The typeid operator

We saw this briefly in S2d. The typeid operator can be used to identify the actual type of an object obtaining this information from RTTI., if it’s attached as it wil be in a polymorphic type. To use this we need the header <typeinfo>. The typeid operator returns a reference to an object in the system class type_info that describes a type.

Dynamic type for polymorphic objects.
Static type for others.
- This is what we saw earlier, there is no RTTI available.

We can use typeid in one of two ways:
typeid(typename)
typeid(Shape)

... returns a reference to a type_info object containing the type of typename.

OR ...

typeid(expression)

... returns a reference to a type_info object

that represents:

A dynamic type of the expression if expression is of polymorphic type.
A static type of the expression if it isn’t polymorphic. Example: Consider that X is a polymorphic type, and that Y is publicly derived from X.

We construct a X pointer pointing to a Y object.

X *pt = new Y(...);

Then ...

typeid( pt ) == typeid( X* ) // true
typeid( pt ) == typeid( Y* ) // false

The left hand side represents the type (X*) declared for pt, not the type of object (Y) which pt points at.

However,

X *pt = new Y(...);
typeid( *pt ) == typeid( X ) // false
typeid( *pt ) == typeid( Y ) // true

The left hand side now represents the type (Y) that pt points to, not the declared data type of pt. typeid is now looking at what type of object is being stored.

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