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

Thiago Macieira thiago.macieira at intel.com
Mon Dec 3 20:42:06 CET 2012

On segunda-feira, 3 de dezembro de 2012 15.37.13, Francisco Lopes wrote:
> Please take a look at my last comment on the bug report (
> http://llvm.org/bugs/show_bug.cgi?id=14486#c10). If my analysis is right,
> it's wrong to use "= {0}" with an inaccessible copy constructor.

Looking at N3337, this is the API required for std::atomic:

atomic() noexcept = default;
constexpr atomic(T) noexcept;
atomic(const atomic&) = delete;
atomic& operator=(const atomic&) = delete;
atomic& operator=(const atomic&) volatile = delete;
T operator=(T) volatile noexcept;
T operator=(T) noexcept;

It also defines ATOMIC_VAR_INIT and in 29.6.5 [atomics.types.operations.req] 
paragraph 6 we find this example:

atomic<int> v = ATOMIC_VAR_INIT(5);

The standard does not define what ATOMIC_VAR_INIT is. GCC defines it as:

#define ATOMIC_VAR_INIT(_VI) { _VI }

And clang defines it (see [1]) as:
#define ATOMIC_VAR_INIT(__v) {__v}

With constructors:
    atomic() _NOEXCEPT {} // = default;
    _LIBCPP_CONSTEXPR atomic(_Tp __d) _NOEXCEPT : __base(__d) {}

    _Tp operator=(_Tp __d) volatile _NOEXCEPT
        {__base::store(__d); return __d;}
    _Tp operator=(_Tp __d) _NOEXCEPT
        {__base::store(__d); return __d;}

    __atomic_base(const __atomic_base&) = delete;
    __atomic_base& operator=(const __atomic_base&) = delete;
    __atomic_base& operator=(const __atomic_base&) volatile = delete;

In other words, libc++'s own std::atomic is supposed to be initialised by 
braces with a deleted copy constructor and assignment operator. It seems to me 
that it's a compiler bug.

[1] http://llvm.org/svn/llvm-project/libcxx/trunk/include/atomic
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
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