20 Memory management library [mem]

20.3 Smart pointers [smartptr]

20.3.4 Smart pointer adaptors [smartptr.adapt] 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 <memory> 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-make int 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.
— end example]
namespace std { template<class Smart, class Pointer, class... Args> class inout_ptr_t { public: explicit inout_ptr_t(Smart&, Args...); inout_ptr_t(const inout_ptr_t&) = delete; ~inout_ptr_t(); 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.
— 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]).
explicit inout_ptr_t(Smart& smart, Args... args);
Effects: Initializes s with smart, a with std​::​forward<Args>(args)..., and p to either
  • smart if is_pointer_v<Smart> 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.
— end note]
Let SP be POINTER_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<SP>(p), std::forward<Args>(args)...); }, std::move(a)); if is_pointer_v<Smart> is true;
  • otherwise, release-statement; if (p) { apply([&](auto&&... args) { s.reset(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a)); } if the expression s.reset(static_cast<SP>(p), std​::​forward<Args>(args)...) is well-
  • otherwise, release-statement; if (p) { apply([&](auto&&... args) { s = Smart(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a)); } if is_constructible_v<Smart, SP, Args...> is true;
  • otherwise, the program is ill-formed.
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<Pointer> 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.
— end note]