История изменений

std::vector<std::vector<int>> xs = {{1, 2, 3}, {1, 2}, {1}, {}};

for (auto const& col : xs) {
    for (auto const& el : col)
        printf("(%p)%d ", &el, el);
    puts("");
}

/*
(0x59f91d0)1 (0x59f91d4)2 (0x59f91d8)3 
(0x59f9220)1 (0x59f9224)2 
(0x59f9270)1 

*/

/*
  total heap usage: 7 allocs, 7 frees, 144 bytes allocated
 */

int xs[][3] = {{1, 2, 3}, {1, 2}, {1}, {}};

for (auto const& col : xs) {
    for (auto const& el : col)
        printf("(%p)%d ", &el, el);
    puts("");
}

/*
(0x7ff0000e0)1 (0x7ff0000e4)2 (0x7ff0000e8)3 
(0x7ff0000ec)1 (0x7ff0000f0)2 (0x7ff0000f4)0 
(0x7ff0000f8)1 (0x7ff0000fc)0 (0x7ff000100)0 
(0x7ff000104)0 (0x7ff000108)0 (0x7ff00010c)0 
 */

/*
  total heap usage: 0 allocs, 0 frees, 0 bytes allocated
 */

std::vector<std::vector<int>> xs = {{1, 2, 3}, {1, 2}, {1}, {}};

for (auto const& col : xs) {
    for (auto const& el : col)
        printf("(%p)%d ", &el, el);
    puts("");
}

/*
(0x59f91d0)1 (0x59f91d4)2 (0x59f91d8)3 
(0x59f9220)1 (0x59f9224)2 
(0x59f9270)1 

*/

/*
  total heap usage: 7 allocs, 7 frees, 144 bytes allocated
 */

int xs[][4] = {{1, 2, 3}, {1, 2}, {1}, {}};

for (auto const& col : xs) {
    for (auto const& el : col)
        printf("(%p)%d ", &el, el);
    puts("");
}

/*
(0x7ff0000d0)1 (0x7ff0000d4)2 (0x7ff0000d8)3 (0x7ff0000dc)0 
(0x7ff0000e0)1 (0x7ff0000e4)2 (0x7ff0000e8)0 (0x7ff0000ec)0 
(0x7ff0000f0)1 (0x7ff0000f4)0 (0x7ff0000f8)0 (0x7ff0000fc)0 
(0x7ff000100)0 (0x7ff000104)0 (0x7ff000108)0 (0x7ff00010c)0 
 */

/*
  total heap usage: 0 allocs, 0 frees, 0 bytes allocated
 */