为什么`T（const T&&&&;）`称为移动构造函数？ [英] Why is `T(const T&&)` called a move constructor?

问题描述

[C ++ 11：12.8 / 3]： code>是一个move构造函数，如果它的第一个参数是 typeX&&& ， const X&&< / code >， volatile X&&& 或 const volatile X&&< / code>，并且没有其他参数否则所有其他参数都有默认参数（8.3.6）。 [..]

为什么是一个 const rvalue引用称为移动构造函数的标准？当然，是不言而喻的这个禁止有意义的移动语义，除了大多数fringey案件？

根据我，正如SO说，由于 ，所以不应将T（const T&&）视为移动构造函数

如果有任何，

解决方案

这里有一些移动构造函数和其他构造函数之间的区别：

• 移动构造函数可以是默认的


• 移动构造函数不会阻止类型成为


• 移动构造函数阻止隐式移动构造函数的生成



• 移动构造函数可以通过标准库函数自动调用

对于所有这些，不调用 X（const X&&）一个移动构造函数会产生不良结果。

你给一个替代：它可能被称为复制构造函数。这似乎没有不良的结果：它会抑制隐式复制构造函数。

移动构造函数是否移动实际上并不重要。 POD类型也可以有移动构造函数。它只是做一个副本，但它仍然称为移动构造函数。

[C++11: 12.8/3]: A non-template constructor for class X is a move constructor if its first parameter is of typeX&&, const X&&, volatile X&&, or const volatile X&&, and either there are no other parameters or else all other parameters have default arguments (8.3.6). [..]

Why is a constructor that takes a const rvalue reference called a "move constructor" by the standard? Surely it's self-evident that this prohibits meaningful move semantics in all but the most fringey cases?

"According to me", as the SO saying goes, T(const T&&) shouldn't be deemed a "move constructor" as such, since it's basically useless.

If anything, shouldn't it be called a copy constructor?

解决方案

Here are some of the differences between move constructors and other constructors:

• Move constructors can be defaulted
• Move constructors don't prevent a type from being a "literal type"
• Non-trivial move constructors prevent a type from being a "trivially copyable type"
• Move constructors prevent the implicit move constructor from being generated
• Move constructors may be automatically called by standard library functions

As far as I can tell, for all of those, not calling X(const X &&) a move constructor gives undesirable results.

You give an alternative: it might be called a copy constructor instead. That too seems to have undesirable results: it would suppress the implicit copy constructor.

Whether a move constructor actually moves doesn't matter. A POD type may have a move constructor too. It'll just be making a copy, but it's still called a move constructor.

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