Error: "undefined reference to 'function'" in C++ - c++

Question

I got an error while compiling C++:
/tmp/ccqs6UN2.o: In function main': PowerModulus.cpp:(.text+0x194): undefined reference to takeModulusLOOP(int, int, int)' collect2: ld returned 1 exit status
The source code:
#include "PowerModulus.h"
#include <iostream>
int modint(int x, int moduint);
int takeModulusLOOP(int x, int n, int moduint);
int main() {
std::cout << takeModulusLOOP(5348, 700, 335);
}
int PowerModulus::takeModulusLOOP(int x, int n, int moduint) {
int total = modint(x, moduint);
n--;
while (--n) {
total = modint(total * x, moduint);
}
return total;
}
int PowerModulus::modint(int x, int moduint) {
while (x < 0) // Deal with negative
x += moduint;
return x % moduint; // Comes out positive now -> %
}
PowerModulus::PowerModulus() {
// TODO Auto-generated constructor stub
}
PowerModulus::~PowerModulus() {
// TODO Auto-generated destructor stub
}
Header file:
#ifndef POWERMODULUS_H_
#define POWERMODULUS_H_
int modint(int x, int moduint);
int takeModulusLOOP(int x, int n, int moduint);
class PowerModulus {
public:
int takeModulusLOOP(int x, int n, int moduint);
int modint(int x, int moduint);
PowerModulus();
virtual ~PowerModulus();
};
#endif /* POWERMODULUS_H_ */
Where is the error?

Answer 1

You have declared a global takeModulusLOOP function, then call it in main, without ever defining it. This is a different function than PowerModulus::takeModulusLOOP.
// main.cpp
#include "PowerModulus.h"
#include <iostream>
int main(){
std::cout << PowerModulus::takeModulusLOOP(5348,700,335) << '\n';
return 0;
}
Changed to a namespace instead of a class, and separated into header and implementation (instead of grouping in main.cpp):
// PowerModulus.cpp
#include "PowerModulus.h"
namespace PowerModulus {
int takeModulusLOOP(int x, int n, int moduint){
int total = modint(x, moduint) ;
n--;
while (--n){
total = modint( total * x, moduint );
}
return total;
}
int modint(int x, int moduint){
while ( x < 0) // deal with negative
x += moduint;
return x % moduint;//comes out positive now -> %
}
}
Header:
// PowerModulus.h
#ifndef POWERMODULUS_H_
#define POWERMODULUS_H_
namespace PowerModulus {
int modint(int x, int moduint);
int takeModulusLOOP(int x, int n, int moduint);
}
#endif

Answer 2

This line:
std::cout << takeModulusLOOP(5348,700,335);
is calling the non-class takeModulusLOOP, which you haven't defined anywhere.
You should either call the class version, by providing an object of the class type and using something like:
PowerModulus p;
std::cout << p.takeModulusLOOP(5348,700,335);
(most likely) or providing a non-class version (least likely).
You could also consider making the function static since it doesn't seem to require an object at all. Then you don't need to instantiate one.

Answer 3

You receive the error, because you do not have such a function.
Actually, you have it in PowerModulus class, so you should call the function from PowerModulus instance.
PowerModulus pM;
pM.takeModulusLoop(5348,700,335);
You do not need to claim the function in the beginning of your .h file or in the beginning of your .cpp file.

Answer 4

If you intended to use the takeModulusLoop function of the PowerModulus class then you need not declare a global function again...
But, if you intended to use a different global function, then you need to define it in its context...