Question

I’m attempting to use std::unique_ptr in order to store the integer handle to some opaque objects. For this purpose, I have defined a custom deleter type which does typedef int pointer in order to override the raw pointer type to int instead of int*. This process is described in the last section of this site: http://asawicki.info/news_1494_unique_ptr_in_visual_c_2010.html

Here is some sample code to better illustrate what I’m attempting to do:

#include <memory>
#include <iostream>

static void close(int p)
{
    std::cout << p << " has been deleted!" << std::endl;
}

struct handle_deleter
{
    typedef int pointer;
    void operator()(pointer p) { close(p); }
};

typedef std::unique_ptr< int, handle_deleter> unique_handle;

int main(int argc, char *argv[])
{
    unique_handle handle(1);

    return 0;
}

When I compile this code using GCC 4.7.2, I get the following error:

In file included from /usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/memory:86:0,
                 from unique_ptr_test.cpp:1:
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/unique_ptr.h: In instantiation of ‘std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = int; _Dp = handle_deleter]’:
unique_ptr_test.cpp:19:23:   required from here
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/unique_ptr.h:172:2: error: invalid operands of types ‘int’ and ‘std::nullptr_t’ to binary ‘operator!=’

The code of the ~unique_ptr procedure reads as follows:

// Destructor.
~unique_ptr() noexcept
{
    auto& __ptr = std::get<0>(_M_t);
    if (__ptr != nullptr)
      get_deleter()(__ptr);
    __ptr = pointer();
}

According to me, the check against nullptr does not make sense since the raw pointer type is int (and not int* due to overriding it in HandleDeleter). Strangely enough, this code compiles with no error under GCC 4.6.1. Upon execution, the sample displays “1 has been deleted!” as expected.

I was wondering if there is any detail I’m overlooking or if it really is a bug within GCC’s STL implementation of unique_ptr.

Thanks,

PMJ

Answer 1

As I said in my comment, if the deleter type you pass has a nested pointer typedef, it must fulfill the NullablePointer requirements:

20.7.1.2 [unique.ptr.single] p3

If the type remove_reference<D>::type::pointer exists, then unique_ptr<T, D>::pointer shall be a synonym for remove_reference<D>::type::pointer. Otherwise unique_ptr<T, D>::pointer shall be a synonym for T*. The type unique_ptr<T, D>::pointer shall satisfy the requirements of NullablePointer (17.6.3.3).

And §17.6.3.3 then lists all the requirements a type has to fulfill to be a NullablePointer. Certain semantics are listed in a table, where:

u denotes an identifier, t denotes a non-const lvalue of type P, a and b denote values of type (possibly const) P, and np denotes a value of type (possibly const) std::nullptr_t.

Expression    Return type                         Operational semantics
P u(np);                                          post: u == nullptr
P u = np;
P(np)                                             post: P(np) == nullptr
t = np        P&                                  post: t == nullptr
a != b        contextually convertible to bool    !(a == b)
a == np       contextually convertible to bool    a == P()
np == a
a != np       contextually convertible to bool    !(a == np)
np != a

Now, the simplest solution is to wrap your int in a type that provides these semantics:

#include <cstddef> // std::nullptr_t

struct handle{
  handle() : value(0){}
  handle(std::nullptr_t) : value(0){}
  /*explicit*/ handle(int v) : value(v){} // make it explicit if you need it
  // special members can be generated

  handle& operator=(std::nullptr_t){ value = 0; return *this; }

  // contextual conversion to bool
  explicit operator bool() const{ return value != 0; }

  int value;
};

bool operator==(handle lhs, handle rhs){ return lhs.value == rhs.value; }
bool operator!=(handle lhs, handle rhs){ return lhs.value != rhs.value; }
// comparision against 'nullptr' is handled by the above operators
// since 'nullptr' can be implicitly converted to 'handle'