Why does sort() function behave differently for following character arrays?

char a[] = {'t' , 'e' , 's' , 't' , 'i' , 'n' , 'g'};
sort(a,a+7);

This works absolutely fine.

char*a = "testing";
sort(a, a+7);

This fails to run. why is it not being sorted?

string a = "testing";
sort(a.begin(),a.end());

This works fine..

string a ="testing";
sort(a , a+7);

But this fails. Here 'a' is a C++ string but why is it necessary to use iterators here ?