Question

Possible Duplicate:
#pragma once vs include guards?

What are the differences (in performance, usability and functionality) in using #pragma once and #if !defined MYHEADER_INCLUDED_ constructs? Or what is the difference between the two?

Answer 1

Wikipedia has all your answers for this and is easy enough to find

From the article

In the C and C++ programming languages, #pragma once is a non-standard
but widely supported preprocessor directive designed to cause the
current source file to be included only once in a single compilation.

Thus, #pragma once serves the same purpose as #include guards, but with
several advantages, including: less code, avoiding name clashes, and
improved compile speed.

It has a more in-depth look at the advantages and disadvantages in the article. If you are really interested I suggest you read it completely, not just the blurb above.