C++11 range-for loop over multidimensional array

Question

I'm jumping into C++11 from C(ANSI). It's a strange world indeed.

I'm trying to come to grips with the following:

int tbl[NUM_ROWS][NUM_COLS] = { 0 };
for (auto row : tbl)
  for (auto col : row) // ERROR - can't iterate over col (type int *)
    // do stuff

The rational here, I'm guessing, is equivalent to the difference between (in C):

int v[] = { 1, 2, 3, 4, 5 };
int *u = v;
// (sizeof v) != (sizeof u);

However, I don't quite see how the following works:

int tbl[NUM_ROWS][NUM_COLS] = { 0 };
for (auto &row : tbl) // note the reference
  for (auto col : row)
    // do stuff

Logically, I'm thinking auto gets typed to int * const - thats what an "array" variable is, a constant pointer to a (possibly) non-constant element. But how is this any more iteratable than a normal pointer? Or, since row is a reference, it's actually typed to int [NUM_COLS], just as if we declared row as int row[NUM_COLS]?

Kerrek SB · Accepted Answer · 2014-09-20 23:24:04Z

11

There are no "multi-dimensional arrays" in C++. There are only arrays. However, the element type of an array can itself be an array.

When you say for (auto x : y), then x is a copy of the range element. However, arrays cannot be copied, so that is not valid when y is an array-valued array. By contrast, it is perfectly fine to form a reference to an array, which is why for (auto & x : y) works.

Maybe it helps to spell out the types:

int a[10][5];  // a is an array of 10 elements of int[5]

for (int (&row)[5] : a)
    for (int & cell : row)
        cell *= 2;

answered Sep 20, 2014 at 23:24

Kerrek SB

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3 Comments

zac Over a year ago

Ya I know there's no multidimensional arrays - its just easier than saying arrays of arrays of ....In anycase, you are correct, arrays cannot be copied - attempts at assignment just copies the address of the first element. OK so, this is a fundamental difference on how a reference isnt just a constant pointer (even if they are internally implemented as such); a reference of an array acts like an array, and not just a reference of a pointer to an element

Kerrek SB Over a year ago

@user70869: Arrays are not pointers. Arrays are arrays. (And they're definitely not "constant pointers". You might be thinking of prvalues.)

zac Over a year ago

Ya, arrays certainly arn't pointers. I was saying references weren't just like constant pointers (top-level const).

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