dynamic_cast conversion

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

Syntax

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.

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;
}

downcast from b2 to d successful

See also

• const_cast
• static_cast
• reinterpret_cast
• explicit cast
• implicit conversions