Add numeric conversions

[unterumarmung]

Suggested API:

    // numeric conversions
    template <size_t N>
    constexpr int stoi(const fixed_string<N>& str, int base = 10);
    
    template <size_t N>
    constexpr unsigned stou(const fixed_string<N>& str, int base = 10);
    
    template <size_t N>
    constexpr long stol(const fixed_string<N>& str, int base = 10);
    
    template <size_t N>
    constexpr unsigned long stoul(const fixed_string<N>& str, int base = 10);
    
    template <size_t N>
    constexpr long long stoll(const fixed_string<N>& str, int base = 10);
    
    template <size_t N>
    constexpr unsigned long long stoull(const fixed_string<N>& str, int base = 10);
    
    template <size_t N>
    constexpr float stof(const fixed_string<N>& str);
    
    template <size_t N>
    constexpr double stod(const fixed_string<N>& str);
    
    template <size_t N>
    constexpr long double stold(const fixed_string<N>& str);
    
    template <auto val> // where decltype(val) is an integral type but not any of char types
    constexpr fixed_string</*...*/> to_fixed_string() noexcept;

Proposed implementation: using to_chars and from_chars from <charconv>
Known issues: cannot be really constexpr

• Create implementation
• Create tests
• Add docs

[abel-mak]

Hi, i implemented stoi can i open pr for it?

[unterumarmung]

@abel-mak sure!

[abel-mak]

@unterumarmung Hey, i didn't use from_chars in the implementation, must i use it?

[unterumarmung]

@abel-mak
from_chars is not constexpr
So:

1. For C++17 we can use from_chars and doesn't allow to call our function in the constant context
2. For C++20 with if (std::is_constant_evaluated()) we can use our own implementation that is constexpr and use from_chars at the run-time

[unterumarmung]

You can open the PR and I will take a look what have so far

[abel-mak]

@unterumarmung this what i've done so far
abel-mak@1f9725e

[unterumarmung]

@abel-mak I've looked on what you already written
Below you can find rules from C++ Core Guidelines that will make your code better:

1. Do not declare before actual use (for example sign variable): https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-init
2. After applying Rule 1, it will be easier to apply this one: https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rconst-immutable
3. Also, please use auto for long type names

[unterumarmung]

You can open the PR: it's easier to review code with opened pull request

[unterumarmung]

Also, there's a tradeoff to be made with C++17, we either should:

1. Provide not as effective stoi implementation for run-time but keep the ability to call the function at compile-time
2. Or remove constexpr in C++17 and provide effective run-time implementation with from_chars

There is no tradeoff in C++20 since we can distinguish run-time and compile-time with if (std::is_constant_evaluated())

[unterumarmung]

@GeorgyFirsov @DymOK93 What do you think about all that (the code and the tradeoff)?

[DymOK93]

1. It is unsafe to use std::is* functions without conversion to unsigned char: for example, see description of the std::isspace
2. I tried to use the __is_constant_evaluated compiler intrinsics, but they seem to have appeared much later than C++17. It might be wise to try to apply them, but in general, you will have to leave the C++17 version without constexpr.
   I'll think about this question... The answer depends on the most promising use of fixed_string

[abel-mak]

@unterumarmung hey, i opened PR and tried to add some changes from what you said about rules.