According to MSVC manual (*1), cl.exe can skip including a header file when that:
contains #pragma once, or
starts with #ifndef, or
starts with #if ! defined.
GCC has a similar trick (*2), but it acts more stricter (e. g. there must be no tokens outside of #ifndef...#endif).
Sun C lacked #pragma once for a looong time. Oracle Developer Studio 12.5 finally implemented it, but we cannot assume such recent version.
This changeset modifies header files so that each of them include strictly one #ifndef...#endif. I believe this is the most portable way to trigger compiler optimizations. [Bug #16770]
Added by shyouhei (Shyouhei Urabe) over 5 years ago
add #include guard hack
According to MSVC manual (*1), cl.exe can skip including a header file
when that:
GCC has a similar trick (*2), but it acts more stricter (e. g. there
must be no tokens outside of #ifndef...#endif).
Sun C lacked #pragma once for a looong time. Oracle Developer Studio
12.5 finally implemented it, but we cannot assume such recent version.
This changeset modifies header files so that each of them include
strictly one #ifndef...#endif. I believe this is the most portable way
to trigger compiler optimizations. [Bug #16770]
*1: https://docs.microsoft.com/en-us/cpp/preprocessor/once
*2: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html