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# dynamic_cast conversion

From cppreference.com
< cpp‎ | language

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

### [edit] 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.

### [edit] 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 cast 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 cast and static_cast can perform this conversion as well.)
4) If expression is a pointer or reference 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 (since C++17) of a complete class type)
- a prvalue otherwise (in this case, if new_type is a pointer type)

### [edit] 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`.

### [edit] Keywords

`dynamic_cast`

### [edit] Example

Run this code

    #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

### [edit] See also

- const_cast
- static_cast
- reinterpret_cast
- explicit cast
- implicit conversions

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