C++ Tip : How to eliminate Temporary Objects

C++ creates temporary objects "behind your back" in several contexts. The overhead of a temporary can be significant because both its constructor and destructor are invoked. You can prevent the creation of a temporary object in most cases, though. In the following example, a temporary is created:

Complex x, y, z;
x=y+z; /* temporary created */

The expression y+z; results in a temporary object of type Complex that stores the result of the addition. The temporary is then assigned to x and destroyed subsequently. The generation of the temporary object can be avoided in two ways:

Complex y,z;
Complex x=y+z; /* initialization instead of assignment */

In the example above, the result of adding x and z is constructed directly into the object x, thereby eliminating the intermediary temporary. Alternatively, you can use += instead of + to get the same effect:

/* instead of x = y+z; */  
x=y;  
x+=z; 

Although the += version is less elegant, it costs only two member function calls: assignment operator and operator +=. In contrast, the use of + results in three member function calls: a constructor call for the temporary, a copy constructor call for x, and a destructor call for the temporary.