how to wrap several files into the same namespace in C++ - c++

Question

In C# it's simple to put all of the classes into an unique namespace. I understand how a namespace works in C++ at a simple level. But when comes to put many files together to appear as one namespace I get really confused.
It is something like this possible:
/* NetTools.cpp
blade 7/12/2014 */
using namespace std;
using namespace NetTools;
#include "NetTools.h"
int main()
{
cout << "testing" << endl;
return 0;
}
//####### EOF
/* NetTools.h
blade 12/7/2014 */
#ifndef NETTOOLS_H
#define NETTOOLS_H
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cmath>
namespace NetTools
{
}
#endif
// #### EOF
/* Commands.h
blade 7/12/2014 */
#include "NetTools.h"
#ifndef COMMANDS_H
#define COMMANDS_H
namespace NetTools
{
}
#endif
// ###### EOF
I'm separating every class declaration in its .h file and the implementation in its cpp file, but I want everything to be in the same namespace.

Answer 1

What you've shown works fine.
One fundamental property of namespaces is that if different files all declare things in a namespace with the same name, the compiler/linker know to put things together so you get one namespace containing everything that was defined in it from all the files.
For example:
// file a
namespace A {
class X;
}
// file b
namespace A {
class Y;
}
// file c
namespace A {
class Z;
}
...is mostly equivalent to:
// one file
namespace A {
class X;
class Y;
class Z;
}
The exception to this is anonymous namespaces. Anonymous namespaces are separated per-file. For example:
// file a
namespace {
void f();
}
// file b
namespace {
int f();
}
These don't conflict because each file has its own, unique anonymous namespace.

Answer 2

What you are doing is correct. There is a problem with your code here though:
/* NetTools.cpp
blade 7/12/2014 */
using namespace std;
using namespace NetTools; // put this AFTER #include "NetTools.h"
#include "NetTools.h"
int main()
{
cout << "testing" << endl;
return 0;
}
It works for using namespace std; (not sure why) but it won't work for namespaces you declare. The compiler needs to see them before you can start using them:
/* NetTools.cpp
blade 7/12/2014 */
#include "NetTools.h"
using namespace std;
using namespace NetTools; // now this should work
int main()
{
cout << "testing" << endl;
return 0;
}
Sidenote:
You should really put everything inside the include guard:
/* Commands.h
blade 7/12/2014 */
#include "NetTools.h" // this should really go after the include guards
#ifndef COMMANDS_H
#define COMMANDS_H
namespace NetTools
{
}
#endif
There is no need to #include "NetTools.h" twice (even if it is include protected).
/* Commands.h
blade 7/12/2014 */
#ifndef COMMANDS_H
#define COMMANDS_H
#include "NetTools.h" // like this
namespace NetTools
{
}
#endif