Bulldozer00's Blog

For most cases, the “assignment” and “function” styles of initializing objects in C++ are the same. However, as the example below shows, in some edge cases, the function style of initialization can be more efficient. Nevertheless, for all practical purposes they are essentially the same since the compiler may optimize away the actual assignment step in the 2 step “assignment” style.

The motivation for this post came from a somewhat lengthy debate with a fellow member of the “C++ Developers Group” on LinkedIn.com. I knew that a subtle difference between the two initialization styles existed, but I couldn’t remember where I read about it. However, after I wrote this post, I browsed through my C++ references again and I found the source. The difference is explained in a much more intelligible and elegant way in “Efficient C++ Performance Programming Techniques“. Specifically, the discussion and example code in Chapter 5, “Temporaries – Object Definition“, does the trick.

Update 12/3/12

As my colleague friend pointed out, the above post is outright wrong with respect to the possibility of the assignment operator being used during initialization. Assignment is only used to copy values from one existing object into another existing object – not when an object is being created. That’s what constructors do. The “Doing Copy Assignment” text in the above code only prints to the console because of the last a2 = a1 statement in main(), which I put there to stop the g++ compiler from complaining about an unused variable. D’oh!

The example in the “Efficient” book that triggered our discussion is provided here:

The authors go on to state:

Only the first form of initialization is guaranteed, across compiler implementations, not to generate a temporary object. If you use forms 2 or 3, you may end up with a temporary, depending on the compiler implementation. In practice, however, most compilers should optimize the temporary away, and the three initialization forms presented here would be equivalent in their efficiency.

Ever since I read that book many years ago, I’ve always preferred to use the  “function” style initialization over the “assignment” style. But it’s just a personal preference.