Using unique_ptr to control a file descriptor

Question

In theory, I should be able to use a custom pointer type and deleter in order to have unique_ptr manage an object that is not a pointer. I tried the following c

Answer 1

Found an answer at cppreference.com. Look in the examples code:

    void close_file(std::FILE* fp) { std::fclose(fp); }
    ...
    {
      std::unique_ptr<std::FILE, decltype(&close_file)> fp(std::fopen("demo.txt",                                                             
                                                                      "r"), 
                                                           &close_file);
      if(fp)     // fopen could have failed; in which case fp holds a null pointer
         std::cout << (char)std::fgetc(fp.get()) << '\n';
    }// fclose() called here, but only if FILE* is not a null pointer
     // (that is, if fopen succeeded)

Tried it in vs2019 and it works! Also tried it with member and lambda:

FileTest.h:

class A
{
  std::unique_ptr<std::FILE, std::function<void(std::FILE*)>> fp;
}

FileTest.cpp

void A::OpenFile(const char* fname)
{
    fp = std::unique_ptr < std::FILE, std::function<void(std::FILE*)>>(
        std::fopen(fname, "wb"),
        [](std::FILE * fp) { std::fclose(fp); });
}

Answer 2

The type exposed by the Deleter::pointer must satisfy the NullablePointer requirements. Chief among them, this expression must be legal: Deleter::pointer p = nullptr;. Of course, nullptr is pretty much defined by the fact that it cannot be implicitly converted to a number, thus this doesn't work.

You'll have to use a type which can be implicitly constructed with std::nullptr_t. Something like this:

struct file_desc
{
  file_desc(int fd) : _desc(fd) {}
  file_desc(std::nullptr_t) : _desc(-1) {}

  operator int() {return _desc;}

  bool operator ==(const file_desc &other) const {return _desc == other._desc;}
  bool operator !=(const file_desc &other) const {return _desc != other._desc;}
  bool operator ==(std::nullptr_t) const {return _desc == -1;}
  bool operator !=(std::nullptr_t) const {return _desc != -1;}

  int _desc;
};

You can use that as the Deleter::pointer type.

Answer 3

The open source Android Framework defines a unique_fd class that might meet your needs: https://android.googlesource.com/platform/system/core/+/c0e6e40/base/include/android-base/unique_fd.h