...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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 | |
---|---|
Because |
#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
:
static_pointer_cast<Derived>(p); dynamic_pointer_cast<Derived>(p);
For polymorphic_pointer_cast
:
dynamic_pointer_cast<Derived>(p); !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); ... }