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c++ - 您是否应该能够从 T 具有非平凡构造函数的 std::optional 移动?

转载 作者:可可西里 更新时间:2023-11-01 15:22:44 25 4
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我正在尝试用 clang 编译 WebKit,我正在点击 compile errors由于本质上是以下模式:

#include <iostream>
#include <optional>

struct X {
X() = default;
X(const X& other) { }
};

struct Y {
std::optional<X> x;;
};

int main() {
Y foo;
Y bar(std::move(foo));
}

所以,他们使用 std::optional<T>其中 T (在他们的情况下, WTF::Variant )具有非平凡的复制/移动构造函数,然后使用 std::optional移动构造函数。使用 GCC 8.1.1 可以很好地编译,但不能使用 clang 6.0.1(使用 GCC 8.1.1 的 libstdc++):

In file included from test.cpp:2:
/bin/../lib64/gcc/x86_64-pc-linux-gnu/8.1.1/../../../../include/c++/8.1.1/optional:276:9: error: call to implicitly-deleted copy constructor of 'std::_Optional_payload<X, true, true, true>'
: _Optional_payload(__engaged
^ ~~~~~~~~~
/bin/../lib64/gcc/x86_64-pc-linux-gnu/8.1.1/../../../../include/c++/8.1.1/optional:739:4: note: in instantiation of member function 'std::_Optional_payload<X, true, true, true>::_Optional_payload' requested here
: _M_payload(__other._M_payload._M_engaged,
^
/bin/../lib64/gcc/x86_64-pc-linux-gnu/8.1.1/../../../../include/c++/8.1.1/optional:985:11: note: in instantiation of member function 'std::_Optional_base<X, false, false>::_Optional_base' requested here
class optional
^
test.cpp:9:8: note: in implicit move constructor for 'std::optional<X>' first required here
struct Y {
^
test.cpp:15:7: note: in implicit move constructor for 'Y' first required here
Y bar(std::move(foo));
^
/bin/../lib64/gcc/x86_64-pc-linux-gnu/8.1.1/../../../../include/c++/8.1.1/optional:288:24: note: copy constructor of '_Optional_payload<X, true, true, true>' is implicitly deleted because variant field '_M_payload' has a
non-trivial copy constructor
_Stored_type _M_payload;

这是有效的 C++,还是 WebKit 损坏并且 clang 正确地拒绝了这个代码?

最佳答案

考虑这个类:

struct X
{
X(int);
X(X&&) = delete;

// does this need to invoke the move constructor??
X() : X(X(0)) { }
};

根据 gcc,答案是否定的:这直接委托(delegate)给 X(int)。根据 clang,答案是肯定的,并且编译失败:

<source>:55:15: error: call to deleted constructor of 'X'    
X() : X(X(0)) { }
^ ~~~~
<source>:52:9: note: 'X' has been explicitly marked deleted here
X(X&&) = delete;
^

这似乎是一个潜在的核心语言问题,因为一方面 [class.base.init]/6说:

The target constructor is selected by overload resolution. Once the target constructor returns, the body of the delegating constructor is executed.

也就是说,我们专门讨论了选择一个构造函数并调用它 - 在这种情况下肯定是 X(X&&)。但另一方面,the very next paragraph说这是初始化:

The expression-list or braced-init-list in a mem-initializer is used to initialize the designated subobject (or, in the case of a delegating constructor, the complete class object) according to the initialization rules of [dcl.init] for direct-initialization.

这看起来很像 [dcl.init]/17.6 中的保证复制省略示例:

If the initializer expression is a prvalue and the cv-unqualified version of the source type is the same class as the class of the destination, the initializer expression is used to initialize the destination object. [ Example: T x = T(T(T())); calls the T default constructor to initialize x. — end example ]

我不确定哪种解释是正确的,但 clang 拒绝对我来说似乎并没有明显的错误。


为什么这个例子是相关的? optional 在 libstdc++ 中的移动构造函数,对于可简单破坏和可简单复制/移动可赋值的类型(如您的 X)通过:this constructor :

  constexpr
_Optional_payload(bool __engaged, _Optional_payload&& __other)
: _Optional_payload(__engaged
? _Optional_payload(__ctor_tag<bool>{},
std::move(__other._M_payload))
: _Optional_payload(__ctor_tag<void>{}))
{ }

此类型已隐式删除了复制和移动构造函数,因为它与不可简单复制构造的成员具有 union 。因此,如果此委托(delegate)构造函数必须调用隐式复制构造函数(如 clang 认为的那样),则这是不正确的。如果不需要,只需调用一个或另一个委托(delegate)构造函数,那么此调用就可以了。

关于c++ - 您是否应该能够从 T 具有非平凡构造函数的 std::optional<T> 移动?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/51379597/

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