局部变量是安全

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// declare global variable
int g_nMode = 1;
 
void doSomething()
{
    g_nMode = 2;
}
 
int main()
{
    g_nMode = 1;
 
    doSomething();
 
    // Programmer expects g_nMode to be 1
    // But doSomething changed it to 2!
 
    if (g_nMode == 1)
        cout << "No threat detected." << endl;
    else
        cout << "Launching nuclear missiles..." << endl;
 
    return 0;
}1
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