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array initialization, is referencing a previous element ok?

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.

like image 321
smerlin Avatar asked Nov 06 '10 19:11

smerlin


1 Answers

C++03/C++11 answer


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.


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 ]

like image 136
Lightness Races in Orbit Avatar answered Oct 25 '22 16:10

Lightness Races in Orbit