fix: use strtod fallback when std::from_chars(float) unavailable

[zhjwpku]

closes #571

[HuaHuaY]

Can we implement this by SFINAE or concept instead of using macro?

[zhjwpku]

Can we implement this by SFINAE or concept instead of using macro?

I tried, but it still requires relying on the _LIBCPP_VERSION macro, I would appreciate your suggestions if there is a better solution.

[HuaHuaY]

Can we implement this by SFINAE or concept instead of using macro?

I tried, but it still requires relying on the _LIBCPP_VERSION macro, I would appreciate your suggestions if there is a better solution.

For examples,

#include <charconv>
#include <string_view>
#include <utility>
#include <iostream>

template <typename T>
concept FromChars = requires(const char* p, T& v) { std::from_chars(p, p, v); };

template <typename T>
    requires std::is_arithmetic_v<T> && (!std::same_as<T, bool>)
    && FromChars<T>
static T ParseNumber(std::string_view str) {
    T value = 0;
    auto [ptr, ec] = std::from_chars(str.data(), str.data() + str.size(), value);
    if (ec == std::errc()) [[likely]] {
        return value;
    }
    std::unreachable();
}

template <typename T>
    requires std::floating_point<T> && (!FromChars<T>)
static T ParseNumber(std::string_view str) {
    char* end = nullptr;
    return std::strtod(str.data(), &end);
}

int main() {
    auto v = ParseNumber<double>("123.12");
    std::cout << v << std::endl;
}

[HuaHuaY]

I think that you need to add FromChars<T> here otherwise old compiler will report error because both requires of the two functions are satisfied.

[zhjwpku]

Yeah, I don't have my old mac with me right now. I'll give it a try once I have access to it.

[zhjwpku]

I just tried it, and it works now, thanks for catching that.

[HuaHuaY]

LGTM

[wgtmac]

Thanks @zhjwpku for fixing this and @HuaHuaY for the review!