// Global and local variables
#include <iostream>
using namespace std;

double a = 128;

int main ()
{
    double a = 256;
    double b=a+a;
    double c=::a+::a;

    cout << "Local a: " << a << endl;
    cout << "Local b: " << b << endl;
    cout << "Global a: " << ::a << endl;
    cout << "Local c: " << c << endl;
    return 0;
}

Local a: 256
Local b: 512
Global a: 128
Local c: 256