NAME

std::type_info::hash_code - std::type_info::hash_code

Synopsis

std::size_t hash_code() const noexcept; (since C++11)

Returns an unspecified value (here denoted by hash code) such that for all
std::type_info objects referring to the same type, their hash code is the same.

No other guarantees are given: std::type_info objects referring to different types
may have the same hash code (although the standard recommends that implementations
avoid this as much as possible), and hash code for the same type can change between
invocations of the same program.

Parameters

(none)

Return value

A value that is identical for all std::type_info objects referring to the same type.

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: