Here's a patch to make shared_ptr use allocator_traits, I don't plan
to commit this (at least not yet) but it shows what's needed to use
allocator_traits instead of the C++03 way of doing things. It uses
allocator_traits<T>::__rebind<U>::__type which is a temporary
substitute for the unsupported allocator_traits<T>::rebind<U> template
alias.
This patch isn't quite backward compatible, because I changed
_Sp_counted_deleter::_My_deleter to derive from the supplied _Alloc
type instead of the rebound type. I think that's more correct, but
changes the type (and possibly size) of the _M_del member. The safe
way to make that change is probably to rename _Sp_counted_deleter to
_Sp_counted_deleter2, so that code built against the old header uses
the old type and code built with the new header uses a new, distinct
type.
I completely rewrote _Sp_counted_ptr_inplace because it doesn't need a
deleter, so doesn't need to derive from _Sp_counted_deleter and can
benefit from the EBO. That could be renamed _Sp_counted_ptr_inplace2
for compatibility.
Apart from demonstrating how to make use of allocator_traits to
support both C++03 and C++11 allocators, this patch allows the
following example to work, using allocate_shared to create a
shared_ptr to a type with no public constructors or destructor, by
using a custom allocator which is declared as a friend:
#include <memory>
#include <new>
template<typename T>
struct MyAlloc;
class Private
{
Private() = default;
Private(const Private&) = default;
~Private() = default;
friend class MyAlloc<Private>;
public:
int get() const { return 0; }
};
template<typename T>
struct MyAlloc : std::allocator<Private>
{
void construct(void* p) { ::new(p) Private(); }
};
int main()
{
MyAlloc<Private> a;
auto p = std::allocate_shared<Private>(a);
return p->get();
}
This allows you to write types which can only be managed
Index: include/bits/shared_ptr_base.h
===================================================================
--- include/bits/shared_ptr_base.h (revision 174358)
+++ include/bits/shared_ptr_base.h (working copy)
@@ -323,18 +323,15 @@
template<typename _Ptr, typename _Deleter, typename _Alloc, _Lock_policy _Lp>
class _Sp_counted_deleter : public _Sp_counted_base<_Lp>
{
- typedef typename _Alloc::template
- rebind<_Sp_counted_deleter>::other _My_alloc_type;
-
// Helper class that stores the Deleter and also acts as an allocator.
// Used to dispose of the owned pointer and the internal refcount
// Requires that copies of _Alloc can free each other's memory.
struct _My_Deleter
- : public _My_alloc_type // copy constructor must not throw
+ : public _Alloc // copy constructor must not throw
{
- _Deleter _M_del; // copy constructor must not throw
+ _Deleter _M_del; // copy constructor must not throw
_My_Deleter(_Deleter __d, const _Alloc& __a)
- : _My_alloc_type(__a), _M_del(__d) { }
+ : _Alloc(__a), _M_del(__d) { }
};
public:
@@ -353,9 +350,11 @@
virtual void
_M_destroy() noexcept
{
- _My_alloc_type __a(_M_del);
- this->~_Sp_counted_deleter();
- __a.deallocate(this, 1);
+ typedef typename allocator_traits<_Alloc>::template
+ __rebind_traits<_Sp_counted_deleter>::__type _My_alloc_traits;
+ typename _My_alloc_traits::allocator_type __a(_M_del);
+ _My_alloc_traits::destroy(__a, this);
+ _My_alloc_traits::deallocate(__a, this, 1);
}
virtual void*
@@ -375,51 +374,44 @@
// helpers for make_shared / allocate_shared
- template<typename _Tp>
- struct _Sp_destroy_inplace
- {
- void operator()(_Tp* __p) const { if (__p) __p->~_Tp(); }
- };
-
struct _Sp_make_shared_tag { };
template<typename _Tp, typename _Alloc, _Lock_policy _Lp>
- class _Sp_counted_ptr_inplace
- : public _Sp_counted_deleter<_Tp*, _Sp_destroy_inplace<_Tp>, _Alloc, _Lp>
+ class _Sp_counted_ptr_inplace : public _Sp_counted_base<_Lp>
{
- typedef _Sp_counted_deleter<_Tp*, _Sp_destroy_inplace<_Tp>, _Alloc, _Lp>
- _Base_type;
+ // Helper class that stores the pointer and also acts as an allocator.
+ // Used to dispose of the owned pointer and the internal refcount
+ // Requires that copies of _Alloc can free each other's memory.
+ struct _Impl
+ : public _Alloc // copy constructor must not throw
+ {
+ _Impl(_Alloc __a) : _Alloc(__a), _M_ptr() { }
+ _Tp* _M_ptr;
+ };
public:
- explicit
- _Sp_counted_ptr_inplace(_Alloc __a)
- : _Base_type(static_cast<_Tp*>(0), _Sp_destroy_inplace<_Tp>(), __a)
- , _M_storage()
- {
- void* __p = &_M_storage;
- ::new (__p) _Tp(); // might throw
- _Base_type::_M_ptr = static_cast<_Tp*>(__p);
- }
-
template<typename... _Args>
_Sp_counted_ptr_inplace(_Alloc __a, _Args&&... __args)
- : _Base_type(static_cast<_Tp*>(0), _Sp_destroy_inplace<_Tp>(), __a)
- , _M_storage()
+ : _M_impl(__a), _M_storage()
{
- void* __p = &_M_storage;
- ::new (__p) _Tp(std::forward<_Args>(__args)...); // might throw
- _Base_type::_M_ptr = static_cast<_Tp*>(__p);
+ _M_impl._M_ptr = static_cast<_Tp*>(static_cast<void*>(&_M_storage));
+ allocator_traits<_Alloc>::construct(__a, _M_impl._M_ptr,
+ std::forward<_Args>(__args)...); // might throw
}
+ virtual void
+ _M_dispose() noexcept
+ { allocator_traits<_Alloc>::destroy(_M_impl, _M_impl._M_ptr); }
+
// Override because the allocator needs to know the dynamic type
virtual void
_M_destroy() noexcept
{
- typedef typename _Alloc::template
- rebind<_Sp_counted_ptr_inplace>::other _My_alloc_type;
- _My_alloc_type __a(_Base_type::_M_del);
- this->~_Sp_counted_ptr_inplace();
- __a.deallocate(this, 1);
+ typedef typename allocator_traits<_Alloc>::template
+ __rebind_traits<_Sp_counted_ptr_inplace>::__type _My_alloc_traits;
+ typename _My_alloc_traits::allocator_type __a(_M_impl);
+ _My_alloc_traits::destroy(__a, this);
+ _My_alloc_traits::deallocate(__a, this, 1);
}
// Sneaky trick so __shared_ptr can get the managed pointer
@@ -429,13 +421,14 @@
#ifdef __GXX_RTTI
return __ti == typeid(_Sp_make_shared_tag)
? static_cast<void*>(&_M_storage)
- : _Base_type::_M_get_deleter(__ti);
+ : 0;
#else
return 0;
#endif
}
private:
+ _Impl _M_impl;
typename aligned_storage<sizeof(_Tp), alignment_of<_Tp>::value>::type
_M_storage;
};
@@ -468,18 +461,21 @@
// The allocator's value_type doesn't matter, will rebind it anyway.
typedef std::allocator<int> _Alloc;
typedef _Sp_counted_deleter<_Ptr, _Deleter, _Alloc, _Lp> _Sp_cd_type;
- typedef std::allocator<_Sp_cd_type> _Alloc2;
- _Alloc2 __a2;
+ typedef typename allocator_traits<_Alloc> ::template
+ __rebind_traits<_Sp_cd_type>::__type _Alloc_traits;
+ typename _Alloc_traits::allocator_type __a;
+ _Sp_cd_type* __mem = 0;
__try
{
- _M_pi = __a2.allocate(1);
- ::new(static_cast<void*>(_M_pi)) _Sp_cd_type(__p, __d);
+ __mem = _Alloc_traits::allocate(__a, 1);
+ _Alloc_traits::construct(__a, __mem, __p, std::move(__d));
+ _M_pi = __mem;
}
__catch(...)
{
__d(__p); // Call _Deleter on __p.
- if (_M_pi)
- __a2.deallocate(static_cast<_Sp_cd_type*>(_M_pi), 1);
+ if (__mem)
+ _Alloc_traits::deallocate(__a, __mem, 1);
__throw_exception_again;
}
}
@@ -488,18 +484,22 @@
__shared_count(_Ptr __p, _Deleter __d, _Alloc __a) : _M_pi(0)
{
typedef _Sp_counted_deleter<_Ptr, _Deleter, _Alloc, _Lp> _Sp_cd_type;
- typedef typename _Alloc::template rebind<_Sp_cd_type>::other _Alloc2;
- _Alloc2 __a2(__a);
+ typedef typename allocator_traits<_Alloc> ::template
+ __rebind_traits<_Sp_cd_type>::__type _Alloc_traits;
+ typename _Alloc_traits::allocator_type __a2(__a);
+ _Sp_cd_type* __mem = 0;
__try
{
- _M_pi = __a2.allocate(1);
- ::new(static_cast<void*>(_M_pi)) _Sp_cd_type(__p, __d, __a);
+ __mem = _Alloc_traits::allocate(__a2, 1);
+ _Alloc_traits::construct(__a2, __mem,
+ __p, std::move(__d), std::move(__a));
+ _M_pi = __mem;
}
__catch(...)
{
__d(__p); // Call _Deleter on __p.
- if (_M_pi)
- __a2.deallocate(static_cast<_Sp_cd_type*>(_M_pi), 1);
+ if (__mem)
+ _Alloc_traits::deallocate(__a2, __mem, 1);
__throw_exception_again;
}
}
@@ -510,18 +510,19 @@
: _M_pi(0)
{
typedef _Sp_counted_ptr_inplace<_Tp, _Alloc, _Lp> _Sp_cp_type;
- typedef typename _Alloc::template rebind<_Sp_cp_type>::other _Alloc2;
- _Alloc2 __a2(__a);
+ typedef typename allocator_traits<_Alloc> ::template
+ __rebind_traits<_Sp_cp_type>::__type _Alloc_traits;
+ typename _Alloc_traits::allocator_type __a2(__a);
+ _Sp_cp_type* __mem = _Alloc_traits::allocate(__a2, 1);
__try
{
- _M_pi = __a2.allocate(1);
- ::new(static_cast<void*>(_M_pi)) _Sp_cp_type(__a,
+ _Alloc_traits::construct(__a2, __mem, std::move(__a),
std::forward<_Args>(__args)...);
+ _M_pi = __mem;
}
__catch(...)
{
- if (_M_pi)
- __a2.deallocate(static_cast<_Sp_cp_type*>(_M_pi), 1);
+ _Alloc_traits::deallocate(__a2, __mem, 1);
__throw_exception_again;
}
}
