Namespaces

CIS-255 Home http://www.c-jump.com/bcc/c255c/c255syllabus.htm

Namespaces

Useful idea: Namespaces
Namespace syntax
Namespace usage
Namespace directives and declarations
The std namespace

1. Useful idea: Namespaces

A simple mechanism for organizing large programs
Named scopes with no other functionality - these are called namespaces.
Good for avoiding collisions and making name connections more explicit.

2. Namespace syntax


// mylibrary.h
namespace mylibrary {

    class IntStack {
    public:
        void push( int value );
        //...
    };

    class Rational {
    public:
        Rational& operator=( Rational const& other );
        //...
    };

} // No closing namespace semicolon

Namespaces can be re-opened:


// IntStack.cpp
namespace mylibrary {
    void IntStack::push( int value )
    {
        // ...
    }
}



// Rational.cpp
mylibrary::Rational rat;

namespace mylibrary {
    Rational& Rational::operator=( Rational const& other )
    {
        //...
    }
}

3. Namespace usage

Classes and functions must be declared in a namespace to be placed inside the namespace
Definitions can use the :: qualifier or be placed inside a namespace.

using declarations provide local aliases for qualified names (this imports symbols):


    using mylibrary::IntStack; // using declaration
    IntStack st;

4. Namespace directives and declarations

Can also import everything at once:


    using namespace mylibrary; // using directive

Namespaces may have using declarations:


    namespace mylibrary {

        using otherlib::IntStack; // using declaration
        //...
    }

5. The std namespace

All C++ standard library stuff is defined inside the std namespace.

Saying


    #include <iostream>

    using namespace std;

means that C++ will give you everything from std without requiring std:: qualification.

For example, cout is really the std::cout:


    #include <iostream>
    int main()
    {
        using namespace std;
        cout << "hello";
        return 0;
    }

Other than I/O, what else is there in std namespace?
- Lots, and all organized using C++ templates!