Does std::unique_ptr support fancy pointers?

• Can std::unique_ptr manage pointers only, or fancy pointers (pointer-like types) too?

I assume "fancy pointers too", because for resources like files/sockets, fancy pointers can be a great tool. And there seems no reason std::unique_ptr shouldn't support them.

• Why are there inconsistent ways to test empty-ness of the managed pointer?

There are different ways to test empty-ness of the managed pointer in std::unique_ptr (GCC):

• In ~unique_ptr(): if (__ptr != nullptr) (code)
• In reset(): if (__old_p) (code)
• In operator bool(): return get() == pointer() ? false : true; (code)

The problem is that they have different requirements to the pointer type:

• bool operator ==(pointer, nullptr_t)
• operator bool()
• bool operator ==(pointer, pointer)

The inconsistent usage means all of them are needed, while one of them would suffice semantically.

#include <memory>using namespace std;class File {    FILE * pf_ = nullptr;public:    File() = default;    File(const char * pathname) { /*...*/ };    void Close() { /*...*/ };    // All are needed to make std::unique_ptr work    explicit operator bool() const { return pf_ == nullptr; }    bool operator ==(nullptr_t) const { return pf_ == nullptr; }    bool operator ==(File) const { /*...*/ return true; };};struct FileCloser {    using pointer = File;    void operator ()(File p) const { p.Close(); }};int main() {    using FilePtr = unique_ptr<File, FileCloser>;    FilePtr p(File("./abc"));    if(p)        p.reset();}

https://godbolt.org/z/fjPjWzzhW

IMHO, std::unique_ptr should require minimum from the pointer type. One solution is to add a static member, e.g. __test_p, and use it to test empty-ness consistently:

template<typename T, typename D = std::default_delete<T>>class unique_ptr{// ...private:    static constexpr bool __test_p(pointer p) noexcept {        if constexpr (is_convertible_v<pointer, bool>)            return bool(p);        else if constexpr (requires {{ p != nullptr} -> convertible_to<bool>; })            return p != nullptr;        else            return p != pointer();    }// ...};