namespaces

I'm confused.

From what I've been reading a namespace allows me to use a name belonging to that same namespace. But only if I have that name already declared. Usually by means of an include statement.

This gives rise to 2 questions:

1. Why then I want the using directive? The simple fact I included the header say, stdlib, already allows me to access it's declarations without any kind of qualification.

2. With my compiler (below), I gained the habit of not including stdlib, but instead using namespace std;. This still allows me to access functions like system() without any qualification. But it clearly goes against what i have been reading now about namespaces.

So, can someone please help me understand exactly what is a namespace and how does it affect && is affected by an include? Take it like I'm 5 years old, will ya

So, I did the following exervice in an attempt to better understand it.

I created a data.cpp file:

Code:

namespace data
{
  int value = 54;
  extern const double pi = 3.1514;
}

And a main.cpp file

Code:

#include <iostream>

namespace data
{
   extern int value;
   extern const double pi;
}

int main(int argc, char* argv[])
{
   using namespace std;
   cout << "The value of pi is " << data::pi;
   using namespace data;
   std::cout << std::endl << "The value of value is " << value;
   std::cout << "\n\n";
   system("pause");
   return 0;
}

Now, I do understand the principle behind namespaces. So don't worry. I'm not asking for a book on it

But why,

1. system("pause") doesn't give me a compile error? I even have to qualify cout and endl even though I have iostream included when under the data namespace... so it should give me an error

2. How do I return to the global namespace? It seems to me as soon as I start the using directive, I'm stuck to it.

3. Above I had to declare the externs. It didn't matter to the compiler I had the using directive for data. With no extern declaration it simply wouldn't compile. I understand this, but why then when I use the std namespace alone with no stdlib.h include does system() still gets recognized?

Once you are using something, it stays used to the end of the file.

If you:

Code:

#include <iostream>

using namespace std;

Then you are doing exactly what

Code:

#include <iostream.h>

does, just explicitly. The difference, would be if you did something like this:

Code:

#include <iostream>

using std::cout;
using std::cin;
using std::system();

Then you could declare an identifier called cerr without clashing. Not that you should, but in a very large project keeping the namespace trim can help avoid name clashing, especially if you have many programmers working together.

Code:

include <iostream.h>

namespace first
{
   int number=5;
};

namespace second
{
   int number=6;
};

int main()
{cout<<first::number<<endl;//outputs 5
  cout<<second::number<<endl;//outputs 6

   return 0;
}

Mostly, an organizational tool that prevents the same name used twice. For example, if you wrote a library you wanted to be distributed, you might want to include your data in a namespace and explicitly call it each time just in case the user has a variable with the same name as the library.