How make a class private inside a namespace in C++?

Question

How make a class private inside a namespace in C++ and prevent other to access the class from outside the namespace, for instance:

namespace company{
    class MyPublicClass{ ... }  // This should be accessible 
    class MyPrivateClass{ ... } // This should NOT be accessible
}

Answer 1

You can use unnamed namespace. Names that appear in an unnamed namespace have internal linkage but names that appear in a named namespace and don't have any storage class specifier, by default, have external linkage.

For example we have a file named source1.cpp:

//source1.cpp
namespace company{
    class MyPublicClass{ ... };
    namespace{
        class MyPrivateClass{ ... };
    }
}
void f1(){
company::MyPrivateClass m;
}

MyPublicClass has external linkage and MyPrivateClass has internal linkage. So there is no problem with source1 and it can be compiled as a library ,but if we have other file named source2.cpp:

//source2.cpp
void f2(){
company::MyPrivateClass m;
}

source2 can't be linked with the compiled source1 and we can not refer to MyPrivateClass.