“仅可移动类型”的问题在VC ++ 2010 [英] Problem with "moveable-only types" in VC++ 2010

问题描述

我最近安装了Visual Studio 2010 Professional RC，试用它并测试在VC ++ 2010中实现的几个C ++ 0x功能。

I recently installed Visual Studio 2010 Professional RC to try it out and test the few C++0x features that are implemented in VC++ 2010.

code> std :: vector 的 std :: unique_ptr ，没有任何问题。然而，当我尝试通过传递临时到 push_back 填充它，编译器抱怨 unique_ptr 的复制构造函数是私人的。我尝试通过移动它插入一个左值，它的工作正常。

I instantiated a std::vector of std::unique_ptr, without any problems. However, when I try to populate it by passing temporaries to push_back, the compiler complains that the copy constructor of unique_ptr is private. I tried inserting an lvalue by moving it, and it works just fine.

#include <utility>
#include <vector>

int main()
{
    typedef std::unique_ptr<int> int_ptr;

    int_ptr pi(new int(1));

    std::vector<int_ptr> vec;

    vec.push_back(std::move(pi));      // OK
    vec.push_back(int_ptr(new int(2))); // compiler error
}

结果是，问题不是 unique_ptr 也不是 vector :: push_back ，但是VC ++在处理右值时解决重载的方式，如下面的代码所示：

As it turns out, the problem is neither unique_ptr nor vector::push_back but the way VC++ resolves overloads when dealing with rvalues, as demonstrated by the following code:

struct MoveOnly
{
    MoveOnly() {}
    MoveOnly(MoveOnly && other) {}

private:

    MoveOnly(const MoveOnly & other);
};

void acceptRValue(MoveOnly && mo) {}

int main()
{
    acceptRValue(MoveOnly()); // Compiler error
}

编译器声称拷贝构造函数不可访问。如果我把它公开，程序编译（即使没有定义复制构造函数）。

The compiler complains that the copy constructor is not accessible. If I make it public, the program compiles (even though the copy constructor is not defined).

我误解了一些关于右值引用的东西，或者是一个）bug在VC ++ 2010中实现这个特性？

Did I misunderstand something about rvalue references, or is it a (possibly known) bug in VC++ 2010 implementation of this feature?

推荐答案

当它不应该（绑定值引用不调用拷贝构造函数，甚至理论上）时，它执行elided-copy-constructor-accessibility检查。因此，不应使用/ Za。

Unfortunately, /Za is buggy. It performs an elided-copy-constructor-accessibility check when it shouldn't (binding rvalue references doesn't invoke copy constructors, even theoretically). As a result, /Za should not be used.

Stephan T. Lavavej，Visual C ++库开发人员（[e-mail]）

Stephan T. Lavavej, Visual C++ Libraries Developer ([e-mail])

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