Thread: std::collate::hash historical question

I'm working with some C++14 code. A library we're using has its own partial port of C++17's std::string_view, but without a corresponding specialization of std::hash.

I assumed that the standard contained no hash function that I could use, aside from std::hash<std::string>, but it turns out I'm wrong.

There is a hash function buried in iostream's locales library: std::collate<CharT>::hash, std::collate<CharT>::do_hash - cppreference.com

The cppreference page mentions its use by std::regex, so I was even more surprised to find that std::collate::hash is defined in C++98, which predates std::hash and std::regex.

My question, for the language lawyers among you, is why was std::collate::hash included in the C++98 standard?

The cppreference page mentions its use by std::regex

You're refering to this page, which is not the one you link. Note that it says the facet is used for comparison as well as hashing. regex is presumably using the locale-aware comparisons.

why was std::collate::hash included in the C++98 standard?

Why not?

Here's the gcc implementation (somewhat simplified by me, mostly just removing leading underscores) original:

Code:

template<typename CharT>
long
collate<CharT>::do_hash(const CharT* lo, const CharT* hi) const
{
    unsigned long val = 0;
    for ( ; lo < hi; ++lo)
        val = *lo + ((val << 7) | (val >> (sizeof(long) * CHAR_BIT - 7)));
    return static_cast<long>(val);
}

Neat, nice find.

> Why not?

Sure, why not, but I would assume that the hashing interface was added for use elsewhere in the standard library. Where, though? In C++98, there was no regex, unordered_map, etc. I also can't find an analogous facility in the C locale library.

It's mysterious to me.