Is C++17 std::to_chars adding a null terminator?

Question

http://en.cppreference.com/w/cpp/utility/to_chars

Reference does not say anything about that, but the example is (for me) clearly using a null-terminated string, otherwise how could it know where to end, since std::array::data returns only a pointer.

#include <iostream>
#include <charconv>
#include <array>

int main()
{
    std::array<char, 10> str{};
    std::to_chars(str.data(), str.data()+str.size(), 42);
    std::cout << str.data();
}

Unfortunately I can't test it myself because AFAIK no compiler supports it yet: https://en.cppreference.com/w/cpp/compiler_support

Edit: Forgot that str is initialized with zeros, however question is still relevant.

Answer 1

The C++17 specification does not state that to_chars adds a NUL terminator:

All functions named to_chars convert value into a character string by successively filling the range [first, last) , where [first, last) is required to be a valid range. If the member ec of the return value is such that the value, when converted to bool, is false, the conversion was successful and the member ptr is the one-past-the-end pointer of the characters written. Otherwise, the member ec has the value errc::value_too_large, the member ptr has the value last, and the contents of the range [first, last) are unspecified.

Nothing is stated about a NUL terminator in that paragraph or in the paragraphs that specifically define the behavior of the individual to_chars overloads. Therefore, it does not write one.

The example works, so long as to_chars does not produce more than 9 characters. Since str is initialized to all NUL characters, anything that is not written into str will be left as NUL characters.

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To add to this, the original paper P0067R0 that proposed it explicitly states that the to_chars functions should not NUL-terminate the strings.

Answer 2

As stated by cpprefrence (your first link)

value is converted to a string as if by std::sprintf in the default ("C") locale.

So no it doesn't add a null terminator, as sprintf doesn't either (when inserting the values).