NaN comparison rule in C/C++

Question

Doing some optimziation on a piece of code, the correctness of the code depending on how the compiler handle NaNs.

I read the IEEE-754 rules on NaN, which states:

Answer 1

C/C++ does not require specific floating-point representation and does not require that any comparison against NaN is false.

In C++ you can check if all floating-point types fulfill IEEE 754 using std::numeric_limits::is_iec559:

static constexpr bool is_iec559;

⁵⁶ True if and only if the type adheres to IEC 559 standard.²¹⁷

⁵⁷ Meaningful for all floating point types.

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217) International Electrotechnical Commission standard 559 is the same as IEEE 754.

For other floating-point representations comparison against NaN may or may not behave the same way.

In fact, even representing NaN itself is not required. See std::numeric_limits::has_quiet_NaN, std::numeric_limits::has_signaling_NaN.