std::out_ptr

 

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std::out_ptr_t

Member functions

out_ptr_t::out_ptr_t

out_ptr_t::~out_ptr_t

out_ptr_t::operator Pointer*out_ptr_t::operator void**

Non-member functions

out_ptr

 

Defined in header <memory>
template< class Pointer = void, class Smart, class... Args > auto out_ptr( Smart& s, Args&&... args );		(since C++23)

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.

Parameters

s	-	the object (typically a smart pointer) to adapt
args...	-	the arguments for resetting to capture

Return value

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

• Pointer, if Pointer is not same as void. 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*.

Notes

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.

Feature-test macro	Value	Std	Feature
__cpp_lib_out_ptr	202106L	(C++23)	std::out_ptr, std::inout_ptr
	202311L	(C++26)	freestanding std::out_ptr and std::inout_ptr

Example

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

Run this code

#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 ...
    }
}

See also

inout_ptr (C++23)	creates an inout_ptr_t with an associated smart pointer and resetting arguments (function template)
make_uniquemake_unique_for_overwrite (C++14)(C++20)	creates a unique pointer that manages a new object (function template)
make_sharedmake_shared_for_overwrite (C++20)	creates a shared pointer that manages a new object (function template)