Question

There are examples of sorting vectors or dynamically allocated arrays but I couldn’t find any help regarding static arrays. Let’s say I have an array

int array[10][10];

and a compare function,

bool compare(const int (*a)[10], const int (*b)[10]);

When I call it like this,

std::sort(array, array + 10, compare);

I have compilation errors: error: cannot convert 'int*' to 'const int (*)[10]' in argument passing

I tried many ways, casting array to (void**) in sort function but then I have segmentation fault. My problem is using arrays as function parameters I guess but I couldn’t figure out how to use this std::sort. Otherwise, I will have to write my own sort function.

Answer 1

When std::sort is called on a container of elements of type T, the comparison function needs to receive arguments of type T or const T&. In this case, you have a 2-dimensional array, so the type of elements is a 1-dimensional array int[10]. Since 1-dimensional arrays decay to pointers, compare can be:

bool compare(int a[10], int b[10]);

or equivalently:

bool compare(int *a, int *b);

This will fix the error you got, but your code still won’t work: std::sort needs the container elements to be assignable (or movable in C++11), but arrays are not assignable.

You can use std::vector<std::vector<int> > instead as people have suggested. Note that your fear of performance problems is misguided: Even if sorting a two-dimensional array was possible, it would involve a lot of copying of one-dimensional arrays which would take a long time. Swapping vectors, on the other hand, is done by simply swapping pointers which is faster. In general, you should not make assumptions about performance if you haven’t tested it first.