| Technical Specification | ||||
| Filesystem library(filesystem TS) | ||||
| Library fundamentals(library fundamentals TS) | ||||
| Library fundamentals 2(library fundamentals TS v2) | ||||
| Library fundamentals 3(library fundamentals TS v3) | ||||
| Extensions for parallelism(parallelism TS) | ||||
| Extensions for parallelism 2(parallelism TS v2) | ||||
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| Extensions for concurrency 2(concurrency TS v2) | ||||
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| Helper classes | ||||
hash | ||||
| Helper objects | ||||
Defined in header <experimental/optional> | ||
template<class T> struct hash<std::experimental::optional<T>>; | (library fundamentals TS) | |
The template specialization ofstd::hash for thestd::experimental::optional class allows users to obtain hashes of the values contained inoptional objects.
| T | - | the type of the value contained inoptional object. The specializationstd::hash<T> must meet the requirements of class templatehash. |
#include <experimental/optional>#include <iostream>#include <string>#include <unordered_set>usingnamespace std::literals; int main(){// hash<optional> makes it possible to use unordered_setstd::unordered_set<std::experimental::optional<std::string>> s={"abc"s,std::experimental::nullopt,"def"s}; for(constauto& o: s)std::cout<< o.value_or("(null)")<<' ';std::cout<<'\n';}
Possible output:
def abc (null)
(C++11) | hash function object (class template)[edit] |
