12/4/2011 1 CS1C – Advanced Programming in C++ Saddleback College Fall 2011 – J Tateyama Namespaces Topic 17 – Shrinkwrap Chapter 14 CS1C – Saddleback College Namespaces  We started out in CS 1A stating that identifiers were used in C++ to name things – variables and constants  As we have progressed we have named – user-defined functions – user-defined data types – classes and so on  There are so many things to name in C++ it is not unreasonable to expect that several things might have the same name.  A namespace is simply a way to divide items into collections so that no two items in a given collection have the same definition  The standard library classes are in the std name space so we have always coded “using namespace std” to avoid coding std:: CS1C – Saddleback College Namespaces  Namespace - A collection of names which are typically class definitions, variables and named constant declarations  When we included directives in our programs such as #include <iostream> using namespace std;  then all global identifiers in the header file iostream became global identifiers in our program – gives us the ability to use identifiers such as cin and cout in our programs  If a global identifier in a program is the same as a global identifier in the header file, the compiler generates an error regarding duplicate identifiers12/4/2011 2 CS1C – Saddleback College Namespaces  To help alleviate this problem of duplicate names, third-party vendors adopted the practice of beginning their global identifiers with the underscore ( _ )  This is why we have said to avoid the use of the underscore as the starting character for identifiers in our programs  Problems can still occur so ANSI/ISO (American National Standard Institution / International Standard Organization) Standard C++ provides a mechanism called namespace to solve the problem of duplicate global identifiers  The general form for defining a namespace in C++ is as follows namespace NamespaceName { namespace members } CS1C – Saddleback College Using Directive  The using directive using namespace std;  tells the compiler that the program is using the std ("standard") namespace  When identifiers such as cin and cout were defined in iostream, it was indicated that they were in the std namespace  If you wished to write your own versions of cin and cout for a particular application, you could omit the reference to the std namespace in your program and the compiler would not know that cin and cout defined in iostream existed  Only your versions of these functions would be known. To date, we have put the using directive as a global directive. This does not always have to be the case.  The scope of a using directive is the block in which it appears  Using directives outside of all blocks apply to the entire file that follows the directive  Review SW14-4 program CS1C – Saddleback College Using Directive  Suppose we wanted to use the DoMath function from NmSpc2 and the constant HELLO from NmSpc1. What would happen if we changed to the following? { // names in this block use HELLO defined in NmSpc1 and DoMath //from NmSpc2 using namespace NmSpc1; using namespace NmSpc2; cout << HELLO; cout << endl << VAL; cout << "\n" << firstNum << " + " << secNum << " = " << DoMath(firstNum, secNum); }  This would not compile because it is an ambiguous reference to a namespace12/4/2011 3 CS1C – Saddleback College Using directive  We need to specify which names from each namespace we want to use in this section. This is done with using declarations. { // names in this block use the following: // HELLO defined in NmSpc1 // DoMath from NmSpc2 using NmSpc1::HELLO; using NmSpc2::DoMath; cout << HELLO; // cout << endl << VAL; cout << "\n" << firstNum << " + " << secNum << " = " << DoMath(firstNum, secNum); } CS1C – Saddleback College Large Software Project Considerations  Style guides suggest that using directives should be avoided in header files because every file that incorporates that header file would also include all the namespaces defined in it.  Examples of fully qualified names – std::cout – std::string  Use long and descriptive names for your namespaces to make them unique  Use the Alias feature to establish short aliases – namespace awesome = AWESOME_Software_MissionViejo_OC_CA_US; – awesome::CalcSomething  Use the using directive for the std namespace only in the implementation file  Use fully qualify names in header