std::out_ptr_t
Defined in header <memory>.
template< class Smart, class Pointer, class... Args >
class out_ptr_t;
out_ptr_t is used to adapt types such as smart pointers for foreign functions that output their results via a Pointer* (usually T** for some object type T) or void** parameter.
out_ptr_t captures additional arguments on construction, provides a storage for the result to which such an aforementioned foreign function writes,
and finally resets the adapted Smart object with the result and the captured arguments when it is destroyed.
out_ptr_t behaves as if it holds following non-static data members:
- a
Smart&reference, which is bound to the adapted object on construction, - for every
TinArgs..., a member of typeT, which is an argument captured on construction and used for resetting while destruction, and - a member subobject that suitable for storing a
Pointerwithin it and providing avoid*object, where thePointerorvoid*object is generally exposed to a foreign function for re-initialization.
Users can control whether each argument for resetting is captured by copy or by reference, by specifying an object type or a reference type in Args... respectively.
Template Parameters
Smart - the type of the object (typically a smart pointer) to adapt
Pointer - type of the object (typically a raw pointer) to which a foreign function writes its result
Args... - type of captured arguments used for resetting the adapted object
Type requirements
- Pointer must meet the requirements of
NullablePointer. - The program is ill-formed if Smart is a std::shared_ptr specialization and
sizeof...(Args) == 0.
Specializations
Unlike most class templates in the standard library, program-defined specializations of out_ptr_t that depend on at least one program-defined type need not meet the requirements for the primary template.
This license allows a program-defined specialization to expose the raw pointer stored within a non-standard smart pointer to foreign functions.
Member functions
| pub | (constructor)(C++23) | constructs an out_ptr_t (public member function) |
| pub | (destructor)(C++23) | resets the adapted smart pointer (public member function) |
| pub | operator=[deleted](C++23) | out_ptr_t is not assignable (public member function) |
| pub | operator Pointer* operator void**(C++23) | converts the out_ptr_t to the address of the storage for output (public member function) |
Non-member functions
| pub | out_ptr(C++23) | creates an out_ptr_t with an associated smart pointer and resetting arguments (function template) |
Notes
out_ptr_t expects that the foreign functions do not used the value of the pointed-to Pointer,
and only re-initialize it. The value of the smart pointer before adaption is not used.
The typical usage of out_ptr_t is creating its temporary objects by std::out_ptr, which resets the adapted smart pointer immediately.
E.g. given a setter function and a smart pointer of appropriate type declared with
int foreign_setter(T**); and std::unique_ptr<T, D> up; respectively,
int foreign_setter(T**);
std::unique_ptr<T, D> up;
if (int ec = foreign_setter(std::out_ptr(up))) {
return ec;
}
is roughly equivalent to
int foreign_setter(T**);
std::unique_ptr<T, D> up;
T* raw_p{};
int ec = foreign_setter(&raw_p);
up.reset(raw_p);
if (ec != 0) {
return ec;
}
It is not recommended to create an out_ptr_t object of a storage duration other than automatic storage duration, because such code is likely to produce dangling references and result in undefined behavior on destruction.
out_ptr_t forbids the usage that would reset a std::shared_ptr without specifying a deleter, because it would call std::shared_ptr::reset and replace a custom deleter later.
Captured arguments are typically packed into a std::tuple<Args...>.
Implementations may use different mechanism to provide the Pointer or void* object they need hold.
| Feature-test macro | Value | Std |
|---|---|---|
| __cpp_lib_out_ptr | 202106L | (C++23) |