Copy object

This is a discussion on Copy object within the C++ Programming forums, part of the General Programming Boards category; If I have this Code: class A { int a; Object B; .... }; class B { int b; AnotherObj ...

  1. #1
    Registered User C_ntua's Avatar
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    Jun 2008
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    1,853

    Copy object

    If I have this
    Code:
    class A
    {
       int a;
       Object B;
       ....
    };
    
    class B
    {
       int b;
       AnotherObj C;
    };
    ...
    and I do
    Code:
    A a = myObjA;
    then the copy constructor is called and a shallow copy is made. Is a.B copied as well?

    I know that a shallow copy in languages like C# or Java don't copy the objects, since they only have references. But what about in C++?

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    14,185
    Each member is copied.

  3. #3
    Captain Crash brewbuck's Avatar
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    Mar 2007
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    Portland, OR
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    Quote Originally Posted by tabstop View Post
    Each member is copied.
    While true, I think that's a bit incomplete.

    Each member is "copied," but what that actually means depends on the type of member.

    Objects held by value are copied, meaning a new instance is created using the copy constructor.

    Pointers are copied, meaning a new pointer is created but the underlying object is not altered or copied.

    References are copied, meaning a new reference is created but again, the underlying object is not altered or copied.

    Generally, if the member objects themselves do "the right thing" when copied, then a class containing these objects will also do the right thing.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  4. #4
    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by C_ntua View Post
    If I have this
    Code:
    class A
    {
       int a;
       Object B;
       ....
    };
    
    class B
    {
       int b;
       AnotherObj C;
    };
    ...
    and I do
    Code:
    A a = myObjA;
    then the copy constructor is called and a shallow copy is made. Is a.B copied as well?

    I know that a shallow copy in languages like C# or Java don't copy the objects, since they only have references. But what about in C++?
    A shallow copy has the same meaning in all languages. The question here is whether this is a shallow copy or not. The answer to that depends on the behaviour of AnotherObj. I.e. whether it is ref-counted, CoW, or just plain duplicated in the copy-constructor etc.
    If AnotherObj's only member were an int for example, then here a shallow copy and a deep copy are the same thing.
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