Scope regions in C++
The scope regions of C++ are somewhat different from those in C. C++ identifies five kinds of scope: function, function prototype, local, namespace, and class. The first two--function scope and function prototype scope--are the same in C++ as they are in C. Local scope corresponds to C's block scope, namespace scope corresponds to C's file scope, and class scope is something new.

In C++, local scope extends the concept of block scope to account for some added features of C++, such as the ability to declare a variable in the initialization step of a for statement, as in:

for (int i = 0; i < N; ++i)
    ...

C++ provides a facility called namespaces for grouping names that would otherwise be crowded into file scope. C++ generalizes the rules for names declared at file scope to include names declared in namespaces as well. In C++, a name has namespace scope if it's declared either in a namespace of the form:

namespace identifier
    {
    ...
    }

or in what C calls file scope. Accordingly, the C++ standard shuns the term file scope in favor of global namespace scope, or just global scope.

C++ also introduces the concept of class scope for names declared within the brace-enclosed body of a class definition. (Classes in C++ include structures and unions, as well.) In C++, each class introduces a new scope, so the same name can be declared as a member in more than one class.

C doesn't quite have a corresponding notion of structure scope. Rather, the C standard says that each structure or union has a separate name space for its members. The C standard uses the term name space (two words) to mean something quite different from the namespace (one word) construct of C++. In C, a name space is a region of the compiler's symbol table. Despite the different verbiage in their respective standards, C and C++ look up structure and union members in much the same way.

Scope and linkage
The concept of scope is meaningful only within a single translation unit. Strictly speaking, a name declared in the global scope is simply a name declared at the outermost scope in a translation unit. That name isn't necessarily known in other translation units.

Despite what you may have learned, neither C nor C++ has any kind of scope that spans from one translation unit to another. Rather, a name declared in one translation unit can refer to a name defined in another translation though a property called external linkage. Stay tuned.

Dan Saks is president of Saks & Associates, a C/C++ training and consulting company. For more information about Dan Saks, visit his website at www.dansaks.com. Dan also welcomes your feedback: e-mail him at dsaks@wittenberg.edu. For more information about Dan click here .

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