Sorting an array of int[2] does not compile - c++

Question

I have a data array with 2*N ints, representing pairs, that is for even i=0,2,4,...,2*N (pairs[i], pairs[i+1]) is such a pair. The data is formatted like this because I use Matlab's mex library. I do:
int N=5;
int data[10] = {1,2,3,4,5,6,7,8,9,10};
struct Pair { int first; int second; };
Pair * pairs = (Pair *)data;
but the problem would be that there is no way to guarantee that Pair aligns as two sizeof(ints) in order first, second. See: Is the member field order of a class "stable"?
I don't want to process and copy all data into a new array, since it should not be necessary, and I need (as far as I can see) to use
typedef int Pair[2];
to be sure that it aligns correctly (no trailing garbage bytes, etc). if I then want to sort the pairs according to the first element, I could do:
#include <iostream>
#include <algorithm>
typedef int Pair[2];
int compare(Pair n1, Pair n2) { return n1[0] < n2[0]; }
int main() {
int N=5;
int data[10] = {1,2, 7,8, 13,14, 4,5, 10,11};
Pair *pairs = (Pair *)((void *)data);
std::cout << "unsorted" << std::endl;
for(int i=0; i<N;++i) std::cout << i << ": (" << pairs[i][0] << ", " << pairs[i][1] << ")" << std::endl;
std::sort(data, data+N, compare);
std::cout << "sorted" << std::endl;
for(int i=0; i<N;++i) std::cout << i << ": (" << pairs[i][0] << ", " << pairs[i][1] << ")" << std::endl;
return 0;
}
see: http://ideone.com/VyBUvc
I could summarize the error message as error: array must be initialized with a brace-enclosed initializer, see below for the complete message. It is caused by the std::sort call.
I wrapped the Pair typedef in a union here ( http://ideone.com/TVmEeZ ), and that seems to work. Why does c++ (or std::sort) not see int[2] in a similar way as a union?
Complete compiler output:
In file included from /usr/include/c++/4.8/bits/stl_pair.h:59:0,
from /usr/include/c++/4.8/bits/stl_algobase.h:64,
from /usr/include/c++/4.8/bits/char_traits.h:39,
from /usr/include/c++/4.8/ios:40,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from prog.cpp:1:
/usr/include/c++/4.8/bits/stl_algo.h: In instantiation of ‘void std::__insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int (*)[2]; _Compare = bool (*)(int*, int*)]’:
/usr/include/c++/4.8/bits/stl_algo.h:2250:70: required from ‘void std::__final_insertion_sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int (*)[2]; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:5514:55: required from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = int (*)[2]; _Compare = bool (*)(int*, int*)]’
prog.cpp:16:35: required from here
/usr/include/c++/4.8/bits/stl_algo.h:2186:11: error: array must be initialized with a brace-enclosed initializer
__val = _GLIBCXX_MOVE(*__i);
^
In file included from /usr/include/c++/4.8/algorithm:62:0,
from prog.cpp:2:
/usr/include/c++/4.8/bits/stl_algo.h:2188:17: error: invalid array assignment
*__first = _GLIBCXX_MOVE(__val);
^
/usr/include/c++/4.8/bits/stl_algo.h: In instantiation of ‘_RandomAccessIterator std::__unguarded_partition(_RandomAccessIterator, _RandomAccessIterator, const _Tp&, _Compare) [with _RandomAccessIterator = int (*)[2]; _Tp = int [2]; _Compare = bool (*)(int*, int*)]’:
/usr/include/c++/4.8/bits/stl_algo.h:2319:78: required from ‘_RandomAccessIterator std::__unguarded_partition_pivot(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int (*)[2]; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:2360:62: required from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = int (*)[2]; _Size = int; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:5513:44: required from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = int (*)[2]; _Compare = bool (*)(int*, int*)]’
prog.cpp:16:35: required from here
/usr/include/c++/4.8/bits/stl_algo.h:2287:35: error: invalid conversion from ‘const int*’ to ‘int*’ [-fpermissive]
while (__comp(*__first, __pivot))
^
/usr/include/c++/4.8/bits/stl_algo.h:2290:34: error: invalid conversion from ‘const int*’ to ‘int*’ [-fpermissive]
while (__comp(__pivot, *__last))
^
In file included from /usr/include/c++/4.8/bits/stl_pair.h:59:0,
from /usr/include/c++/4.8/bits/stl_algobase.h:64,
from /usr/include/c++/4.8/bits/char_traits.h:39,
from /usr/include/c++/4.8/ios:40,
from /usr/include/c++/4.8/ostream:38,
from /usr/include/c++/4.8/iostream:39,
from prog.cpp:1:
/usr/include/c++/4.8/bits/stl_heap.h: In instantiation of ‘void std::make_heap(_RAIter, _RAIter, _Compare) [with _RAIter = int (*)[2]; _Compare = bool (*)(int*, int*)]’:
/usr/include/c++/4.8/bits/stl_algo.h:1970:47: required from ‘void std::__heap_select(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int (*)[2]; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:5363:59: required from ‘void std::partial_sort(_RAIter, _RAIter, _RAIter, _Compare) [with _RAIter = int (*)[2]; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:2355:68: required from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = int (*)[2]; _Size = int; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:5513:44: required from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = int (*)[2]; _Compare = bool (*)(int*, int*)]’
prog.cpp:16:35: required from here
/usr/include/c++/4.8/bits/stl_heap.h:446:25: error: array must be initialized with a brace-enclosed initializer
_ValueType __value = _GLIBCXX_MOVE(*(__first + __parent));
^
/usr/include/c++/4.8/bits/stl_heap.h: In instantiation of ‘void std::__pop_heap(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int (*)[2]; _Compare = bool (*)(int*, int*)]’:
/usr/include/c++/4.8/bits/stl_algo.h:1973:50: required from ‘void std::__heap_select(_RandomAccessIterator, _RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = int (*)[2]; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:5363:59: required from ‘void std::partial_sort(_RAIter, _RAIter, _RAIter, _Compare) [with _RAIter = int (*)[2]; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:2355:68: required from ‘void std::__introsort_loop(_RandomAccessIterator, _RandomAccessIterator, _Size, _Compare) [with _RandomAccessIterator = int (*)[2]; _Size = int; _Compare = bool (*)(int*, int*)]’
/usr/include/c++/4.8/bits/stl_algo.h:5513:44: required from ‘void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = int (*)[2]; _Compare = bool (*)(int*, int*)]’
prog.cpp:16:35: required from here
/usr/include/c++/4.8/bits/stl_heap.h:339:28: error: array must be initialized with a brace-enclosed initializer
_ValueType __value = _GLIBCXX_MOVE(*__result);
^
In file included from /usr/include/c++/4.8/bits/stl_algo.h:61:0,
from /usr/include/c++/4.8/algorithm:62,
from prog.cpp:2:
/usr/include/c++/4.8/bits/stl_heap.h:340:17: error: invalid array assignment
*__result = _GLIBCXX_MOVE(*__first);
^

Answer 1

std::sort(data, data+N, compare);
You are sorting data, not pairs. That said, your new approach is still undefined behaviour, and thus not guaranteed to work1. You are essentially trying to fit a square peg into a round hole. If you want to use std::sort, present valid data – which means copying in your case, or writing a custom iterator which treats an array as a collection of consecutive pairs.
1 That’s a humungous understatement. – Do not do this.

Answer 2

Exchanging your array of two int for a std::pair<int,int> did the trick for me (live at ideone):
#include <iostream>
#include <algorithm>
#include <memory>
typedef std::pair<int,int> Pair;
bool compare(const Pair& i, const Pair& j) { return i.first < j.first; }
int main() {
const int N=5;
Pair pairs[N] = {{1,2}, {7,8}, {13,14}, {4,5}, {10,11}};
std::cout << "unsorted" << std::endl;
for(int i=0; i<N;++i) std::cout << i << ": (" << pairs[i].first << ", " << pairs[i].second << ")" << std::endl;
std::sort(pairs, pairs+N, compare);
std::cout << "sorted" << std::endl;
for(int i=0; i<N;++i) std::cout << i << ": (" << pairs[i].first << ", " << pairs[i].second << ")" << std::endl;
}
An alternative would be encapsulating the array of two int inside a struct. The problem in your code is that std::sort need an array of comparable (you fixed it with your compare function) and copy-or-move-assignable items (arrays are neither)
Maybe even better (less changes to your code) would be using std::array:
#include <iostream>
#include <algorithm>
#include <memory>
typedef std::array<int, 2> Pair;
bool compare(const Pair& i, const Pair& j) { return i[0] < j[0]; }
int main() {
const int N=5;
Pair pairs[N] = {1,2, 7,8, 13,14, 4,5, 10,11};
std::cout << "unsorted" << std::endl;
for(int i=0; i<N;++i) std::cout << i << ": (" << pairs[i][0] << ", " << pairs[i][1] << ")" << std::endl;
std::sort(pairs, pairs+N, compare);
std::cout << "sorted" << std::endl;
for(int i=0; i<N;++i) std::cout << i << ": (" << pairs[i][0] << ", " << pairs[i][1] << ")" << std::endl;
}