c++ - array initialization, is referencing a previous element ok?

Question

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.

Answer 1

_{C++03/C++11 answer}

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No, it's not.

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

• 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.

• 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.

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.

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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; }

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

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]: [..] [ Note: Operations involving indeterminate values may cause undefined behavior. —end note ]