Iterating with foreach through multidimensional arrays

Question

When iterating through a multidimensional array like so:

int arr[2][2] = {{6, 7}, {8, 3}, {5, 2}};
for (auto &row : arr) {
    for (auto &cell : row) {
        // code
    }
}

What is the type of row and cell and why must you use a reference?

jrok · Accepted Answer · 2013-09-09 20:05:10Z

3

When you don't use a reference array-to-pointer conversion kicks in and row is of type pointer to an array of two ints (int(*)[2]). The inner loop is then ill-formed because for it to work you need either:

a type that has begin and end member functions that return iterators,
begin and end free functions that are found by argument dependent lookup and return iterators or
for array types, x and x + bound are used for begin and end, respectively, where x is the range and bound is the array bound.

int(*)[2] doesn't fullfile these conditons. OTOH, when you use auto&, you get int(&)[2] and the third bullet applies.

P.S. You've got too many initializers for int[2][2].

answered Sep 9, 2013 at 20:05

jrok

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1 Comment

zhenguoli Over a year ago

Is the type of row int * or int(*)[2]? According to the book C++ primer, it says that the type of row is int *. Please refer to Using a Range for with Multidimensional Arrays

Jess · Accepted Answer · 2013-09-09 19:52:57Z

0

row is an array of ints. cell is a reference to an int. You need a reference if you want to modify the contents of cell, because otherwise the loop variable will be a copy.

answered Sep 9, 2013 at 19:52

Jess

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