std::to_chars compile but not linking on MacOS / clang - c++

Question

I have problem compiling a project under MacOS with clang.
I did "pinpoint" the problem inside charconv header:
#include <charconv>
#include <array>
#include <iostream>
int main(){
std::array<char, 64> buffer;
auto[p, ec] = std::to_chars(buffer.begin(), buffer.end(), 123);
if (ec != std::errc() )
std::cout << "error" << '\n';
std::cout << (const char *) buffer.data() << '\n';
}
Here is how I am compiling it.
Nikolays-MacBook-Air:~ nmmm$ clang --version
Apple LLVM version 10.0.1 (clang-1001.0.46.4)
Target: x86_64-apple-darwin18.5.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
Nikolays-MacBook-Air:~ nmmm$ clang -std=c++17 x.cc -lstdc++
Undefined symbols for architecture x86_64:
"std::__1::__itoa::__u32toa(unsigned int, char*)", referenced from:
std::__1::__itoa::__traits_base<unsigned int, void>::__convert(unsigned int, char*) in x-9b1746.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Nikolays-MacBook-Air:~ nmmm$
Any help will be appreciated.
update
I tried clang++, but it gives me same error message.

Answer 1

The <charconv> header contains declarations and uses of std::__1::__itoa::__u32toa, a function which is not defined in that header. The function is defined in libc++.dylib. However, Apple does not ship libc++.dylib with the same frequency as it ships headers; and, possibly more important, libc++ failed to include the new symbol in its "abilist" for Apple until 2018-09-23, even though the header change landed on 2018-08-01. So there was a period of about 53 days where (this aspect of) libc++ just didn't work on OSX. Maybe Apple picked it up and shipped it during that period.
One way to make your code work is to build libc++ yourself. Instructions here, as of late 2019.
$ clang++ -std=c++17 test.cc $ROOT/llvm-project/build/lib/libc++.dylib \
-Wl,-rpath,$ROOT/llvm-project/build/lib
$ ./a.out
123
Some commenters on your question are suggesting that you could make your code work by using GNU libstdc++ instead of LLVM libc++. That's vaguely true, but I suspect that building libstdc++ for OSX will be even harder than building libc++ for OSX. (And the latter is not simple!) I am not aware of any way to install libstdc++ on OSX. There is brew install gcc#8, but I bet GCC 8 didn't have <charconv> either.
As of early 2020, no vendor (except just recently Microsoft) provides a full implementation of <charconv> — the floating-point to_chars and from_chars turn out to be difficult, so most vendors don't provide them. ("Why didn't they just copy an existing implementation?" Well, it turns out that <charconv> was standardized as part of C++17 before any implementation existed. Nobody guessed that it would be difficult!)

Answer 2

I wrote a nice long answer to this, to then check and see that it works for me (clang 9.0.0 on linux). You compile with clang and not clang++. The long answer below:
Clang's libc++ has not fully implemented the "Elementary string conversions, revision 5 " (as in to_chars and from_chars) for a long time and some parts are still outstanding, see here. But I think yours should work, so maybe you should update your library.
For usage of to_chars please consider, that it explicitly adds no \0 at the end of the string. Therefore you cannot just use std::cout << (const char *) buffer.data();. You have to use the pointer (named p in your code) to mark the end of your string. Either by inserting a \0, by generating a std::string_view sv(buffer.data(), static_cast<size_t>(p-buffer.data());, using std::copy(buffer.begin(), p, std::ostream_iterator<char>(std::cout, "")); or another of far too many ways.