运行时类型信息的奇怪行为

Question

I've two class Base and Derived as this:

class Base
{
public:
};

class Derived : public Base
{
public:
};

and main function:

int main()
{
    Base* ptr = new Derived;

    std::cout << typeid(*ptr).name() << endl;

    delete ptr;

    system("pause");
}

Program outputs shows class Base where I expected it will show class Derived . But when I've added a virtual method in the Base class, now outputs shows class Derived !

Why RTTI needs at least one virtual method?

Answer 1

Because the language specification says so. RTTI only works on polymorphic types; that is, types with virtual functions. For other types, typeid returns the type info for the static type of its argument.

If you're asking for a rationale for this: it has a run-time cost (typically, a pointer in each object to the per-class metadata, which supports both virtual dispatch and RTTI), and it would be a shame if you had to pay that price for all types, whether or not you want to use RTTI on them.

Answer 2

Too long for a comment.

1) Refers to a std::type_info object representing the type type. If type is a reference type, the result refers to the referenced type.

2) Examines the expression expression

a) If expression is a glvalue expression that identifies an object of a polymorphic type (that is, a class that declares or inherits at least one virtual function), the typeid expression evaluates the expression and then refers to the std::type_info object that represents the dynamic type of the expression . If the result of the evaluated expression is a null pointer, an exception of type std::bad_typeid or a type derived from std::bad_typeid is thrown.

b) If expression is not a glvalue expression of polymorphic type, typeid does not evaluate the expression, and the std::type_info object it identifies represents the static type of the expression . Lvalue-to-rvalue, array-to-pointer, or function-to-pointer conversions are not performed.

Thus the behavior is expected, since in one case the class is polymorphic, and in the other it's not.