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2791. string_view objects and strings should yield the same hash values

Section: 27.3.6 [string.view.hash] Status: Resolved Submitter: Nicolai Josuttis Opened: 2016-11-09 Last modified: 2016-11-21

Priority: Not Prioritized

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Discussion:

Under certain conditions we want to be able to use string_view instead of string. As a consequence both types should behave the same as long we talk about value specific behavior (if possible). For this reason, we should require that both strings and string_view yield the same hash values.

Should be solved in C++17 to ensure that following this use does not require to change hash values (which is allowed but possibly unfortunate).

[2016-11-12, Issaquah]

Resolved by P0513R0

Proposed resolution:

This wording is relative to N4606.

A more formal formulation would be:

For any sv of type SV (being string_view, u16string_view, u32string_view, or wstring_view) and s of the corresponding type S (being string, u16string, u32string, or wstring), hash<SV>()(sv) == hash<S>()(s).

1. Edit 27.3.6 [string.view.hash] as indicated:

   template<> struct hash<string_view>;
   template<> struct hash<u16string_view>;
   template<> struct hash<u32string_view>;
   template<> struct hash<wstring_view>>;
   

   -1- The template specializations shall meet the requirements of class template hash (22.10.19 [unord.hash]). The hash values shall be the same as the hash values for the corresponding string class (27.4.6 [basic.string.hash]).