is build upon compile time.
If you implement your function in your header file, at linker time, you will find multiple times the function content, and that is why you are facing an error.
Move the implementation of randarr and is_prime to another file, and transform you util.h to :
#ifndef UTIL_H
#define UTIL_H
using namespace std;
int* randarr(int size, int min, int max);
template<typename T> void printarr(T* v, int begin, int end) {
for (int i=begin; i<end; i++)
cout << v[i] << " ";
cout << endl;
}
bool is_prime(int n);
#endif