Is #pragma once part of the C++11 standard?

Question

Traditionally, the standard and portable way to avoid multiple header inclusions in C++ was/is to use the #ifndef - #define - #endifpre-compiler directives sch

Answer 1

#pragma once is not standard. It is a widespread (but not universal) extension, which can be used

• if your portability concerns are limited, and
• you can be sure that all of your include files are always on a local disk.

It was considered for standardization, but rejected because it cannot be implemented reliably. (The problems occur when you have files accessible through several different remote mounts.)

It's fairly easy to ensure that there are no include guard conflicts within a single development. For libraries, which may be used by many different developments, the obvious solution is to generate a lot of random characters for the include guard when you create it. (A good editor can be set up to do this for you whenever you open a new header.) But even without this, I've yet to encounter any problems with conflicts between libraries.