Initilize constructor of class from another class

Question

I have class A with constructor A(int a)

A.h

calss A{
  public :  A(int a);
}

A.cpp

#include "a.h"
#include <iostream>
A::A(int a)
{
    std::cout<<a<<end;
}

I need to initialize this class from another class B,

I tried

B.h

class B{
   public :  B();
            A tmp;
            //A tmp(4); //this  giving syntax error          
}

and

B.cpp

 B(){
     tmp = A(4);
}

But I am getting compiler error "error: no matching function for call to 'A::A()'"

(1) Post the complete error message. (2) Constructors don't have return types, so remove the `void`. — Dietrich Epp, Dec 31 '14 at 11:09
Possible duplicate of [In this specific case, is there a difference between using a member initializer list and assigning values in a constructor?](http://stackoverflow.com/questions/4589237/in-this-specific-case-is-there-a-difference-between-using-a-member-initializer) — πάντα ῥεῖ, Dec 31 '14 at 11:22

bolov · Accepted Answer · 2014-12-31T11:56:43.913

3

The problem is that A doesn't have a default constructor (because if you define another constructor and default one is not generated) and so A objects cannot be default constructed.

The solution is to use constructor initialization lists for every constructor of B where you call the appropriate constructor for tmp.

class B {
   A tmp;

public:
   B() : tmp(4) {}
};

If you can use c++11 and in every (or at least most of the) constructors of B you create tmp the same way you can use:

class B {
  A tmp{4};

public:
  B() {};
}

edited Dec 31 '14 at 11:56

answered Dec 31 '14 at 11:15

bolov

72,283
15
145
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tmp is not local variable, I need to use this as class member. – Haris Dec 31 '14 at 11:19

score 1 · Answer 2 · answered Dec 31 '14 at 11:11

1

Constructors should not have a return type. Remove the void in the declaration and you should be good to go. Same goes for your class B.

answered Dec 31 '14 at 11:11

Pzc

88
1
3
8

score 1 · Answer 3 · answered Dec 31 '14 at 11:16

Constructors are special member functions without any return type.

//A tmp(4); //this  giving syntax error

This will for sure give you an error because you mentioned

 void A(int a)

and this is not constructor; because its has return type (void). when you explicitly define parametrized constructor (like A(int a) )the default constructor gets vanished and for this statement

A tmp;

to compile you have to provide your own default constructor.

score 1 · Answer 4 · answered Dec 31 '14 at 11:25

1

It would be great always to post complete error message.

Rather than getting "no matching function for call to" , I am sure that you will be getting below error:

error: no matching function for call to ‘A::A()’
note:                 A::A(int)

This clearly indicates that default constructor A::A() is missing from your code as you have provided your own constructor.

Suggestion: You have defined conversion constructor. It is good practice to use "explicit" keyword.

answered Dec 31 '14 at 11:25

CreativeMind

897
6
19

hi. this should be a comment, not an answer. – bolov Dec 31 '14 at 11:26
@bolov : Yes you are right. But my answer actually contains answer and suggestion both. – CreativeMind Dec 31 '14 at 11:29

score 1 · Answer 5 · answered Dec 31 '14 at 11:57

*.h file

class A
{
    public:
        A(int a);
};
class B
{
    public:
        B();
        A *tmpA;
};

*.cpp file

A::A(int a)
{
    MessageBox(NULL,L"",L"", MB_OK);
}
B::B()
{
    tmpA = new A(4);
}

usage

{
    B *b = new B();
}

is it correct for you?

Initilize constructor of class from another class

5 Answers5