I'm trying to initialise a jagged array. Why doesn't this work:
int *arr[10]; for (int *row : arr) row = new int[10]; it runs but then arr is unreadable.
How can I use this form of "for" properly in this situation?
Otherwise how can I just initialise a jagged array of X rows, each row is Y elements and set all to zero?
Try:
int* arr[10]; for (int*& row : arr) row = new int[10]; Since you are changing the value in the array inside of the loop you need to iterate over references instead of values (which are only copies of what was in the array). This is very similar to function call semantics.
int &* row IIRC is the illegal and nonsensical pointer to reference to int.
(auto& row : arr) instead
int &*).for(auto&row:arr) // note the &. also note the auto row = new int[10]; the compilers should really warn about this stupid error. Happened to me many times.
The provided answer by filmor is correct, but as others have already stated in the comments: If you really want to learn C++11 you should use a container like std::vector.
For example:
std::vector<std::vector<int>> list; And you're done. You can add as many ints as you like. If you want to have a fixed size list of dynamic int-lists, consider using std::array:
std::array<std::vector<int>, 10> arr; Although I would always recommend using std::vector if performance or memory is not an issue. You have to make sure you're not exceeding the maximum number of elements anyway.
Concerning the for-loop, I would always try to use it this way:
for (auto &item : list) If you don't want to modify the list, add const:
for (const auto &item : list) Even if you don't want to modify the list, you're not making copies as you progress through it.
auto const&, because const is best placed on the right-hand side for consistency's sake (it can only be placed on the left-hand side for the outermost level which is awfully confusing)
void foo(const T&). If T is int, then you have a const int& which is a const-reference to an int and everything is well. If T is int*, then you have a const int*& which is a const-reference to an int*, that is the pointer is const, not int, even though const is closer to int and you read const, int, pointer...T const& and substitute int* for T. You then have int* const&, same type as const int*&, put it reads (from right to left) const-reference to pointer to int, which is exactly what it is. Of course, for an experienced developer it's just a quirk to be accounted for, but for a newcomer to the language, the inconsistency is utterly baffling.
std::vector.