c++ - Slow iterating over string using pointers -

i love see why 1 of following solution slower other. lets concider following code:

// create long string int x,y; bool b; char c[10000];  (x=0;x<10000;x++)     c[x]='a'; string s(c); 

now want iterate on string , compare each character other. first solution completes task in 5 seconds:

for (y=0;y<100000;y++){ (x=0;x<10000;x++){     b = (s[x]=='a'); }} 

and second in 21 seconds:

string::iterator begin = s.begin(); string::iterator end   = s.end(); string::iterator i;  (y=0;y<100000;y++){     (i=begin;i<end;i++){         b = (*i=='a'); }} 

why second slower?

it having dereference pointer every time comparing iterator 'a', instead of comparing actual char value. negligible difference in cases, doing 1,000,000,000 iterations noticeable.