Other than being able to dereference a void**, I don't understand the following:
void * foo, **bar;
foo++;//error
bar++;//no error
Why doesn't the first work but the second does? What's the difference?
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@juanchopanza Strictly speaking both of these post increments are UB. – Eugene Sh. Jul 08 '15 at 19:07
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1@juanchopanza `sizeof(void)` is undefined, so arithmetic on `void` pointers is prohibited. – Quentin Jul 08 '15 at 19:07
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@juanchopanza http://i.imgur.com/nnv2UGg.png – shinzou Jul 08 '15 at 19:07
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Actually not both. Only the second one. – Eugene Sh. Jul 08 '15 at 19:08
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possible duplicate of [Pointer arithmetic for void pointer in C](http://stackoverflow.com/questions/3523145/pointer-arithmetic-for-void-pointer-in-c) – dhke Jul 08 '15 at 19:08
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@Quentin: That is a gcc extension (behaves like `char *`) - the most useless imho. – too honest for this site Jul 08 '15 at 19:09
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@Quentin Yes, good point. I got fooled by gcc accepting this. – juanchopanza Jul 08 '15 at 19:09
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Nothing to really pick here, clang accepts it, too. However: `arithmetic on a pointer to void is a GNU extension [-Wpointer-arith]` if `-pendantic` – dhke Jul 08 '15 at 19:11
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Can you post a fully code example that we can actually compile and run? Your foo/bar statement doesn't make sense without context. – QuestionC Jul 08 '15 at 19:11
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@QuestionC I ask specifically about allocated space and pointer arithmetic, this is a bit more specific: http://i.imgur.com/CfqG09s.png – shinzou Jul 08 '15 at 19:14
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NOTE: `void *` is a generic pointer while `void **` is not. – haccks Jul 08 '15 at 19:25
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@haccks really? so why I can return a `void**` from a function that suppose to return `void*`? – shinzou Jul 08 '15 at 19:28
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One is a pointer to the other. – Martin James Jul 08 '15 at 21:43
1 Answers
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First snippet
foo++;//error
because, foo is pointer to void and you cannot have pointer arithmetic on void *, size of the void type is not defined.
Second snippet,
bar++;//no error
because, bar is a pointer to a pointer to void. So, arithmetic operation is permitted, as the size of a pointer to pointer type is well defined.
FWIW, don't get surprised if sometimes, void pointer arithmetic "works" without any error.
Sourav Ghosh
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It worth mentioning that pointer arithmetic resulting in addresses within an unallocated space is UB. So `bar++` is not safe as well. – Eugene Sh. Jul 08 '15 at 19:10
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@EugeneSh.Yes my question is specifically for allocated spaces, I tried to make my question as minimal as possible. – shinzou Jul 08 '15 at 19:12
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@EugeneSh., no arithmetic that goes one item beyond a valid object is allowed, so `bar++` is fine. It is only erroneous when you'd increment further away from a valid object or dereference the pointer. – Jens Gustedt Jul 08 '15 at 20:02