...one of the most highly
regarded and expertly designed C++ library projects in the
world. — Herb Sutter and Andrei
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
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
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
retains the efficiency of
static_cast for non-debug compilations,
but for debug compilations adds safety via an
assert() that a
polymorphic_downcast should be used for downcasts that you are
certain should succeed. Error checking is only performed in translation units
NDEBUG is not defined, via
assert( dynamic_cast<Derived>(x) == x )
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
will fail to compile.
The C++ built-in
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
which works on references, can be used on pointers through the ugly expression
&dynamic_cast<T&>(*p), which causes undefined behavior
. The polymorphic_cast
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:
dynamic_pointer_cast<Derived>(p); !p; // conversion to bool with negation
This includes C++ built-in pointers,