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Polymorphic casts


Pointers to polymorphic objects (objects of classes which define at least one virtual function) are sometimes downcast or crosscast. Downcasting means casting from a base class to a derived class. Crosscasting means casting across an inheritance hierarchy diagram, such as from one base to the other in a Y diagram hierarchy.

Such casts can be done with old-style casts, but this approach is never to be recommended. Old-style casts are sorely lacking in type safety, suffer poor readability, and are difficult to locate with search tools.

The C++ built-in static_cast can be used for efficiently downcasting pointers to polymorphic objects, but provides no error detection for the case where the pointer being cast actually points to the wrong derived class. The polymorphic_downcast template retains the efficiency of static_cast for non-debug compilations, but for debug compilations adds safety via an assert() that a dynamic_cast succeeds.

A polymorphic_downcast should be used for downcasts that you are certain should succeed. Error checking is only performed in translation units where NDEBUG is not defined, via

assert( dynamic_cast<Derived>(x) == x )

where x is the source pointer. This approach ensures that not only is a non-zero pointer returned, but also that it is correct in the presence of multiple inheritance. Attempts to crosscast using polymorphic_downcast will fail to compile.

[Warning] Warning

Because polymorphic_downcast uses assert(), it violates the One Definition Rule (ODR) if NDEBUG is inconsistently defined across translation units. See ISO Std 3.2

#include <boost/polymorphic_cast.hpp>
class Fruit { public: virtual ~Fruit(){}; ... };
class Banana : public Fruit { ... };
void f( Fruit * fruit ) {
  // ... logic which leads us to believe it is a Banana
  Banana * banana = boost::polymorphic_downcast<Banana*>(fruit);

The C++ built-in dynamic_cast can be used for downcasts and crosscasts of pointers to polymorphic objects, but error notification in the form of a returned value of 0 is inconvenient to test, or worse yet, easy to forget to test. The throwing form of dynamic_cast, which works on references, can be used on pointers through the ugly expression &dynamic_cast<T&>(*p), which causes undefined behavior if p is 0. The polymorphic_cast template performs a dynamic_cast on a pointer, and throws an exception if the dynamic_cast returns 0.

For crosscasts, or when the success of a cast can only be known at runtime, or when efficiency is not important, polymorphic_cast is preferred.

The C++ built-in dynamic_cast must be used to cast references rather than pointers. It is also the only cast that can be used to check whether a given interface is supported; in that case a return of 0 isn't an error condition.

While polymorphic_downcast and polymorphic_cast work with built-in pointer types only, polymorphic_pointer_downcast and polymorphic_pointer_cast are more generic versions with support for any pointer type for which the following expressions would be valid:

For polymorphic_pointer_downcast:


For polymorphic_pointer_cast:

!p; // conversion to bool with negation

This includes C++ built-in pointers, std::shared_ptr, boost::shared_ptr, boost::intrusive_ptr, etc.

#include <boost/polymorphic_pointer_cast.hpp>

class Fruit { public: virtual ~Fruit(){} };
class Banana : public Fruit {};

// Use one of these:
typedef Fruit* FruitPtr;
typedef std::shared_ptr<Fruit> FruitPtr;
typedef boost::shared_ptr<Fruit> FruitPtr;
typedef boost::intrusive_ptr<Fruit> FruitPtr;

void f(FruitPtr fruit) {
  // ... logic which leads us to believe it is a banana
  auto banana = boost::polymorphic_pointer_downcast<Banana>(fruit);