What is the reason for #pragma once inside header guards?

c++ boost c-preprocessor header-files pragma
dubnde picture dubnde · Aug 13, 2010 · Viewed 7.3k times · Source

Just seen this inside <boost/asio.hpp>

#ifndef BOOST_ASIO_HPP
#define BOOST_ASIO_HPP

#if defined(_MSC_VER) && (_MSC_VER >= 1200)
# pragma once
#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)

/// ....

#endif // BOOST_ASIO_HPP

Disregarding the _MSC_VER preprocessor checks, what is the benefit of having the #pragma once in this case? Doesn't the preprocessor header guard ensure in all cases and on all platforms, the header contents are only ever included once?

Answer

Aoi Karasu picture Aoi Karasu · Aug 13, 2010

#pragma once specifies that the file will be included (opened) only once by the compiler when compiling a source code file. This can reduce build times as the compiler will not open and read the file after the first #include of the module.

If you don't #pragma once, the file will be opened each time it is needed and compiler will stop parsing it on #ifndef BOOST_ASIO_HPP, if it has been defined.

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