c++ - std::pair<int, int> vs struct with two int's

Question

In an ACM example, I had to build a big table for dynamic programming. I had to store two integers in each cell, so I decided to go for a std::pair<int, int>. However, allocating a huge array of them took 1.5 seconds:

std::pair<int, int> table[1001][1001];

Afterwards, I have changed this code to

struct Cell {
    int first;
    int second;
}

Cell table[1001][1001];

and the allocation took 0 seconds.

What explains this huge difference in time?

Answer 1

std::pair<int, int>::pair() constructor initializes the fields with default values (zero in case of int) and your struct Cell doesn't (since you only have an auto-generated default constructor that does nothing).

Initializing requires writing to each field which requires a whole lot of memory accesses that are relatively time consuming. With struct Cell nothing is done instead and doing nothing is a bit faster.