Range based for-loop on array passed to non-main function

Question

When I try to compile the following code in gcc 4.8.2, I get the following error:

test.cc: In function ‘void foo(int*)’:
test.cc:15:16: error: no matching function for call to ‘begin(int*&)’
   for (int i : bar) {
                ^

Along with a bunch of others from deeper in the template library.

#include <iostream>
using namespace std;

void foo(int*);

int main() {
  int bar[3] = {1,2,3};
  for (int i : bar) {
    cout << i << endl;
  }
  foo(bar);
}

void foo(int* bar) {
  for (int i : bar) {
    cout << i << endl;
  }
}

If I redefine foo to use an indexed for loop, then the code compiles and behaves as expected. Also, if I move the range-based output loop into main, I get the expected behaviour as well.

How do I pass the array bar to foo in such a way that it is capable of executing a range-based for-loop on it?

Community · Accepted Answer · 2017-05-23 12:24:35Z

23

With the array decaying into a pointer you're losing one important piece of information: its size.

With an array reference your range based loop works:

void foo(int (&bar)[3]);

int main() {
  int bar[3] = {1,2,3};
  for (int i : bar) {
    cout << i << endl;
  }
  foo(bar);
}

void foo(int (&bar)[3]) {
  for (int i : bar) {
    cout << i << endl;
  }
}

or, in a generic fashion (i.e. without specifying the array size in the function signature),

template <std::size_t array_size>
void foo(int (&bar)[array_size]) {
  for (int i : bar) {
    cout << i << endl;
  }
}

Try it out

edited May 23, 2017 at 12:24

CommunityBot

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answered Oct 3, 2014 at 16:30

Marco A.

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Comments

Cheers and hth. - Alf · Accepted Answer · 2014-10-03 16:37:40Z

5

For a fixed size array you can

Pass a raw array by reference.
Pass a std::array by reference.
Pass a std::vector by reference.

The natural choice (for a fixed size array) is std::array, i.e.

#include <iostream>
#include <array>
using namespace std;

void foo(array<int, 3> const& bar) {
  for (int i : bar) {
    cout << i << endl;
  }
}

int main() {
  array<int,3> const bar = {1,2,3};
  for (int i : bar) {
    cout << i << endl;
  }
  foo(bar);
}

answered Oct 3, 2014 at 16:37

Cheers and hth. - Alf

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Comments

Guillaume Racicot · Accepted Answer · 2021-03-13 20:15:20Z

2

Using C++20's std::span, you can have a reference to an array or runtime size.

Here's your code with std::span instead:

#include <iostream>
#include <span>

void foo(std::span<int>);

int main() {
  int bar[3] = {1,2,3};
  for (int i : bar) {
    std::cout << i << std::endl;
  }
  foo(bar);
}

void foo(std::span<int> bar) {
  for (int i : bar) { // now works
    std::cout << i << std::endl;
  }
}

answered Mar 13, 2021 at 20:15

Guillaume Racicot

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