std::out_ptr_t

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 T in Args..., a member of type T, which is an argument captured on construction and used for resetting while destruction, and
• a member subobject that suitable for storing a Pointer within it and providing a void* object, where the Pointer or void* 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

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

Non-member functions

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.

Example

See also