static_pointer_cast<T>(..) for weak_ptr<T>s
In the last post I mentioned that the following functions are not defined for weak_ptr<T> (in Boost 1_37_0):
- const_pointer_cast<T>(..)
- static_pointer_cast<T>(..)
- dynamic_pointer_cast<T>(..)
Boost is a very nice and carefully thought-out library, so these functions may be missing for a good reason. Or maybe they’re just considered not very important because weak pointers are not often passed around and copied on the stack. In any case, here is one way to define static_pointer_cast<T>(..) — the same pattern can be used for the other two functions.
# include <boost/assert.hpp>
# include <boost/shared_ptr.hpp>
# include <boost/weak_ptr.hpp>
# include <boost/pointer_cast.hpp>
namespace boost {
template
< typename TRG_T
, typename SRC_T
>
weak_ptr< TRG_T >
static_pointer_cast( weak_ptr< SRC_T > const & wp_src)
//
// This works even if the source weak_ptr is null or
// expired.
//
// No throw - this should never throw an exception.
{
// Get a shared_ptr from the weak_ptr.
// Use weak_ptr.lock( ) so this will not throw
// bad_weak_ptr if the weak_ptr is expired.
shared_ptr< SRC_T >
sp_src(
wp_src.lock( ));
BOOST_ASSERT( wp_src.expired( ) == ! sp_src);
// Cast the shared_ptr to the target type.
shared_ptr< TRG_T >
sp_trg(
static_pointer_cast< TRG_T >( sp_src));
// Auto-cast the shared_ptr< TRG_T > to the
// weak_ptr< TRG_T > return type.
return sp_trg;
}
} /* end namespace boost */
It would probably be more efficient to build this into the weak_ptr<T> class like it is built into the shared_ptr<T> class, but this will do.
Here is how you’d use static_pointer_cast<T>(..).
using boost::shared_ptr;
using boost::weak_ptr;
using boost::static_pointer_cast;
class
base_type
{ public: virtual ~base_type( ) { } };
class
derived_type
: public base_type
{ };
// Make a shared pointer that owns a new object.
shared_ptr< derived_type >
sp_derived(
new derived_type);
// Make a weak pointer to that same object.
weak_ptr< derived_type >
wp_derived(
sp_derived);
// Weak pointers upcast normally, even when they
// are null or expired.
weak_ptr< base_type >
wp_base(
wp_derived);
// Our new static_pointer_cast works with
// weak pointers.
weak_ptr< derived_type >
wp_derived2(
static_pointer_cast< derived_type >( wp_base)
);
I noticed when writing this that weak_ptr<T> does not have an automatic boolean conversion like other pointer types. You cannot write code like if ( weak_ptr_inst ).. or assert( ! weak_ptr_inst). Instead you have to use weak_ptr_inst.expired( ). This is because it would be easy to forget that a weak pointer can expire at any time, which can lead to bugs that fail only occasionally and are hard to reproduce.
// Assume weak_ptr bool operator is defined
// (which it isn't).
template< typename T >
class
weak_ptr
{
... other stuff ...
// Very simple bool operator.
operator bool( ) const { return ! expired( ); }
};
...
weak_ptr< my_class >
weak_ptr_inst(
shared_ptr_inst);
if ( weak_ptr_inst ) {
... Here you might find dangerous code that assumes
... weak_ptr_inst is set (not null, not expired).
... Code should always assume a weak_ptr can
... expire unless the same code is holding a
... shared_ptr to the same target object.
}
if ( ! weak_ptr_inst.expired( ) ) {
... This is the same as above, but it might be safer
... because the explicit call to expired( ) reminds
... us we are working with a weak_ptr, which
... doesn't fit into more common pointer patterns.
}
Using static_pointer_cast<T>(..) and dynamic_pointer_cast<T>(..)
This demonstrates how to use static_pointer_cast<T>(..) and dynamic_pointer_cast<T>(..). It is like the previous post which shows how to use const_pointer_cast<T>(..).
First the headers and some class declarations:
# include <boost/assert.hpp>
# include <boost/pointer_cast.hpp>
# include <boost/shared_ptr.hpp>
using boost::shared_ptr;
using boost::static_pointer_cast;
using boost::dynamic_pointer_cast;
class
base_type
{ public: virtual ~base_type( ) { } };
class
derived_type
: public base_type
{ };
class
derived_type_other
: public base_type
{ };
Notice base_type has a virtual destructor, which makes it a polymorphic class. A class has to have at least one virtual method if you want to use it with dynamic_pointer_cast<T>(..). Otherwise the compiler complains loudly.
Remember that, in general, declaring the destructors virtual is often not necessary if a class is being managed by shared_ptr<T>. If you always immediately attach a new target object to a shared_ptr< MOST_DERIVED_CLASS > after creation, then you don’t have to worry if shared_ptr< BASE_CLASS > later deletes that target object. The most-derived destructor is bound to the shared_ptr<T> during first attach and is passed along to subsequent shared and weak pointers.
Finally, here is the code showing static_pointer_cast<T>(..) and dynamic_pointer_cast<T>(..).
// Make a shared_ptr to a new object. shared_ptr< derived_type > sp_derived( new derived_type); // The shared_ptr knows how to upcast its // inner pointer. shared_ptr< base_type > sp_base( sp_derived); // You can static-downcast the inner pointer. shared_ptr< derived_type > sp_derived2( static_pointer_cast< derived_type >( sp_base)); // You can dynamic-downcast the inner pointer // as long as the inner type is polymorphic. shared_ptr< derived_type > sp_derived3( dynamic_pointer_cast< derived_type >( sp_base)); BOOST_ASSERT( sp_derived3); // You can try to dynamic-downcast to the wrong type. // You will end up with a null shared_ptr because // the inner dynamic-downcast returns a zero pointer. // This does not throw std::bad_cast because it casts // pointers and not refs. shared_ptr< derived_type_other > sp_derived_other( dynamic_pointer_cast< derived_type_other >( sp_base)); BOOST_ASSERT( ! sp_derived_other);
Like const_pointer_cast<T>(..), static_pointer_cast<T>(..) and dynamic_pointer_cast<T>(..) also work with intrusive_ptr<T>s and raw pointers. But they don’t work with weak_ptr<T>, at least not in Boost 1_37_0 (I suspect it’s an oversight). They also do not work with shared_array<T>, which makes sense since array “pointers” should always be to the most-derived type, and of course they don’t work with the single-ownership smart pointers that don’t support normal copy semantics.
There is also a reinterpret_pointer_cast<T>(..) function, but it only works with raw pointers. Reinterpret cast is usually only used at a very low abstraction level, while smart pointers are highly abstracted. If you think you need a reinterpret cast with a smart pointer you are probably doing something wrong.
Using const_pointer_cast<T>(..)
This is a quick demonstration of const_pointer_cast<T>(..).
# include <boost/shared_ptr.hpp>
using boost::shared_ptr;
using boost::const_pointer_cast;
struct test_type { };
// Make an object and give it to a shared_ptr.
shared_ptr< test_type >
sp_non_const(
new test_type);
// Copy the shared_ptr to another, except the
// new shared_ptr holds a pointer to a const.
// This copy can be an implicit cast.
shared_ptr< test_type const >
sp_const(
sp_non_const);
// Copy the shared_ptr to a const to another
// shared_ptr, one that holds a non-const
// pointer. This is not an implicit cast.
// This is what const_pointer_cast is for.
shared_ptr< test_type >
sp_non_const2(
const_pointer_cast< test_type >( sp_const));
And there you have it. const_pointer_cast<T>(..) is used to cast away the const (or volatile) buried inside a smart pointer. It works with shared_ptr<T>s, intrusive_ptr<T>s, and raw pointers. Without it you would have to resort to reinterpret_cast<T>(..) to cast away const buried inside a shared_ptr<T> unless it provided ->shared_from_this().