dynamic_cast conversion

Safely converts pointers and references to classes up, down, and sideways along the inheritance hierarchy.

Syntax

dynamic_cast < new_type > ( expression )

new_type	-	pointer to complete class type, reference to complete class type, or pointer to (optionally cv-qualified) void
expression	-	lvalue (until C++11) glvalue (since C++11) of a complete class type if new_type is a reference, prvalue of a pointer to complete class type if new_type is a pointer.

If the cast is successful, dynamic_cast returns a value of type new_type. If the cast fails and new_type is a pointer type, it returns a null pointer of that type. If the cast fails and new_type is a reference type, it throws an exception that matches a handler of type std::bad_cast.

Explanation

Only the following conversions can be done with dynamic_cast, except when such conversions would cast away constness or volatility.

1) If the type of expression is exactly new_type or a less cv-qualified version of new_type, the result is the value of expression, with type new_type. (In other words, dynamic_cast can be used to add constness. An implicit conversion and static_cast can perform this conversion as well.)

2) If the value of expression is the null pointer value, the result is the null pointer value of type new_type.

3) If new_type is a pointer or reference to Base, and the type of expression is a pointer or reference to Derived, where Base is a unique, accessible base class of Derived, the result is a pointer or reference to the Base class subobject within the Derived object pointed or identified by expression. (Note: an implicit conversion and static_cast can perform this conversion as well.)

4) If expression is a pointer to a polymorphic type, and new_type is a pointer to void, the result is a pointer to the most derived object pointed or referenced by expression.

5) If expression is a pointer or reference to a polymorphic type Base, and new_type is a pointer or reference to the type Derived a run-time check is performed:

a) The most derived object pointed/identified by expression is examined. If, in that object, expression points/refers to a public base of Derived, and if only one subobject of Derived type is derived from the subobject pointed/identified by expression, then the result of the cast points/refers to that Derived subobject. (This is known as a "downcast".)

b) Otherwise, if expression points/refers to a public base of the most derived object, and, simultaneously, the most derived object has an unambiguous public base class of type Derived, the result of the cast points/refers to that Derived (This is known as a "sidecast".)

c) Otherwise, the runtime check fails. If the dynamic_cast is used on pointers, the null pointer value of type new_type is returned. If it was used on references, the exception std::bad_cast is thrown.

6) When dynamic_cast is used in a constructor or a destructor (directly or indirectly), and expression refers to the object that's currently under construction/destruction, the object is considered to be the most derived object. If new_type is not a pointer or reference to the constructor's/destructor's own class or one of its bases, the behavior is undefined.

Similar to other cast expressions, the result is:

an lvalue if new_type is an lvalue reference type (expression must be an lvalue)
an xvalue if new_type is an rvalue reference type (expression may be lvalue or rvalue (until C++17)must be a glvalue (prvalues are materialized) (since C++17) of a complete class type)
a prvalue if new_type is a pointer type

Notes

A downcast can also be performed with static_cast, which avoids the cost of the runtime check, but it's only safe if the program can guarantee (through some other logic) that the object pointed to by expression is definitely Derived.
Some forms of dynamic_cast rely on runtime type identification (RTTI), that is, information about each polymorphic class in the compiled program. Compilers typically have options to disable the inclusion of this information.

Keywords

dynamic_cast.

Example

#include <iostream>
 
struct V {
    virtual void f() {};  // must be polymorphic to use runtime-checked dynamic_cast
};
struct A : virtual V {};
struct B : virtual V {
  B(V* v, A* a) {
    // casts during construction (see the call in the constructor of D below)
    dynamic_cast<B*>(v); // well-defined: v of type V*, V base of B, results in B*
    dynamic_cast<B*>(a); // undefined behavior: a has type A*, A not a base of B
  }
};
struct D : A, B {
    D() : B(static_cast<A*>(this), this) { }
};
 
struct Base {
    virtual ~Base() {}
};
 
struct Derived: Base {
    virtual void name() {}
};
 
int main()
{
    D d; // the most derived object
    A& a = d; // upcast, dynamic_cast may be used, but unnecessary
    D& new_d = dynamic_cast<D&>(a); // downcast
    B& new_b = dynamic_cast<B&>(a); // sidecast
 
 
    Base* b1 = new Base;
    if(Derived* d = dynamic_cast<Derived*>(b1))
    {
        std::cout << "downcast from b1 to d successful\n";
        d->name(); // safe to call
    }
 
    Base* b2 = new Derived;
    if(Derived* d = dynamic_cast<Derived*>(b2))
    {
        std::cout << "downcast from b2 to d successful\n";
        d->name(); // safe to call
    }
 
    delete b1;
    delete b2;
}

Output:

downcast from b2 to d successful