数组初始化时，引用previous元素好吗？ [英] array initialization, is referencing a previous element ok?

问题描述

const QPointF points[] =
{
    QPointF(r.left() - i, r.top() - i),
    QPointF(r.right() + i, r.top() - i),
    QPointF(r.right() + i, r.bottom() + i),
    QPointF(r.left() - i, r.bottom() + i),
    points[0] // is this line valid (according to the C++ standard)?
};

虽然这编译与MS Visual Studio的编译器，我不知道这是否是根据C ++标准有效code。

While this compiles with the MS Visual Studio Compiler, i am not sure if this is valid code according to the C++ Standard.

从标准行情将高AP preciated。

推荐答案

<子> C ++ 03 / C ++ 11的答案

没有，不是这样的。

在确实存在 ¹ 的 = ，点的右侧但其所有的操作数已被评估后初始化器只适用。

On the right-hand side of the =, points does exist¹ but the initialiser is only applied after all its operands have been evaluated.

• 如果点是在命名空间内（因此具有静态存储时间，并已初始化为零 ² ），那么这个是安全的，但你的点使用[0] 有要给你 0 ，而不是 QPointF（r.left（） - 我，r.top（） - I）。再次

• If points is at namespace scope (and thus has static storage duration and has been zero-initialized²), then this is "safe" but your use of points[0] there is going to give you 0, rather than QPointF(r.left() - i, r.top() - i) again.

如果点具有自动存储持续时间MDASH;它尚未初始化，所以你使用点[0] 正在尝试使用未初始化变量，其中点[0] 有一个不确定的值......这是坏 ³ 。

If points has automatic storage duration — it has not yet been initialised so your use of points[0] is attempting to use an uninitialised variable, where points[0] has an indeterminate value... which is bad³.

这很难提供这个参考标准，比其他地说，在没什么8.5的初始化，明确使这成为可能，并且规则其他填写其余部分。

It's difficult to provide standard references for this, other than to say that there is nothing in 8.5 "Initializers" that explicitly makes this possible, and rules elsewhere fill in the rest.

¹ [n3290：3.3.2 / 1]： 申报为名称的点是立即其完整的声明符后（第8条）和它的初始化之前（如果有的话），除如下所述。 [示例：的

¹ [n3290: 3.3.2/1]: The point of declaration for a name is immediately after its complete declarator (Clause 8) and before its initializer (if any), except as noted below. [ Example:

int x = 12;
{ int x = x; }

下面第二个 X 与自己的（不确定）值初始化。的末端例如的]

Here the second x is initialized with its own (indeterminate) value. —end example ]

² [n3290：3.6.2 / 2]：与静态存储时间（3.7.1）或线程存储持续时间的变量（3.7 0.2）应是零初始化（8.5）
  任何其它初始化之前发生。 [..]

² [n3290: 3.6.2/2]: Variables with static storage duration (3.7.1) or thread storage duration (3.7.2) shall be zero-initialized (8.5) before any other initialization takes place. [..]

³ [n3290：17.6.3.3/2]： [..] [注：的操作
  涉及不定值可能会导致不确定的行为。的末端笔记的]

³ [n3290: 17.6.3.3/2]: [..] [ Note: Operations involving indeterminate values may cause undefined behavior. —end note ]

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