Thread: A question about 'Return Value'

  1. #1
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    A question about 'Return Value'

    I have the following code just for fun

    Code:
    class Base{
    public:
    	Base(){cout<<"Constructor"<<endl;}
    	Base(const Base &obj){cout<<"Copy Constructor"<<endl;}
    	Base & operator=(const Base& obj){
    		cout<<"Operator="<<endl;
    		return *this;
    	}
    };
    
    Base foo(){
    	Base x;
    	return x;
    }
    
    void main(){
    	Base tmp = foo();
    }
    =================
    Why the outputs are only

    Constructor
    Copy Constructor

    But I think the outputs should be:
    Constructor
    Copy Constructor // from return x
    Copy Constructor // from Base tmp = foo();
    Last edited by meili100; 06-03-2007 at 11:44 PM.

  2. #2
    Kiss the monkey. CodeMonkey's Avatar
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    tmp called the copy constructor automatically. Nah nah.
    "If you tell the truth, you don't have to remember anything"
    -Mark Twain

  3. #3
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    But I think the outputs should be:
    Constructor
    Copy Constructor // from return x
    Copy Constructor // from Base tmp = foo();

  4. #4
    Massively Single Player AverageSoftware's Avatar
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    Your compiler is probably optimizing out one of the copies.

  5. #5
    Registered User hk_mp5kpdw's Avatar
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    This:
    Code:
    Base tmp = foo();
    Is rearranged by the compiler to become:
    Code:
    Base tmp(foo());
    So, the constructor is called in foo, and then this object is used (copy constructor called) to initialize tmp, so the output is as you've mentioned:
    Constructor (from inside foo function)
    Copy Constructor (initializing tmp from foo's return value)
    "Owners of dogs will have noticed that, if you provide them with food and water and shelter and affection, they will think you are god. Whereas owners of cats are compelled to realize that, if you provide them with food and water and shelter and affection, they draw the conclusion that they are gods."
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  6. #6
    Cat without Hat CornedBee's Avatar
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    And a compiler would be allowed to be even more aggressive and eliminate even the single copy being done.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
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  7. #7
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    Side note: void main() is evil. Use int main().

  8. #8
    "Why use dynamic memory?"
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    u have a mistake in the assignment operator, you should check if the passed object is the same one (memory address)
    Code:
    Base & operator=(const Base& obj){
                   //Check if the object is being assigned to it self
                   //By comparing the memory adress
                   if(this == &obj) {return *this;}
    		cout<<"Operator="<<endl;
    		return *this;
    	}
    Side note: void main() is evil. Use int main().
    Exactly , Bjarne in his website wrote: It has never been "void" for neither C++ nor C
    "C makes it easy to shoot yourself in the foot; C++ makes it harder, but when you do, it blows away your whole leg."-Bjarne Stroustrup
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  9. #9
    Massively Single Player AverageSoftware's Avatar
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    Quote Originally Posted by Hussain Hani View Post
    u have a mistake in the assignment operator, you should check if the passed object is the same one (memory address)
    It's actually pretty wasteful to check for self assignment in cases where it is harmless, such as this case.

  10. #10
    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by AverageSoftware View Post
    It's actually pretty wasteful to check for self assignment in cases where it is harmless, such as this case.
    Indeed.
    This is a good read on the subject of self-assignment:
    http://www.gotw.ca/gotw/011.htm
    Last edited by iMalc; 06-05-2007 at 01:15 AM.
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