从工厂功能就地初始化不可复制的成员（或其他对象） [英] initializing a non-copyable member (or other object) in-place from a factory function
问题描述
一个类必须具有有效的副本或移动构造函数，此语法中的任何一种才合法：
A class must have a valid copy or move constructor for any of this syntax to be legal:
C x = factory();
C y( factory() );
C z{ factory() };
在C ++ 03中，很常见的情况是依靠复制省略来防止编译器接触到复制构造函数。每个类都有一个有效的副本构造函数 signature ，无论是否存在定义。
In C++03 it was fairly common to rely on copy elision to prevent the compiler from touching the copy constructor. Every class has a valid copy constructor signature regardless of whether a definition exists.
在C ++ 11中，不可复制类型应定义 C（C const&）= delete; ，使对该函数的任何引用均无效，无论其使用如何（与不可移动相同）。 （C ++ 11§8.4.3/ 2）。例如，GCC在尝试按值返回这样的对象时会抱怨。复制省略不再提供帮助。
In C++11 a non-copyable type should define C( C const & ) = delete;, rendering any reference to the function invalid regardless of use (same for non-moveable). (C++11 §8.4.3/2). GCC, for one, will complain when trying to return such an object by value. Copy elision ceases to help.
幸运的是，我们还有新的语法来表达意图，而不是依靠漏洞。 factory 函数可以返回 braised-init-list 来临时构造结果：
Fortunately, we also have new syntax to express intent instead of relying on a loophole. The factory function can return a braced-init-list to construct the result temporary in-place:
C factory() {
return { arg1, 2, "arg3" }; // calls C::C( whatever ), no copy
}
编辑：如有疑问，该 return 语句的解析如下：
If there's any doubt, this return statement is parsed as follows:
- 6.6.3 / 2：带有括号初始化列表的return语句通过复制列表初始化来初始化要从函数返回的对象或引用（ 8.5.4）从指定的初始化程序列表中进行。
- 8.5.4 / 1：在复制初始化上下文中的列表初始化称为复制列表初始化。 ¶3：如果T是类类型，则考虑构造函数。枚举适用的构造函数，并通过重载分辨率（13.3、13.3.1.7）选择最佳的构造函数。
不要被名称 copy-list-initialization 所误导。 8.5：
Do not be misled by the name copy-list-initialization. 8.5:
13：初始化形式（使用括号或
=）为通常无关紧要，但是当
初始化程序或要初始化的实体具有类类型时，它确实很重要；见下文。如果要初始化的实体不是
具有类类型，则带括号的初始化器中的expression-list应该是单个表达式。
13: The form of initialization (using parentheses or
=) is generally insignificant, but does matter when the initializer or the entity being initialized has a class type; see below. If the entity being initialized does not have class type, the expression-list in a parenthesized initializer shall be a single expression.
14：发生的初始化形式为
T x = a;
以及参数传递，函数返回，抛出异常（15.1），处理异常（ 15.3），并且聚合成员初始化（8.5.1）称为复制初始化。
14: The initialization that occurs in the form
T x = a;
as well as in argument passing, function return, throwing an exception (15.1), handling an exception (15.3), and aggregate member initialization (8.5.1) is called copy-initialization.
两个副本-initialization及其替代方法 direct-initialization 在初始化程序为大括号init-list时始终遵循列表初始化。添加 = 没有语义影响，这是列表初始化被非正式地称为统一初始化的原因之一。
Both copy-initialization and its alternative, direct-initialization, always defer to list-initialization when the initializer is a braced-init-list. There is no semantic effect in adding the =, which is one reason list-initialization is informally called uniform initialization.
有所不同：直接初始化可能会调用显式构造函数，这与复制初始化不同。复制初始化会初始化一个临时文件，并在转换时将其复制以初始化对象。
There are differences: direct-initialization may invoke an explicit constructor, unlike copy-initialization. Copy-initialization initializes a temporary and copies it to initialize the object, when converting.
复制列表初始化的规范为 return {list} 语句仅将确切的等效语法指定为 temp T = {list}; ，其中 = 表示复制初始化。这并不意味着立即调用了副本构造函数。
The specification of copy-list-initialization for return { list } statements merely specifies the exact equivalent syntax to be temp T = { list };, where = denotes copy-initialization. It does not immediately imply that a copy constructor is invoked.
-结束编辑。
然后可以将函数结果接收到右值引用中，以防止将临时副本复制到本地：
The function result can then be received into an rvalue reference to prevent copying the temporary to a local:
C && x = factory(); // applies to other initialization syntax
问题是，如何从返回不可复制，不可移动类型的工厂函数初始化非静态成员？参考技巧不起作用，因为参考成员不会延长临时人员的寿命。
The question is, how to initialize a nonstatic member from a factory function returning non-copyable, non-moveable type? The reference trick doesn't work because a reference member doesn't extend the lifetime of a temporary.
注意，我不考虑聚合初始化。
Note, I'm not considering aggregate-initialization. This is about defining a constructor.
推荐答案
关于您的主要问题：
问题是，如何从返回非可复制，不可移动类型的工厂函数初始化非静态成员？
The question is, how to initialize a nonstatic member from a factory function returning non-copyable, non-moveable type?
您不知道。
您的问题是您试图混淆两件事：返回值的生成方式和返回值的生成方式在呼叫站点上已使用。这两件事没有相互联系。请记住：函数的定义不会影响使用方式（就语言而言），因为编译器不一定可以使用该定义。因此，C ++不允许使用返回值的生成方式来影响任何东西（省略之外，这是一种优化，而不是语言要求）。
Your problem is that you are trying to conflate two things: how the return value is generated and how the return value is used at the call site. These two things don't connect to each other. Remember: the definition of a function cannot affect how it is used (in terms of language), since that definition is not necessarily available to the compiler. Therefore, C++ does not allow the way the return value was generated to affect anything (outside of elision, which is an optimization, not a language requirement).
换句话说：
C c = {...};
与此不同：
C c = [&]() -> C {return {...};}()
您有一个返回类型的函数值。它返回类型为 C 的prvalue表达式。如果要存储此值并为其命名，则有两种选择：
You have a function which returns a type by value. It is returning a prvalue expression of type C. If you want to store this value, thus giving it a name, you have exactly two options:
-
将其存储为
const&或&。这会将临时设备的寿命延长到控制块的寿命。您不能使用成员变量来做到这一点；
Store it as a
const&or&&. This will extend the lifetime of the temporary to the lifetime of the control block. You can't do that with member variables; it can only be done with automatic variables in functions.
将其复制/移动到一个值中。您可以使用成员变量来执行此操作，但是显然它要求类型是可复制或可移动的。
Copy/move it into a value. You can do this with a member variable, but it obviously requires the type to be copyable or moveable.
这些是如果要存储prvalue表达式，则C ++唯一可用的选项。因此，您可以使该类型可移动，也可以将一个刚分配的指针返回到内存并存储它而不是值。
These are the only options C++ makes available to you if you want to store a prvalue expression. So you can either make the type moveable or return a freshly allocated pointer to memory and store that instead of a value.
此限制是移动原因的很大一部分最初是为了创造：能够按价值传递事物并避免昂贵的复制品。无法更改语言以强制省略返回值。因此，他们在许多情况下都降低了成本。
This limitation is a big part of the reason why moving was created in the first place: to be able to pass things by value and avoid expensive copies. The language couldn't be changed to force elision of return values. So instead, they reduced the cost in many cases.
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