cpp/CPPlusV6/Chapter 9/support.cpp

// support.cpp -- use external variable
// compile with external.cpp
#include <iostream>
extern double warming;  // use warming from another file

// function prototypes
void update(double dt);
void local();

using std::cout;
void update(double dt)      // modifies global variable
{
    extern double warming;  // optional redeclaration
    warming += dt;          // uses global warming
    cout << "Updating global warming to " << warming;
    cout << " degrees.\n";
}

void local()                // uses local variable
{
    double warming = 0.8;   // new variable hides external one

    cout << "Local warming = " << warming << " degrees.\n";
        // Access global variable with the
        // scope resolution operator
    cout << "But global warming = " << ::warming;
    cout << " degrees.\n";
}