Question

I wonder what is the performance difference between fill_n and a for-loop when initializing
an array?

Which one is better?

Answer 1

Just tested these three codes (Compiled by GCC 4.7.2 with -O3 optimization):

I added these two lines to all the codes, because I think without them the GCC optimizer will deceive me.

for (int i = 0; i < SIZE; i++)
    arr[i] = arr[i + 1 % SIZE];

1.cpp

#define SIZE (100*1000*1000)
char arr[SIZE];

int main()
{
    for (int i = 0; i < SIZE; i++)
        arr[SIZE] = 0;

    for (int i = 0; i < SIZE; i++)
        arr[i] = arr[i + 1 % SIZE];
}

2.cpp

...
    std::fill_n(arr, SIZE, 0);
...

3.cpp

...
    memset(arr, 0, SIZE);
...

Measure times:

mostafa@debian:~/Codes/tests/x$ time ./1
real   0m0.073s
user   0m0.008s
sys    0m0.064s

mostafa@debian:~/Codes/tests/x$ time ./2
real   0m0.080s
user   0m0.020s
sys    0m0.056s

mostafa@debian:~/Codes/tests/x$ time ./3
real   0m0.100s
user   0m0.056s
sys    0m0.044s