returning a temporary class object

This is a discussion on returning a temporary class object within the C++ Programming forums, part of the General Programming Boards category; i have a class that returns a temporary object when i overload the addition operator here is the sample code ...

  1. #1
    samurai warrior nextus's Avatar
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    Question returning a temporary class object

    i have a class that returns a temporary object when i overload the addition operator
    here is the sample code

    Code:
    CBox operator+(const CBox& aBox) // CBox is a class
    {
       float h = this->m_height + aBox.m_height;
       float b = this->m_breadth + aBox.m_breath;
       float l = this->m_length + aBox.m_length;
    
       return CBox(h, b, l);
       // returning a temporary CBox object
    }
    help is appreciated...thanks...this is always puzzles me because i thought objects cease to exist after they go out of scope...

  2. #2
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    It's not going out of scope. You'll be invoking some form of copy constructor in the calling expression (or your compiler may perform a return value optimsation).

    If you were returning a reference to your local object your assumption would be correct and I believe the outcome would be undefined.
    Joe

  3. #3
    samurai warrior nextus's Avatar
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    what is a return value optimization?

  4. #4
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    The compiler may notice that you're constructing an object within a function in order to return it. Instead of constructing a local object and then calling a copy constructor / assignment operator when the object is returned it'll construct / assign to the object that accepts the return value. Something like -

    object a = b + c

    Instead of constructing a temporary value within operator+ to store the result of the addition, and using this an argument in 'a's copy constructor 'a' will be constructed from within operator+ as if it had been passed in by reference as a hidden argument.

    This kind of thing may be necessary if you're calling operator+ allot; as multiple unnecessary construction may hurt performance (if this is a consideration). But presently not all C++ compilers carry it out effectively.
    Joe

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