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.