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 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.
dynamic_cast
can be used to add constness. An implicit cast and static_cast
can perform this conversion as well.)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 cast and static_cast
can perform this conversion as well.)new_type
is a pointer to void
, the result is a pointer to the most derived object pointed or referenced by expression.Base
, and new_type
is a pointer or reference to the type Derived
a run-time check is performed: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".)Derived
, the result of the cast points/refers to that Derived
(This is known as a "sidecast".)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.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 construction'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)
- a prvalue otherwise (in this case, if new_type is a pointer type)
Note
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
.
Keywords
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((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
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