Reputation: 8697
In below code I am trying to convert a void* to a shared_ptr of a type:
#include <iostream>
#include <memory>
class A
{
public:
A() { l = 0; }
int l;
void Show() { std::cout << l << "\n"; }
};
void PrintA(void *aptr)
{
std::shared_ptr<A> a1;
a1.reset(aptr);
a1->Show();
}
int main()
{
std::shared_ptr<A> a(new A());
PrintA(a.get());
}
But I get below compilation error:
$ c++ -std=c++14 try20.cpp
In file included from C:/tools/mingw64/x86_64-w64-mingw32/include/c++/bits/shared_ptr.h:52:0,
from C:/tools/mingw64/x86_64-w64-mingw32/include/c++/memory:82,
from try20.cpp:2:
C:/tools/mingw64/x86_64-w64-mingw32/include/c++/bits/shared_ptr_base.h: In instantiation of 'std::__shared_ptr<_Tp, _Lp>::__shared_ptr(_Tp1*) [with _Tp1 = void; _Tp = A; __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2u]':
C:/tools/mingw64/x86_64-w64-mingw32/include/c++/bits/shared_ptr_base.h:1023:4: required from 'void std::__shared_ptr<_Tp, _Lp>::reset(_Tp1*) [with _Tp1 = void; _Tp = A; __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2u]'
try20.cpp:14:14: required from here
C:/tools/mingw64/x86_64-w64-mingw32/include/c++/bits/shared_ptr_base.h:871:39: error: invalid conversion from 'void*' to 'A*' [-fpermissive]
: _M_ptr(__p), _M_refcount(__p)
^
C:/tools/mingw64/x86_64-w64-mingw32/include/c++/bits/shared_ptr_base.h:874:4: error: static assertion failed: incomplete type
static_assert( !is_void<_Tp1>::value, "incomplete type" );
How can I convert a void pointer to a shared pointer of a type?
Upvotes: 2
Views: 1862
Reputation: 217135
Assuming you cannot change declaration of PrintA
, your PrintA
definition should look like:
void PrintA(void *aptr)
{
A* a1 = reinterpret_cast<A*>(aptr);
a1->Show();
}
as you pass to it a pointer.
as you don't reclaim ownership, no need to create std::shared_ptr
. If you need to share ownership, you have to modify A
to allow to use share_from_this
.
Upvotes: 5