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std::enable_shared_from_this(3) | C++ Standard Libary | std::enable_shared_from_this(3) |
NAME¶
std::enable_shared_from_this - std::enable_shared_from_this
Synopsis¶
Defined in header <memory>
template< class T > class enable_shared_from_this; (since
C++11)
std::enable_shared_from_this allows an object t that is currently managed by
a
std::shared_ptr named pt to safely generate additional std::shared_ptr
instances
pt1, pt2, ... that all share ownership of t with pt.
Publicly inheriting from std::enable_shared_from_this<T> provides the
type T with a
member function shared_from_this. If an object t of type T is managed by a
std::shared_ptr<T> named pt, then calling T::shared_from_this will
return a new
std::shared_ptr<T> that shares ownership of t with pt.
Member functions¶
constructor constructs an enable_shared_from_this object
(protected member function)
destructor destroys an enable_shared_from_this object
(protected member function)
operator= returns a reference to *this
(protected member function)
shared_from_this returns a std::shared_ptr which shares ownership of *this
(public member function)
weak_from_this returns a std::weak_ptr which shares ownership of *this
(C++17) (public member function)
Member objects¶
Member name Definition
weak-this (exposition only) std::weak_ptr object tracking the control block
of the
first shared owner of *this.
Notes¶
A common implementation for enable_shared_from_this is to hold a
weak reference
(such as std::weak_ptr) to *this. For the purpose of exposition, the weak
reference
is called weak-this and considered as a mutable std::weak_ptr member.
The constructors of std::shared_ptr detect the presence of an unambiguous and
accessible (i.e. public inheritance is mandatory) enable_shared_from_this
base and
assign the newly created std::shared_ptr to weak-this if not already owned by
a live
std::shared_ptr. Constructing a std::shared_ptr for an object that is already
managed by another std::shared_ptr will not consult weak-this and thus will
lead to
undefined behavior.
It is permitted to call shared_from_this only on a previously shared object,
i.e. on
an object managed by std::shared_ptr<T>. Otherwise, std::bad_weak_ptr
is thrown (by
the shared_ptr constructor from a default-constructed weak-this).
enable_shared_from_this provides the safe alternative to an expression like
std::shared_ptr<T>(this), which is likely to result in this being
destructed more
than once by multiple owners that are unaware of each other (see example
below).
Example¶
// Run this code
#include <iostream>
#include <memory>
class Good : public std::enable_shared_from_this<Good>
{
public:
std::shared_ptr<Good> getptr()
{
return shared_from_this();
}
};
class Best : public std::enable_shared_from_this<Best>
{
struct Private{ explicit Private() = default; };
public:
// Constructor is only usable by this class
Best(Private) {}
// Everyone else has to use this factory function
// Hence all Best objects will be contained in shared_ptr
static std::shared_ptr<Best> create()
{
return std::make_shared<Best>(Private());
}
std::shared_ptr<Best> getptr()
{
return shared_from_this();
}
};
struct Bad
{
std::shared_ptr<Bad> getptr()
{
return std::shared_ptr<Bad>(this);
}
~Bad() { std::cout << "Bad::~Bad() called\n"; }
};
void testGood()
{
// Good: the two shared_ptr's share the same object
std::shared_ptr<Good> good0 = std::make_shared<Good>();
std::shared_ptr<Good> good1 = good0->getptr();
std::cout << "good1.use_count() = " <<
good1.use_count() << '\n';
}
void misuseGood()
{
// Bad: shared_from_this is called without having std::shared_ptr owning the
caller
try
{
Good not_so_good;
std::shared_ptr<Good> gp1 = not_so_good.getptr();
}
catch (std::bad_weak_ptr& e)
{
// undefined behavior (until C++17) and std::bad_weak_ptr thrown
(since C++17)
std::cout << e.what() << '\n';
}
}
void testBest()
{
// Best: Same but can't stack-allocate it:
std::shared_ptr<Best> best0 = Best::create();
std::shared_ptr<Best> best1 = best0->getptr();
std::cout << "best1.use_count() = " <<
best1.use_count() << '\n';
// Best stackBest; // <- Will not compile because Best::Best() is private.
}
void testBad()
{
// Bad, each shared_ptr thinks it's the only owner of the object
std::shared_ptr<Bad> bad0 = std::make_shared<Bad>();
std::shared_ptr<Bad> bad1 = bad0->getptr();
std::cout << "bad1.use_count() = " << bad1.use_count()
<< '\n';
} // UB: double-delete of Bad
int main()
{
testGood();
misuseGood();
testBest();
testBad();
}
Possible output:¶
good1.use_count() = 2
bad_weak_ptr
best1.use_count() = 2
bad1.use_count() = 1
Bad::~Bad() called
Bad::~Bad() called
*** glibc detected *** ./test: double free or corruption
Defect reports
The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.
DR Applied to Behavior as published Correct behavior
LWG 2529 C++11 specification for enable_shared_from_this was clarified and
unclear and maybe unimplementable fixed
See also¶
shared_ptr smart pointer with shared object ownership semantics
(C++11) (class template)
make_shared creates a shared pointer that manages a new object
make_shared_for_overwrite (function template)
(C++20)
2024.06.10 | http://cppreference.com |