C++ alignment of multidimensional array structure
c++, data-structures, memory-alignment, multidimensional-array
Solution
1 Theoretically no. The compiler may decide to add padding to my_inner_array. In practice, I don't see a reason why the compiler would add padding to a struct that has an array in it. In such a case there's no alignment problem creating an array of such structs. You can use a compile time assert:
typedef int my_inner_array_array[3];
BOOST_STATIC_ASSERT(sizeof(my_inner_array) == sizeof(my_inner_array_array));
4 If there are no virtual methods it shouldn't make any difference.
Problem
In my code, I have to consider an array of arrays, where the inner arrays are of a fixed dimension. In order to make use of STL algorithms, it is useful to actually store the data as array of arrays, but I also need to pass that data to a C library, which takes a flattened C-style array. It would be great to be able to convert (i.e. flatten) the multi-dimensional array cheaply and in a portable way. I will stick to a very simple case, the real problem is more general. ``` struct my_inner_array { int data[3]; }; std::vector<my_inner_array> x(15); ``` Is ``` &(x[0].data[0]) ``` a pointer to a continuous block of memory of size 45*sizeof(int) containing the same entries as x? Or do I have to worry about alignment? I am afraid that this will work for me (at least for certain data types and inner array sizes) but that it is not portable. - Is this code portable? - If not, is there a way to make it work? - If not, do you have any suggestions what I could do? - Does it change anything at all if my_inner_array is not a POD struct, but contains some methods (as long as the class does not contain any virtual methods)?