Separate Compilation and Namespaces in Software

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Separate Compilation and Namespaces
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Point 1 is pretty obvious Point 2 has some subtleties For example, suppose the namespaces NS1 and NS2 both provide de nitions for myFunction, but have no other name con icts, then the following will produce no problems:
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using namespace NS1; using namespace NS2;
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provided that (within the scope of these directives) the con icting name myFunction is never used in your code On the other hand, the following is illegal, even if the function myFunction is never used:
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using NS1::myFunction; using NS2::myFunction;
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using
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Sometimes this subtle point can be important, but it does not impinge on most routine code So, we will often use the term using directive loosely to mean either a using directive or a using declaration
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s QUALIFYING NAMES
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This section introduces a way to qualify names that we have not discussed before Suppose that you intend to use the name fun1 as de ned in the namespace NS1, but you only intend to use it one time (or a small number of times) You can name the function (or other item) using the name of the namespace and the scope resolution operator as follows:
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NS1::fun1( );
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This form is often used when specifying a parameter type For example, consider
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int getInput(std::istream inputStream)
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In the function getInput, the parameter inputStream is of type istream, where istream is de ned as in the std namespace If this use of the type name istream is the only name you need from the std namespace (or if all the names you need are similarly quali ed with std::), then you do not need
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using namespace std;
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using std::istream;
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Note that you can use std::istream even within the scope of a using directive for another namespace which also de nes the name istream In this case std::istream and istream will have different de nitions For example, consider
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using namespace MySpace; void someFunction(istream p1, std::istream p2);
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Namespaces
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Assuming istream is a type de ned in the namespace MySpace, then p1 will have the type istream as de ned in MySpace and p2 will have the type istream as de ned in the std namespace
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Self-Test Exercises
8 What is the output produced by the following program
#include <iostream> using namespace std; namespace Hello { void message( ); } namespace GoodBye { void message( ); } void message( ); int main( ) { using GoodBye::message; { using Hello::message; message( ); GoodBye::message( ); } message( ); return 0; } void message( ) { cout << "Global message\n"; } namespace Hello { void message( ) {
Separate Compilation and Namespaces
cout << "Hello\n"; } } namespace GoodBye { void message( ) { cout << "Good-Bye\n"; } }
9 Write the declaration (prototype) for a void function named wow The function wow has two parameters, the first of type speed as defined in the speedway namespace and the second of type speed as defined in the indy500 namespace
Example
A CLASS DEFINITION
IN A
NAMESPACE
In Displays 116 and 117 we have again rewritten both the header file dtimeh for the class DigitalTime and the implementation file for the class DigitalTime This time (no pun intended), we have placed the definition in a namespace called DTimeSavitch Note that the namespace DTimeSavitch spans the two files dtimeh and dtimecpp A namespace can span multiple files If you rewrite the definition of the class DigitalTime as shown in Displays 116 and 117, then the application file in Display 113 needs to specify the namespace DTimeSavitch in some way, such as the following:
using namespace DTimeSavitch;
using DTimeSavitch::DigitalTime;
CHOOSING
NAME
FOR A
NAMESPACE
It is a good idea to include your last name or some other unique string in the names of your namespaces so as to reduce the chance that somebody else will use the same namespace name as you do With multiple programmers writing code for the same project, it is important that namespaces that are meant to be distinct really do have distinct names Otherwise, you can easily have multiple definitions of the same names in the same scope That is why we included the name Savitch in the namespace DtimeSavitch in Display 119