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

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?

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

5 Answers