external.cpp

一本很好的C++学习的丛书！初学者必看的

// external.cpp -- external variables
#include <iostream>
using namespace std;
// external variable
double warming = 0.3;

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

int main()                  // uses global variable
{
    cout << "Global warming is " << warming << " degrees.\n";
    update(0.1);            // call function to change warming
    cout << "Global warming is " << warming << " degrees.\n";
    local();                // call function with local warming
    cout << "Global warming is " << warming << " degrees.\n";
    return 0;
}

void update(double dt)      // modifies global variable
{
    extern double warming;  // optional redeclaration
    warming += dt;
    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";
}