quick access of hash function (without using string object)

Question

The following code snippet can do hash value on a string object. I would like to get hash value a binary string (a pointer and length). I know I can form a string object with pointer and length, but there is extra overhead to form a string only for that. Wonder if it's possible to use the std hash function with two parameters: pointer and length.

Thanks.

#include <iostream>
#include <functional>
#include <string>

int main()
{
    std::string str = "Meet the new boss...";
    std::hash<std::string> hash_fn;
    std::size_t str_hash = hash_fn(str);

    std::cout << str_hash << '\n';
}

specialise std::hash for a custom class that holds a pointer and a length? — Richard Hodges
– Richard Hodges, Commented Aug 22, 2015 at 17:03
Thanks @RichardHodges for the comment, do you eventually need to call std::hash<string>() to get the hash value? — packetie
– packetie, Commented Aug 22, 2015 at 17:13
@codingFun - You might want to look at this answer to another question to see how much time you save by not constructing a short std::string - about 1 ns. — Bo Persson
– Bo Persson, Commented Aug 22, 2015 at 17:31

Community · Accepted Answer · 2017-05-23 12:14:39Z

2

I found this article in stack overflow which shows that the underlying hash function is actually a function of the bytes in the string's internal buffer:

What is the default hash function used in C++ std::unordered_map?

But rather than risk undefined behaviour by calling into internal functions within the standard library, why not ask the question, "how much performance will I lose by creating a std::string"? Given that you can always create such a string as a static const (zero overhead) I wonder what you're actually going to save?

edited May 23, 2017 at 12:14

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answered Aug 22, 2015 at 17:14

Richard Hodges

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3 Comments

packetie Over a year ago

Thanks @RichardHodges for the link. Just tried, I can directly use: std::_Hash_impl::hash(const char*, int size). :-)

Richard Hodges Over a year ago

Gah! You can today but there's no guarantee it will work on all systems or even on the next release of the library. It seems a little knowledge is a dangerous thing...

packetie Over a year ago

Agree. There is a trade off for everything :-) Thanks again.

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