dynamic_cast conversion

 

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

Syntax

dynamic_cast< target-type >( expression )

target-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 target-type is a reference, prvalue of a pointer to complete class type if target-type is a pointer

Explanation

For the convenience of description, “expression or the result is a reference to T” means that “it is a glvalue of type T”, which follows the convention of decltype(since C++11).

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 target-type or a less cv-qualified version of target-type, the result is the value of expression with type target-type.

2) If target-type is “pointer to (possibly cv-qualified) Base” and the type of expression is “pointer to (possibly cv-qualified) Derived” such that Base is a base class of Derived, the result is

• a null pointer value if expression is a null pointer value, or
• a pointer to the unique Base subobject of the Derived object pointed to by expression otherwise.

3) If target-type is “reference to (possibly cv-qualified) Base” and the type of expression is “(possibly cv-qualified) Derived” such that Base is a base class of Derived, the result is the unique Base subobject of the Derived object referred to by expression.

4) Otherwise, expression is a pointer or reference to a polymorphic type.

a) If expression is a pointer:

i) If expression is a null pointer value, the result is the null pointer value of type target-type.

ii) Otherwise, if target-type is “pointer to (possibly cv-qualified) void”, the result is a pointer to the most derived object pointed to by expression.

iii) Otherwise, if the type of expression is “pointer to (possibly cv-qualified) U”, expression must point to an object whose type is similar to U and that is within its lifetime or within its period of construction or destruction, otherwise the behavior is undefined.

b) If the type of expression is “reference to (possibly cv-qualified) U”, expression must refer to an object whose type is similar to U and that is within its lifetime or within its period of construction or destruction, otherwise the behavior is undefined.

c) Otherwise (expression points/refers to a proper object), a runtime check is applied to see if the object pointed/referred to by expression can be converted to the type pointed or referred to by target-type.

Let C be the class type to which target-type points or refers. The runtime check logically executes as follows:

i) If, in the most derived object pointed/referred to by expression, expression points/refers to a public base class subobject of a C object, and if only one object of type C is derived from the subobject pointed/referred to by expression, the result points/refers to that C object.

ii) Otherwise, if expression points/refers to a public base class subobject of the most derived object, and the type of the most derived object has an unambiguous and public base class of type C, the result points/refers to the C subobject of the most derived object.

iii) Otherwise, the runtime check fails.

• If target-type is a pointer type, the result is the null pointer value of target-type.
• If target-type is a reference type, an exception of a type that would match a handler of type std::bad_cast is thrown.

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 target-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 target-type is a reference type an rvalue if target-type is a pointer type	(until C++11)
an lvalue if target-type is an lvalue reference type (expression must be an lvalue) an xvalue if target-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 target-type is a pointer type	(since C++11)

Notes

A downcast can also be performed with static_cast, which avoids the cost of the runtime check, but it is 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 run-time 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

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(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
 
    [[maybe_unused]]
    D& new_d = dynamic_cast<D&>(a); // downcast
    [[maybe_unused]]
    B& new_b = dynamic_cast<B&>(a); // sidecast
 
    Base* b1 = new Base;
    if (Derived* d = dynamic_cast<Derived*>(b1); d != nullptr)
    {
        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); d != nullptr)
    {
        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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
CWG 1269	C++11	the runtime check was not performed for xvalue expression s if target-type is an rvalue reference type	performed
CWG 2861	C++98	expression could point/refer to a type-inaccessible object	the behavior is undefined in this case

References

• C++23 standard (ISO/IEC 14882:2024):

• 7.6.1.7 Dynamic cast [expr.dynamic.cast]

• C++20 standard (ISO/IEC 14882:2020):

• 7.6.1.6 Dynamic cast [expr.dynamic.cast]

• C++17 standard (ISO/IEC 14882:2017):

• 8.2.7 Dynamic cast [expr.dynamic.cast]

• C++14 standard (ISO/IEC 14882:2014):

• 5.2.7 Dynamic cast [expr.dynamic.cast]

• C++11 standard (ISO/IEC 14882:2011):

• 5.2.7 Dynamic cast [expr.dynamic.cast]

• C++98 standard (ISO/IEC 14882:1998):

• 5.2.7 Dynamic cast [expr.dynamic.cast]

• C++03 standard (ISO/IEC 14882:2003):

• 5.2.7 Dynamic cast [expr.dynamic.cast]

See also

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