NAME

std::type_index - std::type_index

Synopsis

Defined in header <typeindex>
class type_index; (since C++11)

The type_index class is a wrapper class around a std::type_info object, that can be
used as index in associative and unordered associative containers. The relationship
with type_info object is maintained through a pointer, therefore type_index is
CopyConstructible and CopyAssignable.

Member functions

constructor constructs the object
(public member function)
destructor destroys the type_index object
(implicitly declared) (public member function)
operator= assigns a type_index object
(implicitly declared) (public member function)
operator==
operator!=
operator<
operator<= compares the underlying std::type_info objects
operator> (public member function)
operator>=
operator<=>
(removed in C++20)
(C++20)
hash_code returns hashed code
(public member function)
returns implementation defined name of the type,
name associated with underlying type_info object
(public member function)

Helper classes

std::hash<std::type_index> hash support for std::type_index
(C++11) (class template specialization)

Example

The following program is an example of an efficient type-value mapping.

// Run this code

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

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

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

int main()
{
std::unordered_map<std::type_index, std::string> type_names;

type_names[std::type_index(typeid(int))] = "int";
type_names[std::type_index(typeid(double))] = "double";
type_names[std::type_index(typeid(A))] = "A";
type_names[std::type_index(typeid(B))] = "B";
type_names[std::type_index(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[std::type_index(typeid(i))] << '\n';
std::cout << "d is " << type_names[std::type_index(typeid(d))] << '\n';
std::cout << "a is " << type_names[std::type_index(typeid(a))] << '\n';
std::cout << "*b is " << type_names[std::type_index(typeid(*b))] << '\n';
std::cout << "*c is " << type_names[std::type_index(typeid(*c))] << '\n';
}

Output: