应该`unique_ptr< T const []&`接受一个`T *`构造函数参数？ [英] Should `unique_ptr< T const [] >` accept a `T*` constructor argument?

问题描述

代码：

#include <memory>
using namespace std;

struct T {};

T* foo() { return new T; }
T const* bar() { return foo(); }

int main()
{
    unique_ptr< T const >       p1( bar() );        // OK
    unique_ptr< T const [] >    a1( bar() );        // OK

    unique_ptr< T const >       p2( foo() );        // OK
    unique_ptr< T const [] >    a2( foo() );        // ? this is line #15
}

使用Visual C ++ 10.0和MinGW g ++的示例错误4.4。 1：

Example errors with Visual C++ 10.0 and MinGW g++ 4.4.1:

[d:\dev\test]
> cl foo.cpp
foo.cpp
foo.cpp(15) : error C2248: 'std::unique_ptr<_Ty>::unique_ptr' : cannot access private member declared in class 'std::unique_ptr<_Ty>'
        with
        [
            _Ty=const T []
        ]
        C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\INCLUDE\memory(2509) : see declaration of 'std::unique_ptr<_Ty>::unique_ptr'
        with
        [
            _Ty=const T []
        ]

[d:\dev\test]
> g++ foo.cpp -std=c++0x
c:\program files (x86)\codeblocks\mingw\bin\../lib/gcc/mingw32/4.4.1/include/c++/bits/unique_ptr.h: In function 'int main()':
c:\program files (x86)\codeblocks\mingw\bin\../lib/gcc/mingw32/4.4.1/include/c++/bits/unique_ptr.h:379: error: deleted function 'std::unique_ptr<_Tp [], _Tp_Deleter>::unique_ptr(_Up*, typename std::enable_if<std::is_convertible::value, void>::type*) [with _Up = T, _Tp = const T, _Tp_Deleter = std::default_delete<const T []>]'
foo.cpp:15: error: used here

[d:\dev\test]
> _

在我看来，数组版本应该接受与非数组版本相同的隐式const添加。

It seems to me that the array version should accept the same implicit const-adding as the non-array version.

不同的是数组版本不应该接受一个派生类的指针，这是显然在上面的机制。

The difference is that the array version should not accept pointer to a derived class, and that's the machinery that apparently kicks in above.

如果代码正式无效，标准的措辞是否反映意图（即，是否适用于DR）？

If the code is formally invalid, does the standard's wording reflect the intent (i.e., is a DR appropriate)?

如果第一个没有，第二个是yes，则意味着有缺陷（即，再次是DR是否适当）？

If no to the first and yes to the second, is the intent defective (i.e., again, is a DR appropriate)?

推荐答案

缺陷报告可能是适当的。 §20.7.1.3.1说，

A defect report may be appropriate. §20.7.1.3.1 says,

explicit unique_ptr(pointer p) noexcept;
unique_ptr(pointer p, see below d) noexcept;
unique_ptr(pointer p, see below d) noexcept;

这些构造函数的行为与主模板中的相同，不接受可转换为指针的指针类型。 [注意：一种实现技术是创建这些成员的专用模板重载。 - end note]

These constructors behave the same as in the primary template except that they do not accept pointer types which are convertible to pointer. [Note: One implementation technique is to create private templated overloads of these members. — end note ]

这个想法显然是为了防止不适用于数组的派生到基础转换。但它是非特定的，cv-qualification转换也被禁止。也许应该更改为禁止指针转换（§4.10），而不是所有的指针转换。

The idea is clearly to prevent derived-to-base conversions that don't work with arrays. But it is unspecific and cv-qualification conversion is forbidden too. Perhaps it should be changed to forbid pointer conversions (§4.10), not all conversions of pointers.

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