copy null terminated char array to std::string respecting buffer length

Maybe it's just the lack of coffee, but I'm trying to create a std::string from a null-terminated char array with a known maximum length and I don't know, how to do it.

auto s = std::string(buffer, sizeof(buffer));

.. was my favorite candidate but since C++ strings are not null-terminated this command will copy sizeof(buffer) bytes regardless of any contained '\0'.

auto s = std::string(buffer);

.. copyies from buffer until \0 is found. This is almost what I want but I can't trust the receive buffer so I'd like to provide a maximum length.

Of course I can now integrate strnlen() like this:

auto s = std::string(buffer, strnlen(buffer, sizeof(buffer)));

But that seems dirty - it traverses the buffer twice and I have to deal with C-artefacts like string.h and strnlen() (and it's ugly).

How would I do this in modern C++?