c++ - What is the memory layout of vector of arrays?

Question

can anybody explaine the memory layout of

std::vector<std::array<int, 5>> vec(2)

does it provide contiguous memory block of a 2D array with 2 rows of 5 elements?

To my understanding, the vector of vectors

std::vector<std::vector<int>> vec(2, std::vector<int>(5))

provide the memory layout of two contiguous arrays of length 5 elements in different locations in memory.

Will it be the same for the vector of arrays?

Answer 1

Arrays do not have any indirection, but just store their data "directly". That is, a std::array<int, 5> literally contains five ints in a row, flat. And, like vectors, they do not put padding between their elements, so they're "internally contiguous".

However, the std::array object itself may be larger than the set of its elements! It is permitted to have trailing "stuff" like padding. So, although likely, it is not necessarily true that your data will all be contiguous in the first case.

An int
+----+
|    |
+----+

A vector of 2 x int
+----+----+----+-----+        +----+----+
| housekeeping | ptr |        | 1  |  2 |
+----+----+----+-----+        +----+----+
                   |          ^
                   -----------

An std::array<int, 5>
+----+----+----+----+----+----------->
| 1  |  2 |  3 |  4 |  5 | possible cruft/padding....
+----+----+----+----+----+----------->

A vector of 2 x std::array<int, 5>
+----+----+----+-----+        +----+----+----+----+----+----------------------------+----+----+----+----+----+----------->
| housekeeping | ptr |        | 1  |  2 |  3 |  4 |  5 | possible cruft/padding.... | 1  |  2 |  3 |  4 |  5 | possible cruft/padding....
+----+----+----+-----+        +----+----+----+----+----+----------------------------+----+----+----+----+----+----------->
                   |          ^
                   -----------

And, even if it were, due to aliasing rules, whether you'd be able to use a single int* to navigate all 10 numbers would potentially be a different matter!

All in all, a vector of ten ints would be clearer, completely packed, and possibly safer to use.

In the case of a vector of vectors, a vector is really just a pointer plus some housekeeping, hence the indirection (as you say).