Header files -- multiple definition

[gunitinug]

I get multiple definition of getSquareSides().

main.cpp

#include <iostream>
#include "square.h"

int main() {
	std::cout << "a square has " << getSquareSides() << "sides" << std::endl;
	std::cout << "a square of length 5 has perimenter" << getSquarePerimeter(5) << std::endl;
	return 0;
}

square.h

#ifndef SQUARE_H
#define SQUARE_H
int getSquareSides() { return 4; }
int getSquarePerimeter(int);
#endif 

square.cpp

#include "square.h"

int getSquarePerimeter(int sideLength) {
	return sideLength * getSquareSides();
}

[liuyang]

Always put implementations in .c or .cpp files.

[JLBorges]

#ifndef SQUARE_H
#define SQUARE_H

    // make it inline http://en.cppreference.com/w/cpp/language/inline
    inline int getSquareSides() { return 4; } // constexpr 
    
    int getSquarePerimeter(int);
#endif  

[E0p9LyTq]

Declaring a function inline in a header file also works.

#ifndef SQUARE_H
#define SQUARE_H

inline int getSquareSides() { return 4; }
int getSquarePerimeter(int);

#endif