C ++中的继承无效。 编译以下C ++代码时发生链接器错误

Question

I am trying to compile a C++ program. I am learning the "virtual functions" concept. So I was trying to see what happens when we override a function without making it "virtual".
I have three directories headers, sources and objects.
In the header directory, I have 2 files, Pokemon.h and Charmander.h . In the sources directory, I have 3 files Pokemon.cpp, Charmander.cpp and Main.cpp.
class Charmander in Charmander.h, inherits from class Pokemon from Pokemon.h.

This is Pokemon.h in the "headers" directory:

/********************************************************************************
 * Pokemon --                                                                   *
 * This is the base class for all pokemons. Every pokemon will inherit from     *
 * this class.                                                                  *
 *                                                                              *
 * Author -- Aditya R.Singh                                                     *
 * Version -- 1.0                                                               *
 * Since -- 2014-06-21                                                          *
 ********************************************************************************/ 



#ifndef POKEMON_H
#define POKEMON_H



using namespace std;



class Pokemon {

    public:
        string type();
        string attack();
        string weakness();
};    



#endif   

This is Charmander.h in the "headers" directory:

/*******************************************************************************
 * Charmander --                                                               *
 * This class is the derived class from class Pokemon. This is specialized for *
 * the pokemon Charmander.                                                     *
 *                                                                             *
 * Author -- Aditya R.Singh                                                    *
 * Version -- 1.0                                                              *
 * Since -- 2014-06-18                                                         *
 *******************************************************************************/



#ifndef CHARMANDER_H
#define CHARMANDER_H



#include "Pokemon.h"  



using namespace std;



class Charmander: public Pokemon {

    public:
        string type();
        string attack();
        string weakness();
};    



#endif  

This is the Pokemon.cpp in the "sources" directory:

/**********************************************************************************
 * Pokemon --                                                                     *
 * This is the implementation of functions in class Pokemon.                      *
 * This is a generic class for every pokemon.                                     *
 *                                                                                *
 * Author -- Aditya R.Singh                                                       *
 * Since -- 1.0                                                                   *
 * Version -- 2014-06-21                                                          *
 **********************************************************************************/



#include <iostream>
#include "../headers/Pokemon.h"



using namespace std;



string Pokemon::type() {

    return "Normal";
}    



string Pokemon::attack() {

    return "Headbutt";
}



string Pokemon::weakness() {

    return "Fighting";
}   

This is the Charmander.cpp file in the "sources" directory:

/**********************************************************************************
 * Charmander --                                                                  *
 * This is the Charmander class's functions implementations.                      *
 * This is specific to Charmander.                                                *
 *                                                                                *
 * Author -- Aditya R.Singh                                                       *
 * Since -- 1.0                                                                   *
 * Version -- 2014-06-21                                                          *
 **********************************************************************************/



#include <iostream>
#include "../headers/Charmander.h"



using namespace std;



string Charmander::type() {

    return "Fire";
} 



string Charmander::attack() {

    return "Fireflame";
}



string Charmander::weakness() {

    return "Water";
}  

And this is the Main.cpp file in the "sources" directory:

/**********************************************************************************
 * Main --                                                                        *
 * This program will show the use of Polymorphism and Inheritance in C++.         *
 *                                                                                *
 * Author -- Aditya R.Singh                                                       *
 * Since -- 1.0                                                                   *
 * Version -- 2014-06-21                                                          *
 **********************************************************************************/



#include <iostream>
#include "../headers/Pokemon.h"
#include "../headers/Charmander.h"



using namespace std;



int main() {

    Pokemon *charmander = new Charmander;   



    cout << "Charmander is a " << charmander->type() << " type pokemon." << endl;
    cout << "Charmander can do a " << charmander->attack() << " attack." << endl;
    cout << "Charmander is weak against " << charmander->weakness() << " type pokemon." << endl;



    return 0;
}  

Now I compiled the Pokemon.cpp and put the object file in the "objects" directory... Currently my terminal is in the "Program" directory...

gcc -c sources/Pokemon.cpp -o objects/Pokemon.o  

I compiled the Charmander.cpp and the put the object file in the "objects" directory... like this ..

gcc -c sources/Charmander.cpp -o objects/Charmander.o  

It's working fine uptill now. I have the object files in the "objects" directory now.

But now I am trying to compile my Main.cpp like this..

gcc sources/Main.cpp -o Main objects/Pokemon.o objects/Charmander.o  

But my GCC compiler gives some big error stack like, undefined symbols for architecture x86_64.

I am using a mac book pro, 64 bit machine. OS Mac OS X maverics.

Answer 1

To get polymorphic behaviour, the base class must have at least one virtual function.

Add to the class definition of Pokemon:

virtual ~Pokemon() { }

Note that since your other functions are not virtual, charmander->type() will call Pokemon::type() , not Charmander::type() .

NB. Your code compiles correctly for me, with the addition of #include <string> . If you still have problems then try moving everything to the same directory , in case you are perhaps having one unit pick up an old version of a header file from a different directory.

Answer 2

You should use g++ instead of gcc to compile and link C++ programs so that the C++ library gets properly linked in.

See https://stackoverflow.com/a/5854712/12711 for the gory details.