C++ namespace Introduction

Introduction

So far, we have seen how to group parts of our C++ code into separate files called headers and source files.

There is another way we can logically group parts of our C++, and that is through namespaces.

A namespace is a scope with a name.

To declare a namespace, we write: 

namespace MyNameSpace 
{ 

}

To declare objects in a namespace, we use: 

namespace MyNameSpace 
{ 
    int x; 
    double d; 
}

To refer to these objects outside the namespace, we use their fully qualified names.

This means we use the namespace_name::our_object notation.

An example where we define the objects outside the namespace they were declared in: 

namespace MyNameSpace 
{ 
    int x; /*from   ww w .  j a v a 2  s  .  co m*/
    double d; 
} 

int main() 
{ 
    MyNameSpace::x = 123; 
    MyNameSpace::d = 456.789; 
}

To introduce an entire namespace into the current scope, we can use the using -directive: 

namespace MyNameSpace 
{ 
    int x; //from  w ww. j a v  a2 s . com
    double d; 
} 

using namespace MyNameSpace; 

int main() 
{ 
    x = 123; 
    d = 456.789; 
}

If we have several separate namespaces with the same name in our code, this means we are extending that namespace. Example: 

namespace MyNameSpace 
{ 
    int x; //ww  w.  j a  v a2s. c o m
    double d; 
} 

namespace MyNameSpace 
{ 
    char c; 
    bool b; 
} 

int main() 
{ 
    MyNameSpace::x = 123; 
    MyNameSpace::d = 456.789; 
    MyNameSpace::c = 'a'; 
    MyNameSpace::b = true; 
}

We now have x, d, c, and b inside our MyNameSpace namespace.

We are extending the MyNameSpace, not redefining it.

A namespace can be spread across multiple files, both headers and source files.

Two namespaces with different names can hold an object with the same name.

Since every namespace is a different scope, they now declare two different unrelated objects with the same name.

It prevents name clashes: 

#include <iostream> 

namespace MyNameSpace 
{ 
    int x; //from   w  w w .j a v a2s  .  co  m
} 
namespace MySecondNameSpace 
{ 
    int x; 
} 

int main() 
{ 
    MyNameSpace::x = 123; 
    MySecondNameSpace::x = 456; 
    std::cout << "1st x: " << MyNameSpace::x << ", 2nd x: " << MySecondNameSpace::x; 
}



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