std::weak_ptr::weak_ptr
std::weak_ptr::weak_ptr
constexpr weak_ptr(); | (1) | (since C++11) |
weak_ptr( const weak_ptr& r ); | (2) | (since C++11) |
template< class Y > weak_ptr( const weak_ptr<Y>& r ); | (2) | (since C++11) |
template< class Y > weak_ptr( const std::shared_ptr<Y>& r ); | (2) | (since C++11) |
weak_ptr( weak_ptr&& r ); | (3) | (since C++14) |
template< class Y > weak_ptr( weak_ptr<Y>&& r ); | (3) | (since C++14) |
Constructs new weak_ptr
that potentially shares an object with r
.
1) Default constructor. Constructs empty
weak_ptr
.
2) Constructs new
weak_ptr
which shares an object managed by r
. If r
manages no object, *this
manages no object too. The templated overloads don't participate in the overload resolution unless Y*
is implicitly convertible to T*
, or Y
is the type "array of N
U
" for some type U
and some number N
, and T
is the type "array of unknown bound of (possibly cv-qualified) U
". (since C++17)
3) Move constructors. Moves a weak_ptr instance from
r
into *this
. After this, r
is empty and r.use_count()==0
. The templated overload doesn't participate in the overload resolution unless Y*
is implicitly convertible to T*
Parameters
r | - | a std::shared_ptr or std::weak_ptr that will be viewed by this std::weak_ptr |
Exceptions
noexcept
specification: noexcept
Notes
Because the default constructor is constexpr
, static weak_ptrs are initialized as part of static non-local initialization, before any dynamic non-local initialization begins. This makes it safe to use a weak_ptr in a constructor of any static object.
Example
#include <memory> #include <iostream> struct Foo {}; int main() { std::weak_ptr<Foo> w_ptr; { auto ptr = std::make_shared<Foo>(); w_ptr = ptr; std::cout << "w_ptr.use_count() inside scope: " << w_ptr.use_count() << '\n'; } std::cout << "w_ptr.use_count() out of scope: " << w_ptr.use_count() << '\n'; std::cout << "w_ptr.expired() out of scope: " << std::boolalpha << w_ptr.expired() << '\n'; }
Output:
w_ptr.use_count() inside scope: 1 w_ptr.use_count() out of scope: 0 w_ptr.expired() out of scope: true
See also
assigns the weak_ptr (public member function) |
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