“无法解析的外部符号”错误

Question

Please have a look at the following code

Main.cpp

#include <iostream>
#include <string>

using namespace std;

int main()
{
    system("pause");

    return 0;
}

Magic.h

#pragma once

class Magic
{
public:
    Magic();
    ~Magic();
    virtual void display()=0;
};

Spell.h

#pragma once
#include "Magic.h"
#include <iostream>
#include <string>

using namespace std;

class Spell :
    public Magic
{
public:
    Spell(void);
    Spell(string words);
    ~Spell(void);
    void display();

private:
    string words;
};

Spell.cpp

#include "Spell.h"
#include "Magic.h"
#include <iostream>
#include <string>

using namespace std;



Spell::Spell(void)
{
}

Spell::Spell(string words)
{
    this->words = words;
}


Spell::~Spell(void)
{
    cout << "Delete Spell" << endl;
}

void Spell::display()
{
    cout << "Spell Words: " << words << endl;
}

Here, I am getting the error

1>------ Build started: Project: Revision1_1, Configuration: Debug Win32 ------
1>Spell.obj : error LNK2019: unresolved external symbol "public: __thiscall Magic::~Magic(void)" (??1Magic@@QAE@XZ) referenced in function __unwindfunclet$??0Spell@@QAE@XZ$0
1>Spell.obj : error LNK2019: unresolved external symbol "public: __thiscall Magic::Magic(void)" (??0Magic@@QAE@XZ) referenced in function "public: __thiscall Spell::Spell(void)" (??0Spell@@QAE@XZ)
1>C:\Users\yohan\Documents\Visual Studio 2010\Projects\Revision1_1\Debug\Revision1_1.exe : fatal error LNK1120: 2 unresolved externals
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

I do not understand what to do here. Why is this happening? Please help! I am new to C++ anyway..

Answer 1

Magic is not implementing its constructor and destructor (which also should be virtual ).

Don't even declare the constructor if not necessary, eg

class Magic {
public:
  virtual ~Magic() {}
  virtual void display() = 0;
};

Unrelated: I didn't know you can display magic.

Answer 2

You didn't implement

Magic();
~Magic();

You'll need to either implement them inline, in an implementation file, or mark them = default .

Answer 3

You have declared a destructor in your Magic class but did not define it. That's why the linker complains (and the compiler doesn't).

Answer 4

You don't have an implementation for Magic. If your intention is for Magic to be an abstract base class, then just change its declaration to:

#pragma once

class Magic
{
public:
    virtual void display()=0;
};

Remember, any method that is not followed by = 0 in the interface must be implemented in the class.