数组初始化，引用前一个元素可以吗？
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)?
};
虽然这是使用 MS Visual Studio 编译器进行编译的，但我不确定这是否是根据 C++ 标准的有效代码。
非常感谢来自该标准的报价。
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)?
};
While this compiles with the MS Visual Studio Compiler, i am not sure if this is valid code according to the C++ Standard.
Quotes from the Standard would be highly appreciated.
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C++03/C++11 答案
不，不是。
在
=的右侧，points确实存在1，但初始化程序仅在其所有操作数都被求值后才应用。如果
points位于命名空间范围内（因此具有静态存储持续时间并且已零初始化2），那么这是“安全的”，但您使用 < code>points[0] 将为您提供0，而不是QPointF(r.left() - i, r.top() - i) 再次。如果
points具有自动存储持续时间 - 它尚未初始化，因此您对points[0]的使用正在尝试使用未初始化的变量，其中point[0]具有不确定的值...这很糟糕3。很难为此提供标准参考，只能说
8.5“Initializers”中没有任何内容明确地使这成为可能，其他地方的规则填补了其余部分。C++03/C++11 answer
No, it's not.
On the right-hand side of the
=,pointsdoes exist1 but the initialiser is only applied after all its operands have been evaluated.If
pointsis at namespace scope (and thus has static storage duration and has been zero-initialized2), then this is "safe" but your use ofpoints[0]there is going to give you0, rather thanQPointF(r.left() - i, r.top() - i)again.If
pointshas automatic storage duration — it has not yet been initialised so your use ofpoints[0]is attempting to use an uninitialised variable, wherepoints[0]has an indeterminate value... which is bad3.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.来自 http://www.comeaucomputing.com/pcgi-bin/compiler.cgi：
from http://www.comeaucomputing.com/pcgi-bin/compiler.cgi:
由于语句中没有 序列点，因此结果是未定义的，就像
i=i++维基百科页面引用的示例。否则，没有任何规定，编译器是否应该首先评估所有内容，然后分配，或者分别以何种顺序对每个元素进行评估分配。
Since there is no sequence point in the statement, the result is undefined, much like for the
i=i++example quoted at that wikipedia page.Otherwise said, nothing specifies, whether the compiler should first evaluate everything, then assign, or do evaluate-assign for each element separately and in which order.
旧答案（没有抓住重点）：
我检查了当前的 C++0x 草案，在那里我找到了句子 8.5.1.17，其中写着：
因此，虽然这句话不是 2003 年 C++ 标准的一部分，但我非常确定，如果这是 C++0x 的一部分，它应该可以在任何最新的编译器中工作。
编辑：
评论让我重新思考这个问题。此行仅确保 QPointF 对象按照它们在数组初始化中出现的顺序创建（如果元素构造函数具有可观察到的副作用，则相关）。问题是，
points的值在数组初始化期间是不确定的。因此，也无法保证points[0]的有效值，至少在您依赖标准的情况下不能保证。Old Answer (misses the point):
I checked the current C++0x draft, and there i found the sentence 8.5.1.17 which says:
So while this sentence is not part of the C++ Standard from 2003, im quite sure that this should be working in any up to date compiler, if this is part of C++0x.
Edit:
The comments made me rethink this matter. This line only ensures that the
QPointFobjects are created in the order in which they occur in the array initialization (relevant if the element constructors have observable side-effects). The problem is, the value ofpointsis indeterminate during its array initialization. So there cant be a guarantee for a valid value ofpoints[0]either, at least not if you rely on the standard.