20 Memory management library [mem]

20.3 Smart pointers [smartptr]

20.3.4 Smart pointer adaptors [smartptr.adapt]

20.3.4.1 Class template out_ptr_t [out.ptr.t]

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out_ptr_t is a class template used to adapt types such as smart pointers ([smartptr]) for functions that use output pointer parameters.
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[Example 1: 
#include <memory> #include <cstdio> int fopen_s(std::FILE** f, const char* name, const char* mode); struct fclose_deleter { void operator()(std::FILE* f) const noexcept { std::fclose(f); } }; int main(int, char*[]) { constexpr const char* file_name = "ow.o"; std::unique_ptr<std::FILE, fclose_deleter> file_ptr; int err = fopen_s(std::out_ptr<std::FILE*>(file_ptr), file_name, "r+b"); if (err != 0) return 1; // *file_ptr is valid return 0; } unique_ptr can be used with out_ptr to be passed into an output pointer-style function, without needing to hold onto an intermediate pointer value and manually delete it on error or failure.
— end example]
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namespace std { template<class Smart, class Pointer, class... Args> class out_ptr_t { public: explicit out_ptr_t(Smart&, Args...); out_ptr_t(const out_ptr_t&) = delete; ~out_ptr_t(); operator Pointer*() const noexcept; operator void**() const noexcept; private: Smart& s; // exposition only tuple<Args...> a; // exposition only Pointer p; // exposition only }; }
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Pointer shall meet the Cpp17NullablePointer requirements.
If Smart is a specialization of shared_ptr and sizeof...(Args) == 0, the program is ill-formed.
[Note 1: 
It is typically a user error to reset a shared_ptr without specifying a deleter, as shared_ptr will replace a custom deleter upon usage of reset, as specified in [util.smartptr.shared.mod].
— end note]
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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.
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Evaluations of the conversion functions on the same object may conflict ([intro.races]).
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explicit out_ptr_t(Smart& smart, Args... args);
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Effects: Initializes s with smart, a with std​::​forward<Args>(args)..., and value-initializes p.
Then, equivalent to:
  • (6.1)
    s.reset(); if the expression s.reset() is well-formed;
  • (6.2)
    otherwise, s = Smart(); if is_constructible_v<Smart> is true;
  • (6.3)
    otherwise, the program is ill-formed.
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[Note 2: 
The constructor is not noexcept to allow for a variety of non-terminating and safe implementation strategies.
For example, an implementation can allocate a shared_ptr's internal node in the constructor and let implementation-defined exceptions escape safely.
The destructor can then move the allocated control block in directly and avoid any other exceptions.
— end note]
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~out_ptr_t();
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Let SP be POINTER_OF_OR(Smart, Pointer) (