How to fix and compile this C++ program - c++

Question

In nestClassDef.h I have written code like this
class A{
public:
class B{
public:
void BTest();
};
};
class B{
};
then in the nestClassDef.cpp I am writing code like this
#include "nestClassDef.h"
#include<iostream>
void A::B::BTest(){
cout<<"Hello World!";
}
int main(){
A a;
A.B b;
b.BTest();
}
But when I am compiling the above code
g++ -o nestClassDef nestClassDef.cpp
I am getting error like this :-
nestClassDef.cpp: In member function ‘void A::B::BTest()’:
nestClassDef.cpp:5: error: ‘cout’ was not declared in this scope
nestClassDef.cpp: In function ‘int main()’:
nestClassDef.cpp:10: error: expected unqualified-id before ‘.’ token
nestClassDef.cpp:11: error: ‘b’ was not declared in this scope
I am at a loss how to fix this. Any understanding shared will be thankfully received.

Answer 1

For the cout error: it's in std namespace, so use std::cout.
For The second error: B is not A's member, it's a nested type, so you have to use A::B b;

Answer 2

nestClassDef.cpp: In member function ‘void A::B::BTest()’:
nestClassDef.cpp:5: error: ‘cout’ was not declared in this scope
Use std::cout instead of cout, or add using namespace std; (probably after your #include statements).
nestClassDef.cpp: In function ‘int main()’:
nestClassDef.cpp:10: error: expected unqualified-id before ‘.’ token
nestClassDef.cpp:11: error: ‘b’ was not declared in this scope
Use A::B instead of A.B.

Answer 3

Add using namespace std; or use std::cout instead of just cout
Use A::B not A.B. Dot operator is used on objects or structs/unions.