error using static function deleter with std::unique_ptr

Question

I have the following code snippet that fails to compile...seems like it should, but it evading me right now. Greatly appreciate any help/suggestions!

#include <atomic>
#include <memory>

struct MyClass {

static void free_lock(std::atomic<int>** lck) { (*lck)->store(0); }

typedef std::unique_ptr<std::atomic<int>*, decltype(&MyClass::free_lock)> lock_scope;

static lock_scope get_lock() {
    static std::atomic<int> lck(0);
    int ref = 0;
    return lock_scope(&lck, &MyClass::free_lock);
}

};

The following error message was reported by Clang 3.2

Compilation finished with errors:
source.cpp:13:23: error: no matching constructor for initialization of 'lock_scope' (aka 'unique_ptr *, decltype(&MyClass::free_lock)>')
        return lock_scope(&lck, &MyClass::free_lock);
                         ^          ~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../include/c++/4.7/bits/unique_ptr.h:130:7: note: candidate constructor not viable: no known conversion from 'std::atomic *' to 'pointer' (aka 'std::atomic **') for 1st argument
      unique_ptr(pointer __p,
      ^
/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../include/c++/4.7/bits/unique_ptr.h:125:7: note: candidate constructor not viable: no known conversion from 'std::atomic *' to 'pointer' (aka 'std::atomic **') for 1st argument
      unique_ptr(pointer __p,
      ^
/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../include/c++/4.7/bits/unique_ptr.h:155:2: note: candidate constructor template not viable: requires single argument '__u', but 2 arguments were provided
        unique_ptr(unique_ptr&& __u) noexcept
        ^
/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../include/c++/4.7/bits/unique_ptr.h:164:2: note: candidate constructor template not viable: requires single argument '__u', but 2 arguments were provided
        unique_ptr(auto_ptr&& __u) noexcept
        ^
/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../include/c++/4.7/bits/unique_ptr.h:114:17: note: candidate constructor not viable: requires 0 arguments, but 2 were provided
      constexpr unique_ptr() noexcept
                ^
/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../include/c++/4.7/bits/unique_ptr.h:120:7: note: candidate constructor not viable: requires single argument '__p', but 2 arguments were provided
      unique_ptr(pointer __p) noexcept
      ^
/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../include/c++/4.7/bits/unique_ptr.h:136:17: note: candidate constructor not viable: requires 1 argument, but 2 were provided
      constexpr unique_ptr(nullptr_t) noexcept
                ^
/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../include/c++/4.7/bits/unique_ptr.h:142:7: note: candidate constructor not viable: requires single argument '__u', but 2 arguments were provided
      unique_ptr(unique_ptr&& __u) noexcept
      ^
/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../include/c++/4.7/bits/unique_ptr.h:262:7: note: candidate constructor not viable: requires 1 argument, but 2 were provided
      unique_ptr(const unique_ptr&) = delete;
      ^
1 error generated.

Answer 1

Your lock_scope type is declared to wrap a pointer to std::atomic<int>*, i.e. a std::atomic<int>**. The free_lock declaration and definition correctly reflects that.

But then you attempt to initialize the lock_scope with a pointer to just a std::atomic<int> (one indirection less).

The compiler messages state that rather clearly:

[...] candidate constructor not viable: 
no known conversion from 'std::atomic *' to 'pointer' (aka 'std::atomic **') [...]

You either need an additional indirection for the wrapped lck or change lock_scope and free_lock to use one indirection less.

Answer 2

You could convert return lock_scope(..); to return std::move( lock_scope(...) );

Because std::unique_ptr::operator = (const std::unique_ptr& ) is private ;