C++ - 2 classes 1 file

Question

Suppose I want something of this sort, in one .cpp source file:

class A {
    public:
        void doSomething(B *b) {};
};

class B {
    public:
        void doSomething(A *a) {};
};

Is there anyway of doing this without splitting it into two separate files, and without receiving a compiler error (syntax error on doSomething(B *b))

@Yuval, accept the person that was the first to answer your question. — Daniel Sloof, Mar 24 '09 at 13:23
@Yuval - accept the first answer that answered your question correctly and helpfully. — Dominic Rodger, Mar 24 '09 at 13:24
Or accept the "best" answer in your opinion. Jiri was fastest and most concise. Others provide links or explanations of why that answer is correct. Whatever you prefer :-) — Steve Jessop, Mar 24 '09 at 13:38
A year has passed, and not a single acknowledgment of the reference in the question title... — Yuval Adam, May 18 '11 at 15:26

score 22 · Accepted Answer · answered Mar 24 '09 at 13:15

22

put at the first line:

class B;

answered Mar 24 '09 at 13:15

Jiri

16,425
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Frederik Gheysels · Answer 2 · 2013-09-30T07:29:47.533

12

If I remember well, you can 'pre-declare' your class B.

class B; // predeclaration of class B

class A
{
   public:
      void doSomething(B* b);
}

class B
{
    public
      void doSomething(A* a) {}
}

public void A::doSomething(B* b) {}

Then, your class 'A' knows that a class 'B' will exists, although it hasn't been really defined yet.

Forward declaration is indeed the correct term, as mentionned by Evan Teran in the comments.

edited Sep 30 '13 at 07:29

answered Mar 24 '09 at 13:16

Frederik Gheysels

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1

good answer, so you get the +1. But for reference the term is "forward declaration" – Evan Teran Mar 24 '09 at 14:46

score 4 · Answer 3 · answered Mar 24 '09 at 13:16

4

forward declare one class before other with

class B;
or
class A;

But still you won't be able to implement

void doSomething(B *b)

using only forward declaration of B. So you have to put definition of doSomething below full class A declaration

answered Mar 24 '09 at 13:16

Mykola Golubyev

57,943
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score 3 · Answer 4 · answered Mar 24 '09 at 13:16

3

Yes. You need a forward declaration:

class B; // add this line before A's declaration

class A {
    public:
        void doSomething(B *b) {};
};

class B {
    public:
        void doSomething(A *a) {};
};

answered Mar 24 '09 at 13:16

Mr Fooz

109,094
6
73
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score 3 · Answer 5 · answered Mar 24 '09 at 13:19

3

The C++ FAQ Lite answers this question and others. I'd seriously considering reading that thing end to end, or getting the book and doing the same.

answered Mar 24 '09 at 13:19

Brian

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score 1 · Answer 6 · answered Mar 24 '09 at 13:18

You can try a forward declaration like

class B;
class A {
  void Method( B* );
};
class B{
};

but you will only be able to declare pointer and reference variables for B then. If you want more (like a method that dereferences B* variable) you can provide a declaration only and define methods later in the same file - at the point where both classes declaration is already available.

score 1 · Answer 7 · answered Mar 24 '09 at 13:18

1

You need to forward declare B.

class B; 

class A
{
public:        
   void doSomething(B *b) {}
};

class B 
{    
public:        
   void doSomething(A *a) {}
};

(And BTW, you don't need the semi-colons after the member function curly braces. :) )

answered Mar 24 '09 at 13:18

Brian Neal

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score 0 · Answer 8 · answered Mar 24 '09 at 13:20

0

Add another declaration of B before A:

class B;

class A {
    public:
        void doSomething(B *b) {};
};

class B {
    public:
        void doSomething(A *a) {};
};

answered Mar 24 '09 at 13:20

Jay Michaud

405
3
18

C++ - 2 classes 1 file

8 Answers8

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