std::hash<std::unique_ptr>

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 >

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

Foo(5)
hash(up):    acac20
hash(foo):   acac20
*up==*foo:   true
 
Foo(5)
hash(up):    acac20
hash(other): acbc50
*up==*other: true
 
~Foo()
~Foo()

See also