2024/constexpr-number-parsing-cpp23/

[utterances-bot]

Parsing Numbers At Compile Time with C++17, C++23, and C++26 - C++ Stories

Thanks to the powerful constexpr keyword and many enhancements in recent C++ standards, we can now perform a lot of computations at compile time. In this text, we’ll explore several techniques for parsing integers, including the “naive” approach, C++23, from_chars, std::optional, std::expected, and even some upcoming features in C++26. But first… why would this even be needed?

https://www.cppstories.com/2024/constexpr-number-parsing-cpp23/

[2kaud]

The C++23 version using std::optional can be 'simplified'. See https://godbolt.org/z/WdThsjYzK

Also similar with the std::expected version.

Note that in this version static_assert(tryParseIntCpp23(" 6789abc").value() == 6789);

does not assert (as the string is not wholly an integer), but does assert in the original.

[fenbf]

Thanks for the comment and for the code. My parsing routines have "loose" requirements, but your code is also looking fine. It's probably nicer than mine and shorter.

[2kaud]

I note that the c++ functions isxxx() and toyyy() (defined in cctype) are still not constexpr. This is probably because many are locale specific. For working in just ASCII with unsigned 8-bit chars, we use these as replacement which are constexpr. These are also faster! See https://godbolt.org/z/zdTM3KhMj This defines these functions for char 0 - 127 but the method is easily extended to 0 - 255 and to add other isxxx() or toyyy() functions as needed.