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

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 takeModulusL开发者_如何学编程OOP(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?

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

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.

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.

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...