7.5 KiB
[inout.ptr.t]
20 Memory management library [mem]
20.3 Smart pointers [smartptr]
20.3.4 Smart pointer adaptors [smartptr.adapt]
20.3.4.3 Class template inout_ptr_t [inout.ptr.t]
inout_ptr_t is a class template used to adapt types such as smart pointers ([smartptr]) for functions that use output pointer parameters whose dereferenced values may first be deleted before being set to another allocated value.
[Example 1: #include struct star_fish* star_fish_alloc();int star_fish_populate(struct star_fish** ps, const char* description);
struct star_fish_deleter {void operator() (struct star_fish* c) const noexcept;};
using star_fish_ptr = std::unique_ptr<star_fish, star_fish_deleter>;
int main(int, char*[]) { star_fish_ptr peach(star_fish_alloc()); // ...// used, need to re-makeint err = star_fish_populate(std::inout_ptr(peach), "caring clown-fish liker"); return err;}
A unique_ptr can be used with inout_ptr to be passed into an output pointer-style function.
The original value will be properly deleted according to the function it is used with and a new value reset in its place.
â end example]
namespace std {template<class Smart, class Pointer, class... Args>class inout_ptr_t {public:constexpr explicit inout_ptr_t(Smart&, Args...); inout_ptr_t(const inout_ptr_t&) = delete; constexpr ~inout_ptr_t(); constexpr operator Pointer*() const noexcept; operator void**() const noexcept; private: Smart& s; // exposition only tuple<Args...> a; // exposition only Pointer p; // exposition only};}
Pointer shall meet the Cpp17NullablePointer requirements.
If Smart is a specialization of shared_ptr, the program is ill-formed.
[Note 1:
It is impossible to properly acquire unique ownership of the managed resource from a shared_ptr given its shared ownership model.
â end note]
Program-defined specializations of inout_ptr_t that depend on at least one program-defined type need not meet the requirements for the primary template.
Evaluations of the conversion functions on the same object may conflict ([intro.races]).
constexpr explicit inout_ptr_t(Smart& smart, Args... args);
Effects: Initializes s with smart,a with std::forward(args)..., andp to either
smart if is_pointer_v is true,
otherwise, smart.get().
Remarks: An implementation can call s.release().
[Note 2:
The constructor is not noexcept to allow for a variety of non-terminating and safe implementation strategies.
For example, an intrusive pointer implementation with a control block can allocate in the constructor and safely fail with an exception.
â end note]
constexpr ~inout_ptr_t();
Let SP bePOINTER_OF_OR(Smart, Pointer) ([memory.general]).
Let release-statement be s.release(); if an implementation does not call s.release() in the constructor.
Otherwise, it is empty.
Effects: Equivalent to:
apply([&](auto&&... args) { s = Smart(static_cast(p), std::forward(args)...); }, std::move(a)); if is_pointer_v is true;
otherwise,release-statement;if (p) { apply([&](auto&&... args) { s.reset(static_cast(p), std::forward(args)...); }, std::move(a));} if the expressions.reset(static_cast(p), std::forward(args)...) is well-
formed;
otherwise,release-statement;if (p) { apply([&](auto&&... args) { s = Smart(static_cast(p), std::forward(args)...); }, std::move(a));} if is_constructible_v<Smart, SP, Args...> is true;
otherwise, the program is ill-formed.
constexpr operator Pointer*() const noexcept;
Preconditions: operator void**() has not been called on *this.
Returns: addressof(const_cast<Pointer&>(p)).
operator void**() const noexcept;
Constraints: is_same_v<Pointer, void*> is false.
Mandates: is_pointer_v is true.
Preconditions: operator Pointer*() has not been called on *this.
Returns: A pointer value v such that:
the initial value v is equivalent to static_cast<void>(p) and
any modification of *v that is not followed by subsequent modification of *this affects the value of p during the destruction of this, such that static_cast<void>(p) == *v.
Remarks: Accessing *v outside the lifetime of *this has undefined behavior.
[Note 3:
reinterpret_cast<void**>(static_cast<Pointer*>(*this)) can be a viable implementation strategy for some implementations.
â end note]