Follow Techotopia on Twitter

On-line Guides
All Guides
eBook Store
iOS / Android
Linux for Beginners
Office Productivity
Linux Installation
Linux Security
Linux Utilities
Linux Virtualization
Linux Kernel
System/Network Admin
Scripting Languages
Development Tools
Web Development
GUI Toolkits/Desktop
Mail Systems
Eclipse Documentation

How To Guides
General System Admin
Linux Security
Linux Filesystems
Web Servers
Graphics & Desktop
PC Hardware
Problem Solutions
Privacy Policy




Thinking in C++
Prev Contents / Index Next

The return optimization

When new objects are created to return by value, notice the form used. In operator+, for example:

return Integer(left.i + right.i);

This may look at first like a “function call to a constructor,” but it’s not. The syntax is that of a temporary object; the statement says “make a temporary Integer object and return it.” Because of this, you might think that the result is the same as creating a named local object and returning that. However, it’s quite different. If you were to say instead:

Integer tmp(left.i + right.i);
return tmp;

three things will happen. First, the tmp object is created including its constructor call. Second, the copy-constructor copies the tmp to the location of the outside return value. Third, the destructor is called for tmp at the end of the scope.

In contrast, the “returning a temporary” approach works quite differently. When the compiler sees you do this, it knows that you have no other need for the object it’s creating than to return it. The compiler takes advantage of this by building the object directly into the location of the outside return value. This requires only a single ordinary constructor call (no copy-constructor is necessary) and there’s no destructor call because you never actually create a local object. Thus, while it doesn’t cost anything but programmer awareness, it’s significantly more efficient. This is often called the return value optimization.

Thinking in C++
Prev Contents / Index Next

   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire