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# std::type_info::hash_code

From cppreference.com
< cpp‎ | types‎ | type info

size_t hash_code() const; |  | (since C++11)

Returns an unspecified value, which is identical for the `type_info` objects referring to the same type. No other guarantees are given. For example, the same value may be returned for different types. The value can also change between invocations of the same program.

### [edit] Parameters

(none)

### [edit] Return value

Some value, which is identical for the same types.

### [edit] Example

The following program is an example of an efficient type-value mapping without using std::type_index.

Run this code

    #include <iostream>
    #include <typeinfo>
    #include <unordered_map>
    #include <string>
    #include <functional>
    #include <memory>

    struct A {
        virtual ~A() {}
    };

    struct B : A {};
    struct C : A {};

    using TypeInfoRef = std::reference_wrapper<const std::type_info>;

    struct Hasher {
        std::size_t operator()(TypeInfoRef code) const
        {
            return code.get().hash_code();
        }
    };

    struct EqualTo {
        bool operator()(TypeInfoRef lhs, TypeInfoRef rhs) const
        {
            return lhs.get() == rhs.get();
        }
    };

    int main()
    {
        std::unordered_map<TypeInfoRef, std::string, Hasher, EqualTo> type_names;

        type_names[typeid(int)] = "int";
        type_names[typeid(double)] = "double";
        type_names[typeid(A)] = "A";
        type_names[typeid(B)] = "B";
        type_names[typeid(C)] = "C";

        int i;
        double d;
        A a;

        // note that we're storing pointer to type A
        std::unique_ptr<A> b(new B);
        std::unique_ptr<A> c(new C);

        std::cout << "i is " << type_names[typeid(i)] << '\n';
        std::cout << "d is " << type_names[typeid(d)] << '\n';
        std::cout << "a is " << type_names[typeid(a)] << '\n';
        std::cout << "b is " << type_names[typeid(*b)] << '\n';
        std::cout << "c is " << type_names[typeid(*c)] << '\n';
    }

Output:

    i is int
    d is double
    a is A
    b is B
    c is C

### [edit] See also

operator==operator!= | checks whether the objects refer to the same type (public member function) [edit]
name | implementation defined name of the type (public member function) [edit]

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