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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

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)
(removed in C++20)(C++20)
 compares the underlying std::type_info objects
(public member function)
returns hashed code
(public member function)
returns implementation defined name of the type,
associated with underlying type_info object
(public member function)

Helper classes

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

Example

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

#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

See also

contains some type's information, generated by the implementation.
This is the class returned by the typeid operator.
(class)

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https://en.cppreference.com/w/cpp/types/type_index