C++ The typeid keyword


Example

The typeid keyword is a unary operator that yields run-time type information about its operand if the operand's type is a polymorphic class type. It returns an lvalue of type const std::type_info. Top-level cv-qualification are ignored.

struct Base {
    virtual ~Base() = default;
};
struct Derived : Base {};
Base* b = new Derived;
assert(typeid(*b) == typeid(Derived{})); // OK

typeid can also be applied to a type directly. In this case, first top-level references are stripped, then top-level cv-qualification is ignored. Thus, the above example could have been written with typeid(Derived) instead of typeid(Derived{}):

assert(typeid(*b) == typeid(Derived{})); // OK

If typeid is applied to any expression that is not of polymorphic class type, the operand is not evaluated, and the type info returned is for the static type.

struct Base {
    // note: no virtual destructor
};
struct Derived : Base {};
Derived d;
Base& b = d;
assert(typeid(b) == typeid(Base)); // not Derived
assert(typeid(std::declval<Base>()) == typeid(Base)); // OK because unevaluated