Chapter 12

Here FirstFile.cpp uses an extern declaration so that it can use x. The compiler needs a declaration of x in order to use it in main(). However, if you declared x without the extern keyword, the compiler would think it’s a definition and would allocate space for x, causing the linkage step to fail (because there are now two x variables in the global scope). With extern, you can make variables globally accessible from multiple source files.

However, we do not recommend using global variables at all. They are confusing and error-prone, especially in large programs. For similar functionality, you should use static class members and methods.

static Variables in Functions

The final use of the static keyword in C++ is to create local variables that retain their values between exits and entrances to their scope. A static variable inside a function is like a global variable that is only accessible from that function. One common use of static variables is to “remember” whether a particular initialization has been performed for a certain function. For example, code that employs this technique might look something like this:

void performTask()

{

static bool inited = false;

if (!inited) {

cout << “initing\n”;

// Perform initialization. inited = true;

}

// Perform the desired task.

}

However, static variables are confusing, and there are usually better ways to structure your code so that you can avoid them. In this case, you might want to write a class in which the constructor performs the required initialization.

Avoid using stand-alone static variables. Maintain state within an object instead.

Order of Initialization of Nonlocal Variables

Before leaving the topic of static data members and global variables, consider the order of initialization of these variables. All global variables and static class data members in a program are initialized before main() begins. The variables in a given source file are initialized in the order they appear in the source file. For example, in the following file Demo::x is guaranteed to be initialized before y.

// source1.cpp

class Demo

{

public:

static int x;

};

336