std::enable_shared_from_this
| 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
constructs an enable_shared_from_this object (protected member function) | |
destroys an enable_shared_from_this object (protected member function) | |
returns a reference to this (protected member function) | |
returns a shared_ptr which shares ownership of *this (public member function) | |
(C++17) |
returns the weak_ptr which shares ownership of *this (public member function) |
Member objects
| Member name | Definition |
weak_this (private)(C++17)
|
std::weak_ptr object tracking the control block of the first shared owner of *this. Exposition only
|
Notes
A common implementation for enable_shared_from_this is to hold a weak reference (such as std::weak_ptr) to this. The constructors of std::shared_ptr detect the presence of an unambiguous and accessible (ie. public inheritance is mandatory)(since C++17) enable_shared_from_this base and assign the newly created std::shared_ptr to the internally stored weak reference if not already owned by a live std::shared_ptr(since C++17). Constructing a std::shared_ptr for an object that is already managed by another std::shared_ptr will not consult the internally stored weak reference 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 the behavior is undefined(until C++17)std::bad_weak_ptr is thrown (by the shared_ptr constructor from a default-constructed weak_this)(since C++17).
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
#include <memory>
#include <iostream>
struct Good: std::enable_shared_from_this<Good> // note: public inheritance
{
std::shared_ptr<Good> getptr() {
return shared_from_this();
}
};
struct Best : std::enable_shared_from_this<Best> // note: public inheritance
{
std::shared_ptr<Best> getptr() {
return shared_from_this();
}
// No public constructor, only a factory function,
// so there's no way to have getptr return nullptr.
[[nodiscard]] static std::shared_ptr<Best> create() {
// Not using std::make_shared<Best> because the c'tor is private.
return std::shared_ptr<Best>(new Best());
}
private:
Best() = default;
};
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
See also
(C++11) |
smart pointer with shared object ownership semantics (class template) |