C++ Primer Plus полностью

• Thread storage duration (C++11)— These days multicore processors are common. These are CPUs that can handle several execution tasks simultaneously. This allows a program to split computations into separate threads that can be processed concurrently. Variables declared with the thread_local keyword have storage that persists for as long as the containing thread lasts. This book does not venture into concurrent programming.

• Dynamic storage duration— Memory allocated by the new operator persists until it is freed with the delete operator or until the program ends, whichever comes first. This memory has dynamic storage duration and sometimes is termed the free store or the heap.

You’ll get the rest of the story now, including fascinating details about when variables of different types are in scope, or visible (that is, usable by the program), and about linkage, which determines what information is shared across files.

Scope and Linkage

Scope describes how widely visible a name is in a file (translation unit). For example, a variable defined in a function can be used in that function but not in another, whereas a variable defined in a file above the function definitions can be used in all the functions. Linkage describes how a name can be shared in different units. A name with external linkage can be shared across files, and a name with internal linkage can be shared by functions within a single file. Names of automatic variables have no linkage because they are not shared.

A C++ variable can have one of several scopes. A variable that has local scope (also termed block scope) is known only within the block in which it is defined. Recall that a block is a series of statements enclosed in braces. A function body, for example, is a block, but you can have other blocks nested within the function body. A variable that has global scope (also termed file scope) is known throughout the file after the point where it is defined. Automatic variables have local scope, and a static variable can have either scope, depending on how it is defined. Names used in a function prototype scope are known just within the parentheses enclosing the argument list. (That’s why it doesn’t really matter what they are or if they are even present.) Members declared in a class have class scope (see Chapter 10, “Objects and Classes”). Variables declared in a namespace have namespace scope. (Now that namespaces have been added to the C++ language, the global scope has become a special case of namespace scope.)

C++ functions can have class scope or namespace scope, including global scope, but they can’t have local scope. (Because a function can’t be defined inside a block, if a function were to have local scope, it could only be known to itself and hence couldn’t be called by another function. Such a function couldn’t function.)

The various C++ storage choices are characterized by their storage duration, their scope, and their linkage. Let’s look at C++’s storage classes in terms of these properties. We begin by examining the situation before namespaces were added to the mix and then see how namespaces modify the picture.

Automatic Storage Duration

Function parameters and variables declared inside a function have, by default, automatic storage duration. They also have local scope and no linkage. That is, if you declare a variable called texas in main() and you declare another variable with the same name in a function called oil(), you’ve created two independent variables, each known only in the function in which it’s defined. Anything you do to the texas in oil() has no effect on the texas in main(), and vice versa. Also each variable is allocated when program execution enters the innermost block containing the definition, and each fades from existence when execution leaves that block. (Note that the variable is allocated when execution enters the block, but the scope begins only after the point of declaration.)

If you define a variable inside a block, the variable’s persistence and scope are confined to that block. Suppose, for example, that you define a variable called teledeli at the beginning of main(). Now suppose you start a new block within main() and define a new variable, called websight, in the block. Then, teledeli is visible in both the outer and inner blocks, whereas websight exists only in the inner block and is in scope only from its point of definition until program execution passes the end of the block:

int main()

{

    int teledeli = 5;

    {                         // websight allocated

        cout << "Hello\n";

        int websight = -2;    // websight scope begins

        cout << websight << ' ' << teledeli << endl;

    }                         // websight expires

    cout << teledeli << endl;

    ...

}  // teledeli expires