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pointer_casts with move semantics for unique_ptr

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This commit is contained in:
Karolin Varner
2015-12-17 15:32:30 +01:00
parent 6b787f1cec
commit ce52fb1045
3 changed files with 222 additions and 15 deletions
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48
include/boost/pointer_cast.hpp
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@ -7,6 +7,8 @@
//
//////////////////////////////////////////////////////////////////////////////
#include <boost/config.hpp>
#ifndef BOOST_POINTER_CAST_HPP
#define BOOST_POINTER_CAST_HPP
@ -44,10 +46,27 @@ inline T* reinterpret_pointer_cast(U *ptr)
#if !defined( BOOST_NO_CXX11_SMART_PTR )
#include <boost/static_assert.hpp>
#include <boost/type_traits/is_base_of.hpp>
#include <boost/type_traits/is_pod.hpp>
#include <boost/type_traits/has_virtual_destructor.hpp>
#include <memory>
namespace boost {
namespace detail {
template<class T, class U>
void assert_safe_moving_upcast() {
BOOST_STATIC_ASSERT_MSG( !(boost::is_base_of<T, U>::value && !boost::is_pod<U>::value && !boost::has_virtual_destructor<T>::value)
, "Upcast from a non-POD child to a base without virtual destructor is unsafe, because the child's destructor "
"will not be called when the base pointer is deleted. Consider using shared_ptr for such types.");
}
}
//static_pointer_cast overload for std::shared_ptr
using std::static_pointer_cast;
@ -68,6 +87,35 @@ template<class T, class U> std::shared_ptr<T> reinterpret_pointer_cast(const std
return std::shared_ptr<T>( r, p );
}
//static_pointer_cast overload for std::unique_ptr
template<class T, class U> std::unique_ptr<T> static_pointer_cast( std::unique_ptr<U> && r ) BOOST_NOEXCEPT
{
detail::assert_safe_moving_upcast<T, U>();
return std::unique_ptr<T>( static_cast<T*>( r.release() ) );
}
//dynamic_pointer_cast overload for std::unique_ptr
template<class T, class U> std::unique_ptr<T> dynamic_pointer_cast( std::unique_ptr<U> && r ) BOOST_NOEXCEPT
{
detail::assert_safe_moving_upcast<T, U>();
T * p = dynamic_cast<T*>( r.get() );
if( p ) r.release();
return std::unique_ptr<T>( p );
}
//const_pointer_cast overload for std::unique_ptr
template<class T, class U> std::unique_ptr<T> const_pointer_cast( std::unique_ptr<U> && r ) BOOST_NOEXCEPT
{
return std::unique_ptr<T>( const_cast<T*>( r.release() ) );
}
//reinterpret_pointer_cast overload for std::unique_ptr
template<class T, class U> std::unique_ptr<T> reinterpret_pointer_cast( std::unique_ptr<U> && r ) BOOST_NOEXCEPT
{
return std::unique_ptr<T>( reinterpret_cast<T*>( r.release() ) );
}
} // namespace boost
#endif // #if !defined( BOOST_NO_CXX11_SMART_PTR )

67
pointer_cast.html
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@ -9,7 +9,7 @@
width="277" align="middle" border="0">pointer_cast</h1>
<p>The pointer cast functions (<code>boost::static_pointer_cast</code> <code>boost::dynamic_pointer_cast</code>
<code>boost::reinterpret_pointer_cast</code> <code>boost::const_pointer_cast</code>)
provide a way to write generic pointer castings for raw pointers and std::shared_ptr. The functions
provide a way to write generic pointer castings for raw pointers, std::shared_ptr and std::unique_ptr. The functions
are defined in <CITE><A href="../../boost/pointer_cast.hpp">boost/pointer_cast.hpp</A>.</CITE></p>
<P>There is test/example code in <CITE><A href="test/pointer_cast_test.cpp">pointer_cast_test.cpp</A></CITE>.</p>
<h2><a name="rationale">Rationale</a></h2>
@ -23,7 +23,7 @@ template&lt;class T, class U&gt;
<P>Pointer cast functions from <CITE><A href="../../boost/pointer_cast.hpp">boost/pointer_cast.hpp</A></CITE>
are overloads of <code>boost::static_pointer_cast</code>, <code>boost::dynamic_pointer_cast</code>,
<code>boost::reinterpret_pointer_cast</code> and <code>boost::const_pointer_cast</code>
for raw pointers and std::shared_ptr. This way when developing pointer type independent classes,
for raw pointers, std::shared_ptr and std::unique_ptr. This way when developing pointer type independent classes,
for example, memory managers or shared memory compatible classes, the same code
can be used for raw and smart pointers.</p>
<H2><A name="synopsis">Synopsis</A></H2>
@ -58,12 +58,71 @@ inline std::shared_ptr&lt;U&gt; const_pointer_cast(std::shared_ptr&lt;T&gt; ptr)
template&lt;class T, class U&gt;
inline std::shared_ptr&lt;U&gt; reinterpret_pointer_cast(std::shared_ptr&lt;T&gt; ptr);
template&lt;class T, class U&gt;
inline std::unique_ptr&lt;U&gt; static_pointer_cast(std::unique_ptr&lt;T&gt; &amp;&amp;ptr);
template&lt;class T, class U&gt;
inline std::unique_ptr&lt;U&gt; dynamic_pointer_cast(std::unique_ptr&lt;T&gt; &amp;&amp;ptr);
template&lt;class T, class U&gt;
inline std::unique_ptr&lt;U&gt; const_pointer_cast(std::unique_ptr&lt;T&gt; &amp;&amp;ptr);
template&lt;class T, class U&gt;
inline std::unique_ptr&lt;U&gt; reinterpret_pointer_cast(std::unique_ptr&lt;T&gt; &amp;&amp;ptr);
} // namespace boost
</PRE>
</BLOCKQUOTE>
<P>As you can see from the above synopsis, the pointer cast functions are just
wrappers around standard C++ cast operators.</P>
<P>As you can see from the above synopsis, the pointer cast functions for raw pointers are just
wrappers around standard C++ cast operators.</P>
<H2><A name="memory_safety">Memory Safety</A></H2>
<P>It is possible to write unsafe code, when upcasting to a base type without virtual destructor.
Consider the following example:</P>
<PRE>
#include &lt;memory&gt;
#include &lt;utility&gt;
#include &lt;boost/pointer_cast.hpp&gt;
#include &lt;boost/make_unique.hpp&gt;
int destructed = 0;
struct base {
~base() {
// ...
}
};
struct child : base {
virtual ~child() {
destructed++;
}
}
int main() {
{
std::unique_ptr<child> tmp = boost::make_unique<child>();
std::unique_ptr<base> sink = boost::static_pointer_cast<base>( std::move(tmp) );
}
// child::~child was never called
assert(destructed == 0);
return 0;
}
</PRE>
<P>In this example, the child destructor child::~child was never called, because the child* in tmp
was downcast to a base* and moved into sink. The destructor of tmp did essentially nothing, because
it contained nullptr during destruction; sink deleted the pointer, but since base::~base is non-virtual
the child destructor was never called.</P>
<P>boost::static_pointer_cast and boost::dynamic_pointer_cast for std::unique_ptr prevent the above scenario
by raising a compiler error when such a cast is detected.</P>
<P>The overloads for std::shared_ptr and boost::shared_ptr are not prone to this problem, since they internally
always store the original pointer with the original type.</P>
<P>The plain pointer casts are in principle also prone to that problem, but it is assumed that raw pointers
are non-owning, so no checking is performed.</P>
<H2><A name="example">Example</A></H2>
<BLOCKQUOTE>
<PRE>

122
test/cpp11_pointer_cast_test.cpp
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@ -1,5 +1,5 @@
//
// cpp11_pointer_cast_test.cpp - a test for boost/pointer_cast.hpp with std::shared_ptr
// cpp11_pointer_cast_test.cpp - a test for boost/pointer_cast.hpp with std::shared_ptr and std::unique_ptr
//
// Copyright (c) 2016 Karolin Varner
//
@ -8,12 +8,28 @@
// http://www.boost.org/LICENSE_1_0.txt)
//
#include <boost/config.hpp>
#include <boost/pointer_cast.hpp>
#include <boost/config.hpp>
#include <boost/detail/lightweight_test.hpp>
#include <boost/get_pointer.hpp>
#include <boost/shared_ptr.hpp>
#include <memory>
#include <utility>
#include <functional>
#if defined( BOOST_NO_CXX11_RVALUE_REFERENCES ) \
|| defined( BOOST_NO_CXX11_HDR_FUNCTIONAL ) \
|| defined( BOOST_NO_CXX11_HDR_UTILITY ) \
|| defined( BOOST_NO_CXX11_LAMBDAS ) \
|| defined( BOOST_NO_CXX11_RVALUE_REFERENCES )
// We expect all the features or none of the features to be
// available, since we should be on C++11
int main() { return 0; }
#else
namespace
{
@ -43,7 +59,7 @@ public:
};
class derived
: public base, public base2
: public base, public base2
{
int filler [5];
};
@ -66,7 +82,7 @@ bool check_dynamic_pointer_cast(const BasePtr &ptr)
//Correct cast with dynamic_pointer_cast
boost::get_pointer(boost::dynamic_pointer_cast<derived>(ptr)) ==
//Correct cast with dynamic_cast
dynamic_cast<derived*>(boost::get_pointer(ptr))
dynamic_cast<derived*>(boost::get_pointer(ptr))
&&
//Incorrect cast with dynamic_pointer_cast
boost::get_pointer(boost::dynamic_pointer_cast<derived_derived>(ptr)) ==
@ -91,7 +107,7 @@ bool check_static_pointer_cast(const BasePtr &ptr)
template <class BasePtr>
bool check_const_pointer_cast(const BasePtr &ptr)
{
return
return
//Unconst and const again using const_pointer_cast
boost::get_pointer(
boost::const_pointer_cast<const base>
@ -101,7 +117,7 @@ bool check_const_pointer_cast(const BasePtr &ptr)
}
template <class BasePtr>
void check_all_casts(const BasePtr &ptr)
void check_all_copy_casts(const BasePtr &ptr)
{
#if !defined( BOOST_NO_RTTI )
BOOST_TEST( check_dynamic_pointer_cast( ptr ) );
@ -110,15 +126,99 @@ void check_all_casts(const BasePtr &ptr)
BOOST_TEST( check_const_pointer_cast( ptr ) );
}
#if !defined( BOOST_NO_RTTI )
template <class BasePtr>
bool check_dynamic_moving_pointer_cast(std::function<BasePtr()> f)
{
BasePtr smart1 = f(), smart2 = f();
derived* expect1 = dynamic_cast<derived*>(boost::get_pointer(smart1));
derived_derived* expect2 = dynamic_cast<derived_derived*>(boost::get_pointer(smart2));
//Check that dynamic_pointer_cast versus dynamic_cast
return
//Correct cast with dynamic_pointer_cast
boost::get_pointer(boost::dynamic_pointer_cast<derived>( std::move(smart1) )) == expect1
&&
//Incorrect cast with dynamic_pointer_cast
boost::get_pointer(boost::dynamic_pointer_cast<derived_derived>( std::move(smart2) )) == expect2;
}
#endif
template <class BasePtr>
bool check_static_moving_pointer_cast(std::function<BasePtr()> f)
{
BasePtr smart = f();
base2 *expect = static_cast<base2*>(static_cast<derived*>(boost::get_pointer(smart)));
return
//Cast base -> derived -> base2 using static_pointer_cast
boost::get_pointer(
boost::static_pointer_cast<base2>(
boost::static_pointer_cast<derived>( std::move(smart) ))) ==
//Now the same with static_cast
expect;
}
template <class BasePtr>
bool check_const_moving_pointer_cast(std::function<BasePtr()> f)
{
BasePtr smart = f();
const base *expect = const_cast<const base*>(const_cast<base*>(boost::get_pointer(smart)));
return
//Unconst and const again using const_pointer_cast
boost::get_pointer(
boost::const_pointer_cast<const base>
(boost::const_pointer_cast<base>( std::move(smart) ))) ==
//Now the same with const_cast
expect;
}
template <class BasePtr>
void check_all_moving_casts(std::function<BasePtr()> f) {
#if !defined( BOOST_NO_RTTI )
BOOST_TEST( check_dynamic_moving_pointer_cast( f ) );
#endif
BOOST_TEST( check_static_moving_pointer_cast( f ) );
BOOST_TEST( check_const_moving_pointer_cast( f ) );
}
}
int main()
{
#if !defined( BOOST_NO_CXX11_SMART_PTR )
std::shared_ptr<base> std_shared(new derived);
check_all_casts(std_shared);
#endif
boost::shared_ptr<base> boost_shared(new derived);
base *plain = boost_shared.get();
return boost::report_errors();
// plain & boost::shared_ptr moving pointer_cast checks; there
// is no specific handleing for those types at the moment; this
// test just makes sure they won't break when std::move() is used
// in generic code
check_all_moving_casts<boost::shared_ptr<base>>([&boost_shared]() {
return boost_shared;
});
check_all_moving_casts<base*>([plain]() {
return plain;
});
// std::shared_ptr casts
check_all_copy_casts(std_shared);
check_all_moving_casts<std::shared_ptr<base>>([&std_shared]() {
return std_shared;
});
// std::unique_ptr casts
check_all_moving_casts<std::unique_ptr<base>>([]() {
return std::unique_ptr<base>(new derived);
});
return boost::report_errors();
}
#endif