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