What is the use of namespaces in C++ ?

Question

I am new to C+ and I wonder why this code compiles fine with g+ and MSVC:

 namespace t {

 class A {
 public:
 int i;
 };

 int b(int k, A a) {
 return k;
 }

 }

 int main()
 {
 t::A cl;
 return b(5, cl);
 }

I thought that b should not be visible from the main function.

Answer 1

namespace is used to avoid name collision.

for Ex;

using namespace std;

that means, Below code has to use std library.
if you don't want to write namespace then you have to use (::) scope resolution operator.