A nested namespace is a namespace that is defined within another namespace. It allows
for organization and grouping of related namespaces.

#include <iostream>

namespace A {
    int x = 1;

    namespace B {
        int y = 2;

        namespace C {
            int z = 3;
        }
    }
}

int main() {
    int a = A::x;
    int b = A::B::y;
    int c = A::B::C::z;

    std::cout << "a: " << a << std::endl;
    std::cout << "b: " << b << std::endl;
    std::cout << "c: " << c << std::endl;
    
    return 0;
}

In this example, namespace "A" contains an integer variable x and another namespace
"B" which contains an integer variable y and it contain another namespace "C" which
contains an integer variable z.You can access the variables of nested namespace by
using the scope resolution operator "::" . In this case, to access the variable y
which is in namespace B, we have to use A::B::y and to access the variable z which
is in namespace C, we have to use A::B::C::z.

This way you can organize your code in a more readable and maintainable way.