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You might gain a better understanding of automatic variables if you look at how a typical C++ compiler implements them. Because the number of automatic variables grows and shrinks as functions start and terminate, the program has to manage automatic variables as it runs. The usual means is to set aside a section of memory and treat it as a stack for managing the flow and ebb of variables. It’s called a stack because new data is figuratively stacked atop old data (that is, at an adjacent location, not at the same location) and then removed from the stack when a program is finished with it. The default size of the stack depends on the implementation, but a compiler typically provides the option of changing the size. The program keeps track of the stack by using two pointers. One points to the base of the stack, where the memory set aside for the stack begins, and one points to the top of the stack, which is the next free memory location. When a function is called, its automatic variables are added to the stack, and the pointer to the top points to the next available free space following the variables. When the function terminates, the top pointer is reset to the value it had before the function was called, effectively freeing the memory that had been used for the new variables.

A stack is a LIFO (last-in, first-out) design, meaning the last variables added to the stack are the first to go. The design simplifies argument passing. The function call places the values of its arguments on top of the stack and resets the top pointer. The called function uses the description of its formal parameters to determine the addresses of each argument. For example, Figure 9.3 shows a fib() function that, when called, passes a 2-byte int and a 4-byte long. These values go on the stack. When fib() begins execution, it associates the names real and tell with the two values. When fib() terminates, the top-of-stack pointer is relocated to its former position. The new values aren’t erased, but they are no longer labeled, and the space they occupy will be used by the next process that places values on the stack. (Figure 9.3 is somewhat simplified because function calls may pass additional information, such as a return address.)

Figure 9.3. Passing arguments by using a stack.

Register Variables

C originally introduced the register keyword to suggest that the compiler use a CPU register to store an automatic variable:

register int count_fast;  // request for a register variable

The idea was that this would allow faster access to the variable.

Prior to C++11, C++ used the keyword in the same fashion, except that as hardware and compilers developed in sophistication, the hint was generalized to mean that the variable was heavily used and perhaps the compiler could provide some sort of special treatment. With C++11, even that hint is being deprecated, leaving register as just a way to explicitly identify a variable as being automatic. Given that register can only be used with variables that would be automatic anyway, one reason to use this keyword is to indicate that you really do want to use an automatic variable, perhaps one with the same name as an external variable. This is the same purpose the original use of auto served. The more important reason for retaining register, however, is to avoid invalidating existing code that uses that keyword.

Static Duration Variables

C++, like C, provides static storage duration variables with three kinds of linkage: external linkage (accessible across files), internal linkage (accessible to functions within a single file), and no linkage (accessible to just one function or to one block within a function). All three last for the duration of the program; they are less ephemeral than automatic variables. Because the number of static variables doesn’t change as the program runs, the program doesn’t need a special device such as a stack to manage them. Instead, the compiler allocates a fixed block of memory to hold all the static variables, and those variables stay present as long as the program executes. Also if you don’t explicitly initialize a static variable, the compiler sets it to 0. Static arrays and structures have all the bits of each element or member set to 0 by default.

Note

Classic K&R C does not allow you to initialize automatic arrays and structures, but it does allow you to initialize static arrays and structures. ANSI C and C++ allow you to initialize both kinds. But some older C++ translators use C compilers that are not fully ANSI C-compliant. If you are using such an implementation, you might need to use one of the three varieties of static storage classes for initializing arrays and structures.

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C++ Primer Plus
C++ Primer Plus

C++ Primer Plus is a carefully crafted, complete tutorial on one of the most significant and widely used programming languages today. An accessible and easy-to-use self-study guide, this book is appropriate for both serious students of programming as well as developers already proficient in other languages.The sixth edition of C++ Primer Plus has been updated and expanded to cover the latest developments in C++, including a detailed look at the new C++11 standard.Author and educator Stephen Prata has created an introduction to C++ that is instructive, clear, and insightful. Fundamental programming concepts are explained along with details of the C++ language. Many short, practical examples illustrate just one or two concepts at a time, encouraging readers to master new topics by immediately putting them to use.Review questions and programming exercises at the end of each chapter help readers zero in on the most critical information and digest the most difficult concepts.In C++ Primer Plus, you'll find depth, breadth, and a variety of teaching techniques and tools to enhance your learning:• A new detailed chapter on the changes and additional capabilities introduced in the C++11 standard• Complete, integrated discussion of both basic C language and additional C++ features• Clear guidance about when and why to use a feature• Hands-on learning with concise and simple examples that develop your understanding a concept or two at a time• Hundreds of practical sample programs• Review questions and programming exercises at the end of each chapter to test your understanding• Coverage of generic C++ gives you the greatest possible flexibility• Teaches the ISO standard, including discussions of templates, the Standard Template Library, the string class, exceptions, RTTI, and namespaces

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