英文:
How to call rvalue methods of members?
问题
我正在编写一个类（Interface），它封装了一个类（Impl），该类恰好具有一个右值引用函数（参见Impl::Close）。
class Impl
{
public:
/// Called from Dtor.
/// You can't reuse this after it has been closed
/// Therefore && added so need temporary / rvalue ref.
void Close(int why) && noexcept { /*...*/ }
};
class Interface
{
public:
void Close(int why) && noexcept { _i.Close(why); }
private:
Impl _i;
};
Lintian 给了我以下错误：
> error: 'this' argument to member function 'Close' is an lvalue, but function has rvalue ref-qualifier
g++ 给了我以下错误：
main.cpp: 在成员函数‘void Interface::Close(int) &&’中：
main.cpp:15:51: error: passing ‘Impl’ as ‘this’ argument discards qualifiers [-fpermissive]
15 | void Close(int why) && noexcept { _i.Close(why); }
| ^
main.cpp:4:10: note: in call to ‘void Impl::Close(int) &&’
7 | void Close(int why) && noexcept { }
|
我不太理解这个反馈。我明白在调用Close后对象不再存在，所以我不确定为什么Interface和Impl中都不适用&&。
英文:
I'm writing a class (Interface), which encapsulates a class (Impl) which happens to have an rvalue function (see Impl::Close).
class Impl
{
public:
/// Called from Dtor.
/// You can't reuse this after it has been closed
/// Therefore && added so need temporary / rvalue ref.
void Close(int why) && noexcept { /*...*/ }
};
class Interface
{
public:
void Close(int why) && noexcept { _i.Close(why); }
private:
Impl _i;
};
Lintian gives me:
> error: 'this' argument to member function 'Close' is an lvalue, but function has rvalue ref-qualifier
g++ gives me:
main.cpp: In member function ‘void Interface::Close(int) &&’:
main.cpp:15:51: error: passing ‘Impl’ as ‘this’ argument discards qualifiers [-fpermissive]
15 | void Close(int why) && noexcept { _i.Close(why); }
| ^
main.cpp:4:10: note: in call to ‘void Impl::Close(int) &&’
7 | void Close(int why) && noexcept { }
|
I don't really understand this feedback. I get that the object doesn't really exist after calling Close so I'm not sure why the && doesn't make sense in both Interface and the Impl.
答案1
得分: 5
_i 本身仍然是一个 lvalue，因此需要将其转换为 rvalue，以调用其 rvalue 限定的成员函数
void Close(int why) && noexcept { std::move(_i).Close(why); }
英文:
_i itself is still an lvalue, so it needs to be converted to an rvalue to call its rvalue-qualified member function
void Close(int why) && noexcept { std::move(_i).Close(why); }
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