std::type_index

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

Helper classes

std::hash<std::type_index>

(C++11)

hash support for std::type_index
(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:

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

Language
Standard Library Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

Member functions
type_index::type_index
type_index::operator=type_index::operator!=type_index::operator<type_index::operator<=type_index::operator>type_index::operator>=
type_index::hash_code
type_index::name

std::type_index

Member functions

Helper classes

Example

See also