std::hash<std::unique_ptr>

Declaration

C++11

template<class T, class Deleter> struct hash<unique_ptr<T, Deleter>>;

The template specialization of std::hash for std::unique_ptr<T, Deleter> allows users to obtain hashes of objects of type std::unique_ptr<T, Deleter>.

The specialization std::hash<std::unique_ptr<T,D>> is enabled (see std::hash) if std::hash<typename std::unique_ptr<T,D>::pointer> is enabled, and is disabled otherwise.

When enabled, for a given std::unique_ptr<T, D> p, this specialization ensures that std::hash<std::unique_ptr<T, D>>()(p) == std::hash<typename std::unique_ptr<T, D>::pointer>()(p.get()).

The member functions of this specialization are not guaranteed to be noexcept because the pointer may be a fancy pointer and its hash might throw.

Example

#include <iostream>#include <memory>#include <functional> struct Foo {    Foo(int nr) { std::cout << "Foo(" << nr << ")\n"; }    ~Foo() { std::cout << "~Foo()\n"; }    bool operator==(const Foo &other) { return nr == other.nr; };    int nr;}; int main(){    std::cout << std::boolalpha;    Foo* foo = new Foo(5);    std::unique_ptr<Foo> up(foo);     std::cout         << "     hash(up): "         << std::hash<std::unique_ptr<Foo>>()(up)         << '\n';    std::cout         << "    hash(foo): "         << std::hash<Foo*>()(foo)         << '\n';    std::cout         << "    *up==*foo: "         << (*up == *foo)         << '\n';     std::unique_ptr<Foo> other = std::make_unique<Foo>(5);    std::cout         << "     hash(up): "         << std::hash<std::unique_ptr<Foo>>()(up)         << '\n';    std::cout         << "  hash(other): "         << std::hash<std::unique_ptr<Foo>>()(other)         << '\n';    std::cout         << "  *up==*other: "         <<(*up == *other)         << '\n';}

Possible Result

Foo(5)     hash(up): 7167008    hash(foo): 7167008    *up==*foo: trueFoo(5)     hash(up): 7167008  hash(other): 7171152  *up==*other: true~Foo()~Foo()