CODEWITHSUNDEEP
c++copy-constructorassignment-operator
abhi4eternity
abhi4eternity

Reputation: 440

Why is copy constructor not called if the overloaded + and = operators return by value in the following case?

When [abc e = a+b] is called, the copy constructor is not called. 

class abc{
        int i;
        public:
        abc()
            {
            i = 10;
            cout<<"constructor"<<endl; 
        }
        abc(const abc &a)
        {   
            cout<<"copy constructor"<<endl;
            i = a.i; 
        }
        abc operator=(const abc &a)
        {
            cout<<"operator="<<endl;
            abc temp;
            temp.i = a.i;
            return temp;
        }
        abc operator+(abc& a)
        {   
            cout <<"Operator+ called"<<endl;
            abc temp;
            temp.i = i+a.i;
            return temp ;
        }
    };

    int main()
    {
        abc a,b;
        cout <<"----------------------------------------------"<<endl;
        a = b;
        cout <<"----------------------------------------------"<<endl;
        abc c = a;
        cout <<"-----------------------------------------------"<<endl;
        abc d(a);
        cout <<"-------------------------------------------"<<endl;
        **abc e = a+b;** 
    }

However if the overload operators methods are replaced with the following methods that return references to the object of class abc, copy constructor gets called.

abc& operator=(const abc &a)
    {
        cout<<"operator="<<endl;
        i = a.i;
        return *this;
    }
    abc& operator+(const abc& a)
    {   
        cout <<"Operator+ called"<<endl;
        i = i+a.i;
        return *this ;
    }

Can some one please explain why does this happen?

Upvotes: 2

Views: 240

Answers (3)

thanga
thanga

Reputation: 183

You can disable optimization options on your compiler. Please check for the optimization flag used while compiling your code.

Upvotes: 0

Loki Astari
Loki Astari

Reputation: 264351

Its an optimization explicitly allowed by the standard.

In n3376
12.8 paragraph 31:

When certain criteria are met, an implementation is allowed to omit the copy/move construction of a class object, even if the constructor selected for the copy/move operation and/or the destructor for the object have side effects.

This optimization can be achieved via RVO (this case) and under certain conditions NRVO.

Upvotes: 0

SingerOfTheFall
SingerOfTheFall

Reputation: 29966

This is happening due to return value optimization.

Since no costly memory-copying operations are being made if your constructor returns a reference, it works as you would expect, because no optimization is being made in this case. If it returns a value, the optimization kicks in and it works in a somewhat unexpected (for you) manner, but it is allowed by the standard.

Upvotes: 1

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