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std::enable_shared_from_this

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Gestione della memoria dinamica
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std::enable_shared_from_this
Protetto funzioni membro
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enable_shared_from_this::enable_shared_from_this
enable_shared_from_this::~enable_shared_from_this
enable_shared_from_this::operator=
Funzioni membro pubbliche
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Public member functions
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enable_shared_from_this::shared_from_this
 
Defined in header <memory>
template< class T > class enable_shared_from_this;
(dal C++11)
std::enable_shared_from_this permette un t oggetto che è attualmente gestito da un std::shared_ptr nome pt per generare in modo sicuro altre istanze std::shared_ptr pt1, pt2, ... che tutti azionariato di t con pt.
Original:
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.
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Eredità da std::enable_shared_from_this<T> fornisce un T tipo con shared_from_this funzione membro. Se un oggetto di t T tipo è gestito da un std::shared_ptr<T> nome pt, quindi T::shared_from_this chiamata restituirà un nuovo std::shared_ptr<T> che la proprietà di parti t con pt.
Original:
Inheriting from std::enable_shared_from_this<T> provides a 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.
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Notare che prima di chiamare shared_from_this su un t oggetto, ci deve essere un std::shared_ptr che possiede t.
Original:
Note that prior to calling shared_from_this on an object t, there must be a std::shared_ptr that owns t.
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Si noti inoltre che enable_shared_from_this fornisce un'alternativa a un'espressione come std::shared_ptr<T>(this), che è suscettibile di provocare this stati distrutti più di una volta da più proprietari che non sono a conoscenza della vicenda.
Original:
Also note that enable_shared_from_this provides an 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 eachother.
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Indice

[modifica] Membri funzioni

costruisce un oggetto enabled_shared_from_this
Original:
constructs an enabled_shared_from_this object
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(protetto funzione membro)
distrugge un oggetto enable_shared_from_this
Original:
destroys an enable_shared_from_this object
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(protetto funzione membro)
restituisce un riferimento this
Original:
returns a reference to this
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(protetto funzione membro)
restituisce un shared_ptr che condivide la proprietà di *this
Original:
returns a shared_ptr which shares ownership of *this
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(membro pubblico funzione)

[modifica] Note

Un'implementazione comune per enable_shared_from_this è quello di tenere un riferimento debole (come std::weak_ptr) a this. I costruttori di std::shared_ptr in grado di rilevare la presenza di una base enable_shared_from_this e gli assetti proprietari con i std::shared_ptrs esistenti, invece di assumere il puntatore non è gestito da nessun altro.
Original:
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 can detect presence of a enable_shared_from_this base and share ownership with the existing std::shared_ptrs, instead of assuming the pointer is not managed by anyone else.
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[modifica] Esempio

#include <memory>
#include <iostream>
 
struct Good: std::enable_shared_from_this<Good>
{
    std::shared_ptr<Good> 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"; }
};
 
int main()
{
    // Good: the two shared_ptr's share the same object
    std::shared_ptr<Good> gp1(new Good);
    std::shared_ptr<Good> gp2 = gp1->getptr();
    std::cout << "gp2.use_count() = " << gp2.use_count() << '\n';
 
    // Bad, each shared_ptr thinks it's the only owner of the object
    std::shared_ptr<Bad> bp1(new Bad);
    std::shared_ptr<Bad> bp2 = bp1->getptr();
    std::cout << "bp2.use_count() = " << bp2.use_count() << '\n';
} // UB: double-delete of Bad

Output:

gp2.use_count() = 2
bp2.use_count() = 1
Bad::~Bad() called
Bad::~Bad() called
*** glibc detected *** ./test: double free or corruption

[modifica] Vedi anche

smart pointer with shared object ownership semantics
(classe template) [edit]