Re: STL Map uses hashing?

Pete Becker <>
Sat, 31 May 2008 13:13:08 -0400
On 2008-05-31 12:42:50 -0400, "Thomas J. Gritzan"
<> said:

Pete Becker schrieb:

  Creating good-quality hashing functions are not a trivial task at all
(and the subject of continuing extensive research). One cannot expect
the average user to invent a good-quality function without extensive
knowledge and experience on the subject.

Indeed. That's why I've never thought that standardizing hashed
containers was a good idea. There's just too much flexibility, making
them hard to specify well, with the result that naive users can get
horrible performance without knowing why.

But, to answer your question, no, there is no requirement for "high
quality" hashing functions. Implementations will be required to provide
hashing functions for the builtin types, pointers, std::string,
std::u16string, std::u32string, std::wstring, std::error_code, and

What about std::pair<>, tuple<>, etc.?

They're not on the list.

How to do a good hashing function for a combined type like

struct key
    int key1;
    std::string key2;


It depends strongly on how the values are distributed.

Roundhouse Consulting, Ltd. ( Author of "The
Standard C++ Library Extensions: a Tutorial and Reference

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