Unary Scope Resolution Operator

It is possible to declare local and global variables of the same name. C++ provides the unary scope resolution operator (::) to access a global variable when a local variable of the same name is in scope. The unary scope resolution operator cannot be used to access a local variable of the same name in an outer block. A global variable can be accessed directly without the unary scope resolution operator if the name of the global variable is not the same as that of a local variable in scope.

Figure 6.23 demonstrates the unary scope resolution operator with local and global variables of the same name (lines 7 and 11). To emphasize that the local and global versions of variable number are distinct, the program declares one variable of type int and the other double.

Figure 6.23. Unary scope resolution operator.

 1 // Fig. 6.23: fig06_23.cpp
 2 // Using the unary scope resolution operator.
 3 #include 
 4 using std::cout;
 5 using std::endl;
 6
 7 int number = 7; // global variable named number
 8
 9 int main()
10 {
11 double number = 10.5; // local variable named number
12
13 // display values of local and global variables
14 cout << "Local double value of number = " << number
15 << "
Global int value of number = " << ::number << endl;
16 return 0; // indicates successful termination
17 } // end main
 
 Local double value of number = 10.5
 Global int value of number = 7
 

Using the unary scope resolution operator (::) with a given variable name is optional when the only variable with that name is a global variable.

Common Programming Error 6.20

It is an error to attempt to use the unary scope resolution operator (::) to access a nonglobal variable in an outer block. If no global variable with that name exists, a compilation error occurs. If a global variable with that name exists, this is a logic error, because the program will refer to the global variable when you intended to access the nonglobal variable in the outer block.

Good Programming Practice 6.7

Always using the unary scope resolution operator (::) to refer to global variables makes programs easier to read and understand, because it makes it clear that you are intending to access a global variable rather than a nonglobal variable.


Software Engineering Observation 6.17

Always using the unary scope resolution operator (::) to refer to global variables makes programs easier to modify by reducing the risk of name collisions with nonglobal variables.

Error-Prevention Tip 6.4

Always using the unary scope resolution operator (::) to refer to a global variable eliminates possible logic errors that might occur if a nonglobal variable hides the global variable.

Error-Prevention Tip 6.5

Avoid using variables of the same name for different purposes in a program. Although this is allowed in various circumstances, it can lead to errors.


Introduction to Computers, the Internet and World Wide Web

Introduction to C++ Programming

Introduction to Classes and Objects

Control Statements: Part 1

Control Statements: Part 2

Functions and an Introduction to Recursion

Arrays and Vectors

Pointers and Pointer-Based Strings

Classes: A Deeper Look, Part 1

Classes: A Deeper Look, Part 2

Operator Overloading; String and Array Objects

Object-Oriented Programming: Inheritance

Object-Oriented Programming: Polymorphism

Templates

Stream Input/Output

Exception Handling

File Processing

Class string and String Stream Processing

Web Programming

Searching and Sorting

Data Structures

Bits, Characters, C-Strings and structs

Standard Template Library (STL)

Other Topics

Appendix A. Operator Precedence and Associativity Chart

Appendix B. ASCII Character Set

Appendix C. Fundamental Types

Appendix D. Number Systems

Appendix E. C Legacy Code Topics

Appendix F. Preprocessor

Appendix G. ATM Case Study Code

Appendix H. UML 2: Additional Diagram Types

Appendix I. C++ Internet and Web Resources

Appendix J. Introduction to XHTML

Appendix K. XHTML Special Characters

Appendix L. Using the Visual Studio .NET Debugger

Appendix M. Using the GNU C++ Debugger

Bibliography



C++ How to Program
C++ How to Program (5th Edition)
ISBN: 0131857576
EAN: 2147483647
Year: 2004
Pages: 627

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