Nested classes

[LB]

http://stackoverflow.com/a/14607402/1959975

Do you think you will use nested classes more often for this? This certainly makes a lot of things easier for my code.

[helios]

Nope. I only nest simple classes leaning towards the POD side of things.

[LB]

Huh, I've got some complex relationships that are simplified by this (mainly it means I don't have to use friend declarations any longer).

[Cheraphy]

I'm with helios on this one. I've found that when I'm nesting a class that is anything but simple, I'm usually doing something wrong and need to rethink the structure of my program.

[MiiNiPaa]

The same result could be achieved by use of friend class declaration. So there is little actual use of nesting classes. And I think they looks a little messy.

[JLBorges]

Nested classes are used to encapsulate pure implementation-support types. This is relatively unimportant in amateur software; in a large production code base, proliferation of unencapsulated implementation types is something one would want to avoid. The classic example being the fully insulated concrete class idiom.

#include <memory>

struct A
{
    A() ;
    // ....
    private:
        struct implementation ;
        std::unique_ptr<implementation> impl ;
};

If a pure implementation-support-type is not a nested non-public type, it should be placed in a nested namespace with a tell-tale name mandated by the style-guide.

namespace A_stuff
{
    namespace detail { struct implementation ; }

    struct A
    {
        A() ;
        // ....
        private:
            std::unique_ptr<detail::implementation> impl ;
    };
}

This is obviously even more important for types which are compile-time polymorphic (or where inline access from client code is required for performance reasons); ie. where a compilation firewall is (as in the fully insulated concrete class) not a possibility.