When to return a reference and when to return a copy?

  • When to return a reference and when to return a copy? dotminic

    I was looking at the d3dx math .h and noticed that a difference between the "+" and "+=" operators:

    D3DXVECTOR3& operator += ( CONST D3DXVECTOR3& );
    D3DXVECTOR3 operator + ( CONST D3DXVECTOR3& ) const;

    Why is it returning a reference in the case of a += operation and a copy in the case of + operation ?

    I'm guessing in the first case it's modifying the vector on which you call the += operation, something like this:

    this->_x += x;
    return *this;

    and in the case of a + operation it would be doing something like this:

    return D3DXVECTOR3(this->_x + x, this->_y + y, this->_z + z);

    That being said, if that's what happens in the D3DXVECTOR3 implementation, why not return a reference in the case of + operation ?

  • In the operator += case, the method is probably modifying the calling object in place. On the other hand, operator + is creating a new D3DXVECTOR3 that is the sum of the calling object and the argument.

  • You return a reference to the original vector in a += because you are modifying the actual vector itself and the vector will not fall out of scope.

    While in a + you do not want any modification of the original vector. Instead a + operator computes a new value and uses copy constructors to return the value.

    If you would try something like

    D3DXVECTOR3 newVal = D3DXVECTOR3(this->_x + x, this->_y + y, this->_z + z);
    return &newVal;

    You would return a reference to a vector which fell out of scope and therefore will be cleaned up very soon.

  • Why is a reference returned for += ?

    A reference is returned because the object is (likely) being modified directly, and as such it wouldn't make sense to create a copy for the sole purpose of returning one.

    Note that the reference can still be converted to a copy by other operations (such as =):

    D3DXVECTOR3 a (1,2,3);
    D3DXVECTOR3 b (1,2,3);
    a+=b;                 //no copy made
    D3DXVECTOR3 c = a+=b; //one copy made

    So this approach (returning by reference) allows for the least amount of work to be done in each situation.

    Why is a reference NOT returned for + ?

    A reference is not returned for + because a copy is made as part of the operation, so it's returned. Consider what would happen if you added two D3DXVECTOR3's together, like this: (a + b). Which of the two objects should the result be referencing? Well obviously the answer is neither - the result should be a copy. Hence a copy is returned.

c++ directx vector
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