---
description: "Learn more about: Casting"
title: "Casting"
ms.date: "11/19/2018"
helpviewer_keywords: ["casting [C++]", "coercion [C++]", "virtual functions [C++], in derived classes [C++]", "static cast operator", "dynamic cast operator", "polymorphic classes [C++]", "classes [C++], polymorphism"]
ms.assetid: 3dbeb06e-2f4b-4693-832d-624bc8ec95de
---
# Casting

The C++ language provides that if a class is derived from a base class containing virtual functions, a pointer to that base class type can be used to call the implementations of the virtual functions residing in the derived class object. A class containing virtual functions is sometimes called a "polymorphic class."

Since a derived class completely contains the definitions of all the base classes from which it is derived, it is safe to cast a pointer up the class hierarchy to any of these base classes. Given a pointer to a base class, it might be safe to cast the pointer down the hierarchy. It is safe if the object being pointed to is actually of a type derived from the base class. In this case, the actual object is said to be the "complete object." The pointer to the base class is said to point to a "subobject" of the complete object. For example, consider the class hierarchy shown in the following figure.

![Diagram of a class hierarchy where C derives from B, which derives from A.](../cpp/media/vc38zz1.gif "Class hierarchy") <br/>
Class hierarchy

An object of type `C` could be visualized as shown in the following figure.

![Diagram of Class C with subobjects B and A.](../cpp/media/vc38zz2.gif "Class C with subobjects B and A") <br/>
Class C with sub-objects B and A

Given an instance of class `C`, there is a `B` subobject and an `A` subobject. The instance of `C`, including the `A` and `B` subobjects, is the "complete object."

Using run-time type information, it is possible to check whether a pointer actually points to a complete object and can be safely cast to point to another object in its hierarchy. The [dynamic_cast](../cpp/dynamic-cast-operator.md) operator can be used to make these types of casts. It also performs the run-time check necessary to make the operation safe.

For conversion of nonpolymorphic types, you can use the [static_cast](../cpp/static-cast-operator.md) operator (this topic explains the difference between static and dynamic casting conversions, and when it is appropriate to use each).

This section covers the following topics:

- [Casting operators](../cpp/casting-operators.md)

- [Run-time type information](../cpp/run-time-type-information.md)

## See also

[Expressions](../cpp/expressions-cpp.md)