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Simple C++ Class problem

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Join Date: Nov 2008

Posts: 2

Reputation: Photomotoz is an unknown quantity at this point

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Photomotoz Photomotoz is offline

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Newbie Poster

Simple C++ Class problem

Nov 24th, 2008

Hey, guys I think this is a very simple problem and that I just can't get it is due to some gap in my knowledge. Any way, what is wrong with this :

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 C++ Syntax (Toggle Plain Text) 
class A{
private: int value;
public : A(int v);
};
 
A::A(int v)
{value = v;}
 
//Does not work
class B {A ob(5);};
//Works
main(){A ob(5);}
class A{
private: int value;
public : A(int v);
};

A::A(int v)
{value = v;}

//Does not work
class B {A ob(5);};
//Works
main(){A ob(5);}

By does not work, I mean compile.

Thanks up front.

Last edited by Photomotoz; Nov 24th, 2008 at 3:50 pm.

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Join Date: Jun 2008

Posts: 182

Reputation: mrboolf will become famous soon enough

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mrboolf

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Re: Simple C++ Class problem

Nov 24th, 2008

You should write your B class like this:

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 C++ Syntax (Toggle Plain Text) 
class B {
     A *ob;
};
class B {
     A *ob;
};

and then in B constructor add ob = new A(5);
I think you can't call constructors from within a class declaration.

Last edited by mrboolf; Nov 24th, 2008 at 4:25 pm.

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Join Date: Nov 2008

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Photomotoz Photomotoz is offline

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Re: Simple C++ Class problem

Nov 24th, 2008

Hey, thanks for the reply.

What do you mean by
"in B constructor add ob = new A(5);"

What I am trying to do is make several objects of a class with several parameters within another class, which then in return gets called in main.

Last edited by Photomotoz; Nov 24th, 2008 at 4:35 pm.

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Join Date: Jun 2008

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Reputation: mrboolf will become famous soon enough

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mrboolf

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Junior Poster

Re: Simple C++ Class problem

Nov 24th, 2008

I don't understand what you are trying to do - but my suggestion was to write a constructor for your B class like this

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 C++ Syntax (Toggle Plain Text) 
B::B() {
     ob = new A(5);
}
B::B() {
     ob = new A(5);
}

so that every time an object of type B is created, its member A object will be initialized with the chosen value. Of course you may pass parameters to the constructor of B and forward them to the constructor of A - just don't forget to free the memory by putting delete A; in B's destructor.

Hope this helps.

Last edited by mrboolf; Nov 24th, 2008 at 4:53 pm.

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Join Date: Nov 2008

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StuXYZ

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Re: Simple C++ Class problem

Nov 24th, 2008

I think a little care in needed here. Poster #4 suggested making a heap object of A. [new A(5)] but that is maybe not what you want. It adds a level of complexity that might not be needed. You have to delete ob in B's destructor and manage the memory carefully in the copy constructor.

So what about

 Help with Code Tags 
 c++ Syntax (Toggle Plain Text) 
class A
{
  private: 
    int value;
  public: 
    A(int);        // constructor taking a value
};
 
class B
{
   private:
       A obj;
  public:
     B();           // Default Constructor
};
 
A::A(int v) : value(v)
{}
 
B::B() : Obj(5)
{}
 
 
int
main()
{
  A ob(8);
  B bItem;
}
class A
{
  private: 
    int value;
  public: 
    A(int);        // constructor taking a value
};

class B
{
   private:
       A obj;
  public:
     B();           // Default Constructor
};

A::A(int v) : value(v)
{}

B::B() : Obj(5)
{}


int
main()
{
  A ob(8);
  B bItem;
}

That way you have a A object in each B, and it goes out of scope on with with the corresponding B object.

It you want to pass parameters to B from A, try this.

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 c++ Syntax (Toggle Plain Text) 
class B 
{
 private:
       A obj;
       A secondObj;
  public:
     B(int,int);           // Constructor
};
 
B::B(int a,int b) : 
   Obj(a),SecondObj(b)
 
int
main()
{
   B bItem(3,4);       // Make Obj(3) and secondObj(4)
}
class B 
{
 private:
       A obj;
       A secondObj;
  public:
     B(int,int);           // Constructor
};

B::B(int a,int b) : 
   Obj(a),SecondObj(b)

int
main()
{
   B bItem(3,4);       // Make Obj(3) and secondObj(4)
}