[Development] Error "invokes deleted constructor" while building Qt5 with clang and libc++

Thiago Macieira thiago.macieira at intel.com
Mon Dec 3 22:28:25 CET 2012

On segunda-feira, 3 de dezembro de 2012 18.08.01, Francisco Lopes wrote:
> Can you point out where in the standard brace assignment initialization is
> not supposed to invoke a copy constructor? Because from what I've pointed,
> I only see this case, and so I see both the compiler not working as
> expected as both the qt and all this library code as ill-formed.

That's not my point. The standard leaves ATOMIC_VAR_INIT as an implementation-
defined value, so long as this works:

	atomic<int> v = ATOMIC_VAR_INIT(1);

I then pointed out that libc++'s own std::atomic has the deleted copy 
constructor and assignment operator, as required, and their ATOMIC_VAR_INIT is 
using brace initialisation.

If you're right and brace initialisation is not permitted, then they'll have 
to fix ATOMIC_VAR_INIT in their implementation. But given that their 
implementation as well as GCC's uses brace initialisation to call the int 
constructor, it seems to me it should be permitted.

An alternative I can see is to add an intermediate type such as

template <typename T> struct atomic_initializer
    constexpr atomic_initializer(T t) : value(t) {}
    T value;
template <typename T> 
atomic_initializer<T> atomic_initializer_helper(T v)
{ return atomic_initializer<T>(v); }

And add a constructor that does allow copying from this type:
    template <typename X>
    constexpr atomic(atomic_initializer<X> i) noexcept;
    template <typename X>
    T operator=(atomic_initializer<T> i) noexcept;

And finally change the macro to use that:
#define ATOMIC_VAR_INIT(v)  (atomic_initializer_helper(v))

However, I'm not sure that we wouldn't end up at exactly the same problem 
we're at right now due to the deleted copy constructor.
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
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