7 Expressions [expr]

7.6 Compound expressions [expr.compound]

7.6.1 Postfix expressions [expr.post]

7.6.1.7 Dynamic cast [expr.dynamic.cast]

The result of the expression dynamic_­cast<T>(v) is the result of converting the expression v to type T.
T shall be a pointer or reference to a complete class type, or “pointer to cv void.
The dynamic_­cast operator shall not cast away constness ([expr.const.cast]).
If T is a pointer type, v shall be a prvalue of a pointer to complete class type, and the result is a prvalue of type T.
If T is an lvalue reference type, v shall be an lvalue of a complete class type, and the result is an lvalue of the type referred to by T.
If T is an rvalue reference type, v shall be a glvalue having a complete class type, and the result is an xvalue of the type referred to by T.
If the type of v is the same as T (ignoring cv-qualifications), the result is v (converted if necessary).
If T is “pointer to cv1 B” and v has type “pointer to cv2 D” such that B is a base class of D, the result is a pointer to the unique B subobject of the D object pointed to by v, or a null pointer value if v is a null pointer value.
Similarly, if T is “reference to cv1 B” and v has type cv2 D such that B is a base class of D, the result is the unique B subobject of the D object referred to by v.55
In both the pointer and reference cases, the program is ill-formed if B is an inaccessible or ambiguous base class of D.
[Example 1: struct B { }; struct D : B { }; void foo(D* dp) { B* bp = dynamic_cast<B*>(dp); // equivalent to B* bp = dp; } — end example]
Otherwise, v shall be a pointer to or a glvalue of a polymorphic type.
If v is a null pointer value, the result is a null pointer value.
If T is “pointer to cv void”, then the result is a pointer to the most derived object pointed to by v.
Otherwise, a runtime check is applied to see if the object pointed or referred to by v can be converted to the type pointed or referred to by T.
Let C be the class type to which T points or refers.
The runtime check logically executes as follows:
  • If, in the most derived object pointed (referred) to by v, v points (refers) to a public base class subobject of a C object, and if only one object of type C is derived from the subobject pointed (referred) to by v the result points (refers) to that C object.
  • Otherwise, if v points (refers) to a public base class subobject of the most derived object, and the type of the most derived object has a base class, of type C, that is unambiguous and public, the result points (refers) to the C subobject of the most derived object.
  • Otherwise, the runtime check fails.
The value of a failed cast to pointer type is the null pointer value of the required result type.
A failed cast to reference type throws an exception of a type that would match a handler of type std​::​bad_­cast.
[Example 2: class A { virtual void f(); }; class B { virtual void g(); }; class D : public virtual A, private B { }; void g() { D d; B* bp = (B*)&d; // cast needed to break protection A* ap = &d; // public derivation, no cast needed D& dr = dynamic_cast<D&>(*bp); // fails ap = dynamic_cast<A*>(bp); // fails bp = dynamic_cast<B*>(ap); // fails ap = dynamic_cast<A*>(&d); // succeeds bp = dynamic_cast<B*>(&d); // ill-formed (not a runtime check) } class E : public D, public B { }; class F : public E, public D { }; void h() { F f; A* ap = &f; // succeeds: finds unique A D* dp = dynamic_cast<D*>(ap); // fails: yields null; f has two D subobjects E* ep = (E*)ap; // error: cast from virtual base E* ep1 = dynamic_cast<E*>(ap); // succeeds } — end example]
[Note 1:
Subclause [class.cdtor] describes the behavior of a dynamic_­cast applied to an object under construction or destruction.
— end note]
55)55)
The most derived object ([intro.object]) pointed or referred to by v can contain other B objects as base classes, but these are ignored.