How are the __cplusplus directive defined in various compilers?

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The 199711L stands for Year=1997, Month = 11 (i.e., November of 1997) — the date when the committee approved the standard that the rest of the ISO approved in early 1998.

For the 2003 standard, there were few enough changes that the committee (apparently) decided to leave that value unchanged.

For the 2011 standard, it’s required to be defined as 201103L, (again, year=2011, month = 03) again meaning that the committee approved the standard as finalized in March of 2011.

For the 2014 standard, it’s required to be defined as 201402L, interpreted the same way as above (February 2014).

For the 2017 standard, it’s required to be defined as 201703L (March 2017).

For the 2020 standard, the value has been updated to 202002L (February 2020).

Before the original standard was approved, quite a few compilers normally defined it to 0 (or just an empty definition like #define __cplusplus) to signify “not-conforming”. When asked for their strictest conformance, many defined it to 1.

I almost forgot to mention, but one more tidbit about ancient compilers: a few of the earliest versions of cfront (and probably a few others copying it) defined c_plusplus instead of __cplusplus. I don’t recall it’s being defined to any meaningful value though.

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