Unresolved External Symbol, Header File Cause

Question

I've searched (and found) topics to this error, but wasn't able to apply them to my specific situation, yet. So here it goes:

Rational.h

#include <iostream>
class Rational{
public:
    Rational(int a = 0, int b = 1);
    Rational(const Rational &number);
    ~Rational();

    static Rational add(const Rational &a, const Rational &b);
    static Rational sub(const Rational &a, const Rational &b);
    static Rational mult(const Rational &a, const Rational &b);
    static Rational div(const Rational &a, const Rational &b);

    void reduce(Rational a);

    int get_nom() const;
    int get_denom() const;
    void set_nom(int a);
    void set_denom(int b);

    void printOut();

private:
    int nom;
    int denom;

    int greatCommonDiv(int a, int b);
};

Rational.cpp

#include <iostream>

class Rational{
public:
    Rational(int a = 0, int b = 1):
        nom(a), denom(b){
    }
    Rational(const Rational &number):
        nom(number.get_nom()), denom(number.get_denom()){
    }
    ~Rational(){
    }

    static Rational add(const Rational &a, const Rational &b){
        Rational sum( ((a.get_nom() * b.get_denom()) + (a.get_denom() * b.get_denom())), (a.get_denom() * b.get_denom()) );
        sum.reduce();
        return sum;
    }
    static Rational sub(const Rational &a, const Rational &b){
        Rational diff( ((a.get_nom() * b.get_denom()) - (a.get_denom() * b.get_denom())), (a.get_denom() * b.get_denom()) );
        diff.reduce();
        return diff;
    }
    static Rational mult(const Rational &a, const Rational &b){
        Rational product(a.get_nom() * b.get_nom(), a.get_denom() * b.get_denom());
        product.reduce();
        return product;
    }
    static Rational div(const Rational &a, const Rational &b){
        Rational quotient(a.get_nom() * b.get_denom(), a.get_denom() * b.get_nom());
        quotient.reduce();
        return quotient;
    }
    void reduce(){
        int ggT = greatCommonDiv(nom, denom);
        nom = nom / ggT;
        denom = denom / ggT;
    }

    int get_nom() const { return nom; }
    int get_denom() const { return denom; }
    void set_nom(int a){ nom = a; }
    void set_denom(int b){ denom = b; }

    void printOut(){
        std::cout << nom << "/" << denom << std::endl;
        return;
    }

private:
    int nom;
    int denom;

    int greatCommonDiv(int a, int b){           
        if(b == 0)
            return a;
        else return greatCommonDiv(b, a % b);
    }
};

Source.cpp

#include <iostream>
#include <Rational.h>

int main(){
Rational a(5,3);
a.printOut();
}

MSVS gives me 3 errors:

1>Source.obj : error LNK2019: unresolved external symbol "public: __thiscall Rational::Rational(int,int)" (??0Rational@@QAE@HH@Z) referenced in function _main

1>Source.obj : error LNK2019: unresolved external symbol "public: __thiscall Rational::~Rational(void)" (??1Rational@@QAE@XZ) referenced in function _main

1>Source.obj : error LNK2019: unresolved external symbol "public: static void __cdecl Rational::printOut(void)" (?printOut@Rational@@SAXXZ) referenced in function _main

I can't figure out, why this happens as I'm pretty certain that he can find the .h and .cpp files at the right places.

Did you compile Rational.cpp to get a Rational.obj? Did you link in Rational.obj when trying to create an executable? — user93353
possible duplicate of Why am I getting this error ? "unresolved external symbol" — user93353
I just hit F7 in Visual Studio and aiming to have it run in a console windows. I added '#include "Rational.h" to the .cpp file, the errors are gone but instead I'm getting 'Rational: 'class' type redefinition'. — DenverCoder21
You cannot have the class declaration in both Rational.h and Rational.cpp - the syntax of your Rational.cpp is totally inappropriate. Rational.cpp should contain only method definitions of the class and not the class definition again - buy a book - these are utter basics. — user93353

firefrorefiddle firefrorefiddle · Accepted Answer · 2013-08-06T09:21:30

So you solved your linker problem yourself. Regarding the "class type redefinition":

The compiler is right. You can/shall/must only define the class once, as you did correctly in the header file. The cpp file should look like this instead:

#include "Rational.h"

Rational Rational::add(const Rational &a, const Rational &b){
    Rational sum( ((a.get_nom() * b.get_denom()) + (a.get_denom() * b.get_denom())), (a.get_denom() * b.get_denom()) );
    sum.reduce();
    return sum;
}

...

You get the basic idea: Whatever is not defined (only declared) in the header, you define in the cpp. Always prefixed with Classname::. Things you defined in the header (like your constructor) you don't have to defined again.

Unresolved External Symbol, Header File Cause

1 Answers