Experiments with namespace in class protected area - c++

Question

#include <iostream>
class A
{
public:
int func();
protected:
namespace B { enum { D = 0, E = 1 }; }
namespace C { enum { D = 0, E = 1 }; }
}
int A::func()
{
int x = A::B::D;
int y = A::C::E;
return x + y;
}
int main() {
A a;
int x = a.func();
std::cout << x << std::endl; // 1
return 0;
}
What's wrong with this code?
I am just curious and experimenting with namespace, because I'd like to have enums with same names of values.
I don't want to use enum class, because I cannot do operations with integer without overloading operators or casting.

Answer 1

You can't declare a namespace inside a class. Instead, what you can use is a struct like
#include <iostream>
class A
{
public:
int func();
protected:
struct B { enum { D = 0, E = 1 }; };
struct C { enum { D = 0, E = 1 }; };
};
int A::func()
{
int x = A::B::D;
int y = A::C::E;
return x + y;
}
int main() {
A a;
int x = a.func();
std::cout << x << std::endl; // 1
return 0;
}
which you can see working in this live example.

Answer 2

From the C++ 14 Standard (7.3.1 Namespace definition)
2 Every namespace-definition shall appear in the global scope or in a
namespace scope
So these namespace definitions within the class scope (where you forgot to place a semicolon)
class A
{
public:
int func();
protected:
namespace B { enum { D = 0, E = 1 }; }
namespace C { enum { D = 0, E = 1 }; }
};
^^^
are invalid.