How to create an object of a derived class in a base class in c++

Question

Hello I am trying to create a program but am having some trouble I have 3 classes called Goku vegeta Beerus with Goku being the base class and vegeta and Beerus inherting from Goku I want to create an object of vegeta called rose and an object of Beerus called god inside Goku but am getting error can anyone point out how i would do this any and all help is appreciated the program is below:

class Goku
{    public:
     Goku(); 
   ˜ Goku();
   string get_Name(void); 
   int get_power(void)
 // This is what i want to do but keep getting errors
   vegeta* rose;
   Beerus* god;


};
class vegeta: public Goku
{
  public:
    vegeta(); //an intitializtion constructor
 ˜ vegeta();//destructor
   string get_Name(void); 
   int get_power(void)

};
class Beerus: public Goku
{      public:
       Beerus(); //an intitializtion constructor
      ˜ Beerus();//destructor
      string get_Name(void); 
     int get_power(void)

};

Note the error i get are: vegeta cannot name a type Beerus cannot name a type

Have you tried "rubber duck debugging"? When you hit the line `GokuBlack* rose;` Do you have any idea what `GokuBlack` is? — John3136, Sep 11 '16 at 11:47
Oh sorry Gokublack is a mistake should be vegeta i will edit it straight away thank you for pointing it out — LOLtricker, Sep 11 '16 at 11:48
Same question applies: when you hot `Beerus* god;`, do you have any idea what `Beerus` is? — John3136, Sep 11 '16 at 11:52

Sergey Tyulenev · Answer 1 · 2016-09-11T11:56:41.823

1

use pre-declaring class
A class definition may reside somewhere else. But this class can be used for a pointer to a memory area.

class Beerus;         // pre-declaring class
class GokuBlack;      // pre-declaring class

class Goku
{
public:
    Goku();
    ~Goku();

    string get_Name(void);
    int get_power(void);


    GokuBlack* rose;
    Beerus* god;
};

class vegeta : public Goku
{
public:
    vegeta(); //an intitializtion constructor
    ~vegeta();
    string get_Name(void);
    int get_power(void);
};

class Beerus : public Goku
{
public:
    Beerus(); //an intitializtion constructor
    ~Beerus();

    string get_Name(void);
    int get_power(void);
};

edited Sep 11 '16 at 11:56

answered Sep 11 '16 at 11:48

Sergey Tyulenev

73
1
8

Just a code dump without explanation is not a good answer. – John3136 Sep 11 '16 at 11:52
i'm edit answer – Sergey Tyulenev Sep 11 '16 at 11:57

score 0 · Accepted Answer · answered Sep 11 '16 at 12:07

You have an error in your class. Currently, when you do this:

vegeta* rose;
Beerus* god;

At that point, the compiler doesn't know what vegeta and Beerus are. To solve this, add the following 2 lines all the way at the beginning of your code:

class Beerus;    
class vegeta;

This tells the compiler, that if one of the above 2 classes is referenced, before being completely defined, as in instantiated, then the compiler can look ahead for the name in the rest of the file.

Live Example

use forward declaration to use incomplete types. but you'll face problems I recommend use templates — Raindrop7, Sep 11 '16 at 12:23

score 0 · Answer 3 · answered Sep 11 '16 at 13:13

let's not talk about containment just about forwarding declaration consider:

#include "stdafx.h"
#include <iostream>
using namespace std;


class A;
class B;

class B
{
public:
    B();
    void print() { aObj->print(); } // error
    private:
        A* aObj; // it's ok because it's declared
};

B::B()
{
    aObj = new A; // error
}

class A
{
 public:
    A(){}
    void print() { cout << "A prints" << endl; }
};

now the solution:

just move the definition of constructor B under class A and print() of B under A and everything is ok! because when it comes to initialize aObj it looks above and find a complete A this what we use when use header files containing implementations of classes so the code becomes like this:

class A;
class B;

class B
{
    public:
        B();
        void print(); // just prototype
    private:
        A* aObj;
};

class A
{
     public:
     A(){}
     void print() { cout << "A prints" << endl; }
};

B::B()
{
    aObj = new A; // correct
}

void B::print()
{
     aObj->print(); // ok
}

How to create an object of a derived class in a base class in c++

3 Answers3