There is such code:
int tab[14][2];
int (*wskk)[2] = tab; // &tab makes error
int tab2[2];
wskk = &tab2; // tab2 makes error
Why is it possible to use one pointer to point at two arrays of different dimensions?
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Matteo Italia
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scdmb
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1What do you mean by *arrays of different type*? – Dennis Sep 30 '11 at 16:57
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tab is int[14][2], and tab2 is int[2] – scdmb Sep 30 '11 at 16:58
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I think he's referring to different dimensions – Robb Sep 30 '11 at 16:58
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1In C/C++, a pointer does not really point to an array; it points to an element inside the array. In this case, the first element. – Nemo Sep 30 '11 at 17:01
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1@Nemo: What about pointers to arrays? – K-ballo Sep 30 '11 at 17:03
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Why would wskk = &tab2 cause an error? Aren't they both of type int[2]? – Tom Sep 30 '11 at 17:03
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@Tom: I suppose he meant `tab2 makes error`. – Matteo Italia Sep 30 '11 at 17:09
2 Answers
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To understand what's going on you must be familiar with a few key-concepts:
- a multidimensional array is an array of arrays;
- the name of an array decays to a pointer to its first element;
- the type of
wskkis "pointer to an array of 2ints".
Thus, if you write tab you're getting a pointer to the first element of tab, which is its first row; the row has type int[2], so a pointer to it has type int (*)[2], which is exactly the type of your pointer. Because of this you can assign tab to wskk, which will now point to the first row of tab.
You can't assign &tab to it, because that yields you a pointer to the whole multidimensional array, which is of type int (*)[14][2].
As for the second piece, it's even simpler: tab2 is an array of two ints, so its type is int[2]. If you get a pointer to it via the & operator, you get a int (*)[2], which is the type of your pointer. Actually, it makes sense: tab2 and a row of tab are effectively the same stuff (an array of 2 ints).
You can't assign tab2 to it because tab2 decays to a pointer to its first element, i.e. an int *.
Matteo Italia
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Make the array-to-pointer conversion explicit, it may become more clear:
int tab[14][2];
int (*wskk)[2] = &tab[0]; // point at tab[0], which has type array of 2 int
int tab2[2];
wskk = &tab2; // point at tab2, which has type array of 2 int
See also: How do I use arrays in C++?
