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The name following the namespace keyword identifies a namespace separate from the global namespace within which we can place entities we wish to declare outside a function or class The namespace does not change the meaning of the declarations within it; it changes only their visibility Before continuing, let's extend our set of available namespaces:
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namespace IBM_Canada_Laboratory { template <class elemType> class Array { }; class Matrix { }; // } namespace Disney_Feature_Animation { class Point { }; template <class elemType, int size> class Array { }; // }
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If the declarations within a namespace are not immediately visible to the program, how do we access them We use a qualified name notation of the form
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namespace_identifier::entity_name;
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as in
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Cplusplus_Primer_3E::Array<string> text; IBM_Canada_Laboratory::Matrix mat; Disney_Feature_Animation::Point origin( 5000, 5000 );
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Although Disney_Feature_Animation, IBM_Canada_Laboratory, and Cplusplus_Primer_3E uniquely identify each respective namespace, they are somewhat cumbersome to use often within our programs Using namespace identifiers such as P3E, DFA, or IBM_CL is more convenient, but it conveys considerably less information and increases the possibility of name collision To provide for both meaningful namespace identifiers and programmer convenience in accessing the entities declared within the namespace, an alias facility is provided A namespace alias allows us to associate an alternative, shorter or generic name with an existing namespace For example:
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// provide a generic alias namespace LIB = IBM_Canada_Laboratory; // simply provide a shorter alias namespace DFA = Disney_Feature_Animation;
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This alias can then be used as a synonym to the original namespace For example:
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#include "IBM_Canadah" namespace LIB = IBM_Canada_Laboratory; int main() { LIB::Array<int> ia(1024); // }
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Potentially, an alias can also serve to encapsulate the actual namespace being used In this scenario, for example, by changing the namespace assigned to the alias, we change the set of declarations we use without having to change the actual code accessing those declarations through the alias For example:
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namespace LIB = Cplusplus_Primer_3E; int main() { // in this case, this declaration need not change LIB::Array<int> ia(1024); // }
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For this technique to work in practice, however, the declarations within the two namespaces must provide the exact interface For example, the following does not work because the Disney Array class wants both a type and a size parameter for its Array class:
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namespace LIB = Disney_Feature_Animation; int main() { // no longer a valid declaration LIB::Array<int> ia(1024); // }
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More often, programmers would prefer unqualified access to the names declared within a namespace Even with the ability to provide a shorter alias for a namespace identifier, it can often prove cumbersome to qualify every access of every name declared within a namespace The using directive makes the declarations within a namespace visible so that they can be referred to without qualification For example:
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#include "IBM_Canada_Laboratoryh" // makes all names visible using namespace IBM_Canada_Laboratory; int main() { // ok: IBM_Canada_Laboratory::Matrix Matrix mat( 4,4 );
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// ok: IBM_Canada_Laboratory::Array Array<int> ia( 1024 ); // }
Both using and namespace are keywords The namespace referred to must already have been declared, or a compile-time error results The using declaration provides a more selective name visibility mechanism It allows for a single declaration within a namespace to be made visible For example:
#include "IBM_Canada_Laboratoryh" // only makes Matrix visible using IBM_Canada_Laboratory::Matrix; int main() { // ok: IBM_Canada_Laboratory::Matrix Matrix mat(4,4); // error: IBM_Canada_Laboratory::Array not visible Array<int> ia( 1024 ); // }
To prevent the components of the C++ standard library from polluting the global namespace of users' programs, all the components of the C++ standard library are declared within a namespace called namespace std As we mentioned in 1, even though we include a C++ library header file in a program text file, the components declared within that header file are not automatically visible in the text file For example, under Standard C++, the following code sample does not compile properly:
#include <string> // error: string is not visible string current_chapter = "A Tour of C++";
All the declarations in the <string> header file are enclosed in namespace std As mentioned in 1, we can use a using directive following the #include preprocessor directive to make the components of namespace std declared in the C++ header file <string> visible in the text file:
#include <string> using namespace std; // ok: string is visible string current_chapter = "A Tour of C++";
A using directive is usually seen as a poor choice for making the names declared in namespace std visible in our programs In the example, the using directive makes all the components of namespace std declared in the header file <string> visible in the program text file This brings back the global namespace pollution problem that namespace std tries to avoid in the first place and increases the chance that names of the C++ standard library components will collide with some of the global names declared in our
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