Thread: problem understanding namespace

in your case the problem is that
getchar is defined in stdio.h

thus getchar exists on global namespace and NOT in std.
thus std::getchar would look for "getchar" in namespace std - and of course its not there.

with using namespace std the comiler looks up getchar in both, global namespace and std, it wont find it in std, but it WILL find it on global namespace. thus it works

>getchar is defined in stdio.h
Which is a deprecated header. The standard header for getchar is cstdio.

>thus std::getchar would look for "getchar" in namespace std - and of course its not there.
getchar exists in the global namespace if you use stdio.h, but the std namespace if you use cstdio.

One issue that you may encounter is with non-conforming compilers (*cough* VC++6 *cough*) and the standard C functions not being properly declared in the std namespace. So this code, while perfectly correct and legal, would fail to compile because the compiler wouldn't find getchar or putchar in the std namespace:

Code:

#include <cstdio>

int main()
{
  int c;

  while ( ( c = std::getchar() ) != EOF )
    std::putchar ( c );
}

On those compilers, it's actually easier to write code with a using directive rather than try to figure out the quirks and leave off a namespace qualification for those standard names that were inconveniently left out.

the global namespace is simply the primary namespace youre in

if your file looks like:

Code:

#include <iostream>

int main() {
  return 0;
}

then main is part of the global namespace
you can think of namespaces like they were directories.
instead of having all your files in one big directory you move them to many directories (e.g. c:\windows, c:\programs)
thats basically the same what you do with namespaces
it avoids conflicts with entities (classes, functions, variables) that have the same name.

e.g. there is a list class in namespace std.
now you make your own list and since its a list you want to call it list too.
so you put it into your own namespace and call it list there.
so you can access the stl list via std::list
and your own list via your_namespace::list