std::hash <std::experimental::optional>

 

C++

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Utilities library

Strings library

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Algorithms library

Iterators library

Numerics library

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Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Technical Specifications

 

Technical specifications

Filesystem library (filesystem TS)

Library fundamentals (library fundamentals TS)

Library fundamentals 2 (library fundamentals 2 TS)

Extensions for parallelism (parallelism TS)

Concepts (concepts TS)

Extensions for concurrency (concurrency TS)

 

Library fundamentals

experimental::optional

experimental::any

experimental::basic_string_view

experimental::sample

experimental::apply

experimental::search

experimental::default_searcher experimental::make_default_searcher

experimental::boyer_moore_searcher experimental::make_boyer_moore_searcher

experimental::boyer_moore_horspool_searcher experimental::make_boyer_moore_horspool_searcher

type-erased and polymorphic allocators

variable templates for type traits

 

std::experimental::optional

Member functions

optional::optional

optional::~optional

optional::operator=

Observers

optional::operator->optional::operator*

optional::operator bool

optional::value

optional::value_or

Modifiers

optional::emplace

optional::swap

Non-member functions

operator==operator<

make_optional

swap

Helper classes

hash

nullopt_t

in_place_t

bad_optional_access

Helper objects

nullopt

in_place

 

Defined in header <experimental/optional>
template<class T> struct hash<std::experimental::optional<T>>;		(library fundamentals TS)

The template specialization of std::hash for the std::experimental::optional class allows users to obtain hashes of the values contained in optional objects.

Template parameters

T

-

the type of the value contained in optional object. The specialization std::hash<T> must meet the requirements of class template hash.

Example

Run this code

#include <experimental/optional>
#include <unordered_set>
#include <string>
#include <iostream>
using namespace std::literals;
int main()
{
    // hash<optional> makes it possible to use unordered_set
    std::unordered_set<std::experimental::optional<std::string>> s = {
            "abc"s, std::experimental::nullopt, "def"s
    };
 
    for(const auto& o : s)
        std::cout << o.value_or("(null)") << ' ';
}

Possible output:

def abc (null)

See also

hash (C++11)

hash function object (class template)