dynamic_cast Operator from MSDN

Visual C++ Language Reference

dynamic_cast Operator

Converts the operand expression to an object of type type-id.

dynamic_cast < type-id > ( expression )

 Remarks

The type-id must be a pointer or a reference to a previously defined class type or a "pointer to void". The type of expression must be a pointer if type-id is a pointer, or an l-value if type-id is a reference.

For more information about dynamic_cast, see Breaking Changes in dynamic_cast.

See static_cast for an explanation of the difference between static and dynamic casting conversions, and when it is appropriate to use each.

In Visual C++ 2005, there are two breaking changes in the behavior of dynamic_cast in managed code:

• dynamic_cast to a pointer to the underlying type of a boxed enum will fail at runtime, returning 0 instead of the converted pointer.

• dynamic_cast will no longer throw an exception when type-idis an interior pointer to a value type, with the cast failing atruntime. The cast will now return the 0 pointer value instead ofthrowing.

If type-id is a pointer to an unambiguous accessible direct or indirect base class of expression, a pointer to the unique subobject of type type-id is the result. For example:

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// dynamic_cast_1.cpp
// compile with: /c
class B { };
class C : public B { };
class D : public C { };

void f(D* pd) {
   C* pc = dynamic_cast<C*>(pd);   // ok: C is a direct base class
                                   // pc points to C subobject of pd 
   B* pb = dynamic_cast<B*>(pd);   // ok: B is an indirect base class
                                   // pb points to B subobject of pd
}

Thistype of conversion is called an "upcast" because it moves a pointer upa class hierarchy, from a derived class to a class it is derived from.An upcast is an implicit conversion.

If type-id is void*, a run-time check is made to determine the actual type of expression. The result is a pointer to the complete object pointed to by expression. For example:

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// dynamic_cast_2.cpp
// compile with: /c /GR
class A {virtual void f();};
class B {virtual void f();};

void f() {
   A* pa = new A;
   B* pb = new B;
   void* pv = dynamic_cast<void*>(pa);
   // pv now points to an object of type A

   pv = dynamic_cast<void*>(pb);
   // pv now points to an object of type B
}

If type-id is not void*, a run-time check is made to see if the object pointed to by expression can be converted to the type pointed to by type-id.

If the type of expression is a base class of the type of type-id, a run-time check is made to see if expression actually points to a complete object of the type of type-id. If this is true, the result is a pointer to a complete object of the type of type-id. For example:

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// dynamic_cast_3.cpp
// compile with: /c /GR
class B {virtual void f();};
class D : public B {virtual void f();};

void f() {
   B* pb = new D;   // unclear but ok
   B* pb2 = new B;

   D* pd = dynamic_cast<D*>(pb);   // ok: pb actually points to a D
   D* pd2 = dynamic_cast<D*>(pb2);   // pb2 points to a B not a D
}

Thistype of conversion is called a "downcast" because it moves a pointerdown a class hierarchy, from a given class to a class derived from it.

Incases of multiple inheritance, possibilities for ambiguity areintroduced. Consider the class hierarchy shown in the following figure.

For CLR types, dynamic_cast results in either a no-op if the conversion can be performed implicitly, or an MSIL isinst instruction, which performs a dynamic check and returns nullptr if the conversion fails.

The following sample uses dynamic_cast to determine if a class is an instance of particular type:

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// dynamic_cast_clr.cpp
// compile with: /clr
using namespace System;

void PrintObjectType( Object^o ) {
   if( dynamic_cast<String^>(o) )
      Console::WriteLine("Object is a String");
   else if( dynamic_cast<int^>(o) )
      Console::WriteLine("Object is an int");
}

int main() {
   Object^o1 = "hello";
   Object^o2 = 10;

   PrintObjectType(o1);
   PrintObjectType(o2);
}

Class Hierarchy Showing Multiple Inheritance

A pointer to an object of type D can be safely cast to B or C. However, if D is cast to point to an A object, which instance of Awould result? This would result in an ambiguous casting error. To getaround this problem, you can perform two unambiguous casts. For example:

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// dynamic_cast_4.cpp
// compile with: /c /GR
class A {virtual void f();};
class B {virtual void f();};
class D : public B {virtual void f();};

void f() {
   D* pd = new D;
   B* pb = dynamic_cast<B*>(pd);   // first cast to B
   A* pa2 = dynamic_cast<A*>(pb);   // ok: unambiguous
}

Furtherambiguities can be introduced when you use virtual base classes.Consider the class hierarchy shown in the following figure.

Class Hierarchy Showing Virtual Base Classes

In this hierarchy, A is a virtual base class. See Virtual Base Classes for the definition of a virtual base class. Given an instance of class E and a pointer to the A subobject, a dynamic_cast to a pointer to B will fail due to ambiguity. You must first cast back to the complete E object, then work your way back up the hierarchy, in an unambiguous manner, to reach the correct B object.

Consider the class hierarchy shown in the following figure.

Class Hierarchy Showing Duplicate Base Classes

Given an object of type E and a pointer to the D subobject, to navigate from the D subobject to the left-most A subobject, three conversions can be made. You can perform a dynamic_cast conversion from the D pointer to an E pointer, then a conversion (either dynamic_cast or an implicit conversion) from E to B, and finally an implicit conversion from B to A. For example:

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// dynamic_cast_5.cpp
// compile with: /c /GR
class A {virtual void f();};
class B : public A {virtual void f();};
class C : public A { };
class D {virtual void f();};
class E : public B, public C, public D {virtual void f();};

void f(D* pd) {
   E* pe = dynamic_cast<E*>(pd);
   B* pb = pe;   // upcast, implicit conversion
   A* pa = pb;   // upcast, implicit conversion
}

The dynamic_castoperator can also be used to perform a "cross cast." Using the sameclass hierarchy, it is possible to cast a pointer, for example, fromthe B subobject to the D subobject, as long as the complete object is of type E.

Considering cross casts, it is actually possible to do the conversion from a pointer to D to a pointer to the left-most A subobject in just two steps. You can perform a cross cast from D to B, then an implicit conversion from B to A. For example:

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// dynamic_cast_6.cpp
// compile with: /c /GR
class A {virtual void f();};
class B : public A {virtual void f();};
class C : public A { };
class D {virtual void f();};
class E : public B, public C, public D {virtual void f();};

void f(D* pd) {
   B* pb = dynamic_cast<B*>(pd);   // cross cast
   A* pa = pb;   // upcast, implicit conversion
}

A null pointer value is converted to the null pointer value of the destination type by dynamic_cast.

When you use dynamic_cast < type-id > ( expression ), if expression cannot be safely converted to type type-id, the run-time check causes the cast to fail. For example:

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// dynamic_cast_7.cpp
// compile with: /c /GR
class A {virtual void f();};
class B {virtual void f();};

void f() {
   A* pa = new A;
   B* pb = dynamic_cast<B*>(pa);   // fails at runtime, not safe;
   // B not derived from A
}

The value of a failed cast to pointer type is the null pointer. A failed cast to reference type throws a bad_cast Exception.   If expression does not point to or reference a valid object, a __non_rtti_object exception is thrown.

See typeid for an explanation of the __non_rtti_object exception.

 Example

Thefollowing sample creates the base class (struct A) pointer, to anobject (struct C). This, plus the fact there are virtual functions,enables runtime polymorphism.

The sample also calls a non-virtual function in the hierarchy.

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// dynamic_cast_8.cpp
// compile with: /GR /EHsc
#include <stdio.h>
#include <iostream>

struct A {
    virtual void test() {
        printf_s("in A/n");
   }
};

struct B : A {
    virtual void test() {
        printf_s("in B/n");
    }

    void test2() {
        printf_s("test2 in B/n");
    }
};

struct C : B {
    virtual void test() {
        printf_s("in C/n");
    }

    void test2() {
        printf_s("test2 in C/n");
    }
};

void Globaltest(A& a) {
    try {
        C &c = dynamic_cast<C&>(a);
        printf_s("in GlobalTest/n");
    }
    catch(std::bad_cast) {
        printf_s("Can't cast to C/n");
    }
}

int main() {
    A *pa = new C;
    A *pa2 = new B;

    pa->test();

    B * pb = dynamic_cast<B *>(pa);
    if (pb)
        pb->test2();

    C * pc = dynamic_cast<C *>(pa2);
    if (pc)
        pc->test2();

    C ConStack;
    Globaltest(ConStack);

   // will fail because B knows nothing about C
    B BonStack;
    Globaltest(BonStack);
}

Output

in C
test2 in B
in GlobalTest
Can't cast to