C++ question!

Please try out the following testing program, and you will see the "difference"!

#include
#include
using namespace std;

void main() {
double a=3;
double b=3.0;
cout << "a = " << a << endl;
cout << "b = " << b << endl;
printf("a = %f\n", a);
printf("b = %f\n", b);
}

The output is :

a = 3
b = 3
a = 3.000000
b = 3.000000