C++ runtime error?

Question

Point V[rows];

Is this allowed in C++? rows is a variable whose value is given at runtime and Point is my class.

strictly speaking, this would be a compile time error, not a run-time error. — SingleNegationElimination, Dec 06 '10 at 07:08

score 4 · Answer 1 · answered Dec 06 '10 at 05:53

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In C++ the comparable idiom is:

std::vector<Point> V(rows);

It's not 100% identical, because it still calls new Point[] (c99 can use the stack), but it still gives you the vector without performing multiple allocs.

answered Dec 06 '10 at 05:53

SingleNegationElimination

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score 0 · Accepted Answer · edited May 23 '17 at 12:26

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Only in C99 - it's a new feature called "variable length arrays". Normally, no.

I would strongly recommend against using this feature. If you have to do it, either use alloca, or allocate them properly, i.e. Point *V = new Point V[rows];.

BTW: Many people discourage Alloca as well. See here.

edited May 23 '17 at 12:26

Community

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answered Dec 06 '10 at 05:30

EboMike

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but then can i acces it as V[i] or not ? – abbas Dec 06 '10 at 05:31
Yes. A pointer works essentially like an array. You just have to delete the array when you're done. – EboMike Dec 06 '10 at 05:32
Eh, `Point *V = new Point[rows];`, obviously. – EboMike Dec 06 '10 at 05:38
Glad to help! By the way, if you accept this answer, we both get rep points. Good luck with your program! – EboMike Dec 06 '10 at 05:40
Thanks I just did that...i was tring it before thanking u but it said ur suppose to wait – abbas Dec 06 '10 at 05:43
Assigning the result of a new to a pointer is very bad idea (not exception safe and liable to be leaked by inexperienced users). – Martin York Dec 06 '10 at 08:34
Not a good idea to speak about alloca to people who asks such questions. – Sergey Miryanov Dec 07 '10 at 08:46

C++ runtime error?

2 Answers2