How to define a 2D array in C++ and STL without memory manipulation?

Question

There are several ways to define a 2D array in C++ and STL without memory manipulation, and the following codes illustrate two different methods:

int main () 
{
         


        

Answer 1

There are a lot of trade-offs here.

If you declare a C-style 2D array int array[height][width], then you really get a single contiguous block of memory. The compiler converts indexes to their 1D address

array[row][col] == *(array + row * width + col)

• Advantages: cache coherency. All the memory is in the same place.
• Disadvantages: you need a multiply for every indexing. Indirection might be faster.

If you use a vector of vectors, then each row is allocated separately. The outer vector stores pointers to the inner vectors. Indexing becomes an indirection followed by an addition:

array[row][col] == *(*(array + row) + col)

• Advantages: indirection may be faster than multiplication.
• Disadvantages: not cache coherent, since each row is allocated separately (unless the implementation optimizes for vector).

If performance is truly important, you need to test both and figure out which is faster on your data.