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std::out_ptr - cppreference.com

template< class Pointer = void, class Smart, class... Args >
auto out_ptr( Smart& s, Args&&... args );

Returns an std::out_ptr_t with deduced template arguments that captures arguments for resetting by reference.

The program is ill-formed if construction of the return value (see below) is ill-formed.

std::out_ptr_t<Smart, P, Args&&>(s, std::forward<Args>(args)...), where P is

• Pointer, if Pointer is not a void type. Otherwise,
• Smart::pointer, if it is valid and denotes a type. Otherwise,
• Smart::element_type*, if Smart::element_type is valid and denotes a type. Otherwise,
• std::pointer_traits<Smart>::element_type*.

Users may specify the template argument for the template parameter Pointer, in order to interoperate with foreign functions that take a Pointer*.

As all arguments for resetting are captured by reference, the returned out_ptr_t should be a temporary object destroyed at the end of the full-expression containing the call to the foreign function, in order to avoid dangling references.

Use std::out_ptr to adapt a smart pointer for sqlite3_open, which expects a sqlite3** as an out parameter.

#include <memory>
#include <sqlite3.h>
 
int main()
{
    auto close_db = [](sqlite3* db) { sqlite3_close(db); };
 
    {
        // open an in-memory database, and manage its lifetime with std::unique_ptr
        std::unique_ptr<sqlite3, decltype(close_db)> up;
        sqlite3_open(":memory:", std::out_ptr(up));
 
        sqlite3* db = up.get();
        // do something with db ...
    }
    {
        // same as above, but use a std::shared_ptr
        std::shared_ptr<sqlite3> sp;
        sqlite3_open(":memory:", std::out_ptr(sp, close_db));
 
        sqlite3* db = sp.get();
        // do something with db ...
    }
}

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