CPPReference is not the C++ standard. Its a wiki. It gets things wrong. It doesn't always give you the full information. Probably best not to rely on it, for things that matter.

But, for example, LLVM does not fully support IEEE 754 [0].

And nor does GCC - who list it as unsupported, despite defining the macro and having partial support. [1]

The biggest caveat is in Annex F of the C standard:

> The C functions in the following table correspond to mathematical operations recommended by IEC 60559. However, correct rounding, which IEC 60559 specifies for its operations, is not required for the C functions in the table.

The C++ standard [2] barely covers support, but if a type supports any of the properties of ISO 60559, then it gets is_iec559 - even if that support is _incomplete_.

This paper [3] is a much deeper dive - but the current state for C++ is worse than C. Its underspecified.

> When built with version 18.1.0 of the clang C++ compiler, without specifying any compiler options, the output is:

> distance: 0.0999999

> proj_vector_y: -0.0799999

> Worse, if -march=skylake is passed to the clang C++ compiler, the output is:

> distance: 0.1

> proj_vector_y: -0.08

[0] https://github.com/llvm/llvm-project/issues/17379

[1] https://www.gnu.org/software/gcc/projects/c-status.html

[2] https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/n49...

[3] https://isocpp.org/files/papers/P3375R2.html

Big-Endian Testing with QEMU