并非所有类型的成员都初始化时，可以复制普通类型的类吗? [英] Can a trivial type class be copied when not all its members are initialized?

问题描述

(我刚刚意识到我首先需要通过复制联合来解决一个更基本的问题:

(I just realized I first need to solve a much more basic issue with copying unions: When a union object is copied, is a member subobject created?. Please see that other question first.)

类的隐式生成的复制操作(构造函数和赋值)执行成员复制(初始化或赋值). (对于普通类型，它们是相同的.)

The implicitly generated copy operations (constructor and assignment) of a class perform member by member copy (initialization or assignment). (For a trivial type these are the same.)

由于某些未初始化成员的类是非法的，因此无法复制该类.

So a class with some members not initialized cannot be copied, as accessing uninitialized objects is illegal.

struct C {
  int m1, m2;
};

void f() {
  C c1, c2;

  c1.m1 = 1;
  c2 = c1; // not initialized
}

但是，即使工会包含类成员，但其中的某些成员未初始化(因为……根据定义，不是工会的两个成员都被初始化)，也可以始终复制该工会.

But a union can always be copied, even if it contains class members, some of which aren't initialized (because... by definition not two members of a unions are initialized).

这是否意味着复制具有未初始化成员的类的并集是合法的:

Does that mean that copying a union of a class with uninitialized members is legal:

union U {
  C m;
};

void g() {
  U u1, u2;
  u1.m.m1 = 1;
  u2 = u1;
}

，如果是这样，是否可以通过强制转换为此类联合来复制类?

and if so, can classes be copied by casting to such union?

void f2() {
  C c1, c2;

  c1.m1 = 1;
  (U&)c2 = (U&)c1; // not initialized?
}

推荐答案

是的，您可以通过定义为复制对象表示形式的默认复制/移动赋值运算符来复制具有未初始化(间接)成员的联合. (您也可以编写自己的使用std::memcpy的运算符.如果默认的运算符未建立正确的活动成员，显然这是措辞上的缺陷，并且std::memcpy也应该这样做.)

Yes, you can copy the union with the uninitialized (indirect) member—via the default copy/move assignment operator that is defined to copy the object representation. (You could also write your own operator that used std::memcpy. It is obviously a wording defect if the defaulted operator doesn’t establish the correct active member, and is std::memcpy should do so as well.)

但是，您不能将强制转换为普通类对象的更安全副本.根据特殊规则，对赋值运算符的调用具有未定义的行为必需，因为它不涉及对任何(标量)对象的访问.即使通过reinterpret_cast(！)，您自己的基于std::memcpy的实现也可以，但这不具有新闻价值-通过unsigned char(可能是char)和std::byte读取/复制不确定的值是始终允许).

However, you cannot use cast-to-union as a safer copy for normal class objects; the call to the assignment operator has undefined behavior per a special rule needed because it doesn’t involve an access to any (scalar) object. Your own std::memcpy-based implementation would be fine, even via reinterpret_cast(!), but that’s not newsworthy—reading/copying indeterminate values via unsigned char (and perhaps char) and std::byte is always allowed).

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