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Depending on the order of includes math.h may be included before we have a chance to set the define
Because we don't control the order of includes and we use M_PI/M_PI_2 in public headers we need to make sure including <cmath> will provide the proper defines.
We already include <cmath>
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The other alternative is to not use |
Not to be confused with the C one that requires math.h.
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BTW, there are C++ variants for these math values, but it requires C++20. |
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The public headers use
M_PIandM_PI_2which are defined when including<cmath>(ormath.h) and_USE_MATH_DEFINESis defined (and some other defines are possible as well:#if !defined(__STRICT_ANSI__) || defined(_POSIX_C_SOURCE) || defined(_POSIX_SOURCE) || defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE) || defined(_USE_MATH_DEFINES))In my LLVM 22 toolchain
<string>includes<cmath>. And it's included before the local header in many cases (as well as most toolchain headers are included before the local headers). And even if it was not the case, an app using the public headers could be including<cmath>before including our headers. In that caseM_PIandM_PI_2won't be defined. So we need to make sure_USE_MATH_DEFINESis defined before using our headers.