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

Question

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 1

#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.

Answer 2

Specifies that the file will be included (opened) only once by the compiler in a build. This can reduce build times as the compiler will not open and read the file after the first #include of the module

And one more related question from SO