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

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#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.

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