Trying to store complex array into a class c++

Question

I am not very good with c++ and was wondering why an error happens with arrays inside of arrays, but works fine with a normal array (and how to fix it possibly).

Heres the code (just trying to store an array inside a class upon constructing it):

class foo {
    int* stored;
public:
    foo(int* ptr) {
        stored = ptr;
    }
};

int main() {
    int arr[][2] = {{1,4},{5,7},{2,5}};
    foo obj(arr);
}

When the array is something like int arr[] = {1,2,3} it works without errors, but the normal code gives me this error:

error: no matching function for call to 'foo::foo(int [3][2])'

I searched around for a while, but I don't really know what to search for in the first place, which is why I am asking here (but I feel it has something to do with pointers). Thanks for any help.

@texasbruce An array of arrays is **not** the same as a pointer to a pointer. See e.g. [this answer of mine to see why](http://stackoverflow.com/a/18440456/440558). — Some programmer dude, Sep 22 '13 at 00:13

Some programmer dude · Accepted Answer · 2013-09-22T00:14:54.923

1

You try to pass an array of arrays of integers as a single integer pointer, which will not work. While it's true that an array decay to pointer when passed, an array of arrays can not be a single pointer, and not a pointer to pointer either.

It can however be used as an pointer to array:

class foo
{
    int (*stored)[2];

public:
    foo(int (*ptr)[2]) : stored(ptr) {}
};

The parentheses in the declaration is needed because otherwise it would be an array of pointers.

However, I would recommend against using raw arrays, and move to std::vector or std::array, which of course can be nested:

std::vector<std::array<int, 2>> arr = {
    {{ 1, 2 }},
    {{ 3, 4 }}
};

edited Sep 22 '13 at 00:14

answered Sep 22 '13 at 00:07

Some programmer dude

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Thanks, I'll look more into vectors and arrays from the std thing, I never really knew anything about them. – Lemon Drop Sep 22 '13 at 00:15
@lemondrop I've updated my answer with links to reference pages for the standard containers. – Some programmer dude Sep 22 '13 at 00:16
@lemondrop: There is `std::complex` as well. – Siyuan Ren Sep 22 '13 at 00:21
@JoachimPileborg How would you pass a vector thing like that to a class? Do they work the same way as normal arrays? Edit: Apparently you can just do `foo(vector> ptr) : stored(ptr) {}`, so nevermind – Lemon Drop Sep 22 '13 at 01:06

score 1 · Answer 2 · answered Sep 22 '13 at 00:11

Firstly your code (even if it worked) would not store an array. In your class you have a pointer so all you are storing is a pointer to an array. It is important to get these distinctions clear in your head.

You have to declare a pointer that is compatible with the array you are trying to 'store'. That pointer is int (*stored)[2], that is a pointer to an array of two ints.

Its actually pretty simple

int* is compatible with int[]
int (*)[N] is compatible with int[][N]
int (*)[N][M] is compatible with int[][N][M]

etc. etc. But the parentheses do confuse people. They are necessary because the * operator has a lower priority than the [] operator.

Trying to store complex array into a class c++

2 Answers2