Pros and cons of using nested C++ classes and enumerations?

Question

What are the pros and cons of using nested public C++ classes and enumerations? For example, suppose you have a class called printer, and this class also stores information on output trays, you could have:

class printer
{
public:
    std::string name_;

    enum TYPE
    {
        TYPE_LOCAL,
        TYPE_NETWORK,
    };

    class output_tray
    {
        ...
    };
    ...
};

printer prn;
printer::TYPE type;
printer::output_tray tray;

Alternatively:

class printer
{
public:
    std::string name_;
    ...
};

enum PRINTER_TYPE
{
    PRINTER_TYPE_LOCAL,
    PRINTER_TYPE_NETWORK,
};

class output_tray
{
    ...
};

printer prn;
PRINTER_TYPE type;
output_tray tray;

I can see the benefits of nesting private enums/classes, but when it comes to public ones, the office is split - it seems to be more of a style choice.

So, which do you prefer and why?

Answer 1

Nested classes

There are several side effects to classes nested inside classes that I usually consider flaws (if not pure antipatterns).

Let's imagine the following code :

class A
{
   public :
      class B { /* etc. */ } ;

   // etc.
} ;

Or even:

class A
{
   public :
      class B ;

   // etc.
} ;

class A::B
{
   public :

   // etc.
} ;

So:

• Privilegied Access: A::B has privilegied access to all members of A (methods, variables, symbols, etc.), which weakens encapsulation
• A's scope is candidate for symbol lookup: code from inside B will see all symbols from A as possible candidates for a symbol lookup, which can confuse the code
• forward-declaration: There is no way to forward-declare A::B without giving a full declaration of A
• Extensibility: It is impossible to add another class A::C unless you are owner of A
• Code verbosity: putting classes into classes only makes headers larger. You can still separate this into multiple declarations, but there's no way to use namespace-like aliases, imports or usings.

As a conclusion, unless exceptions (e.g. the nested class is an intimate part of the nesting class... And even then...), I see no point in nested classes in normal code, as the flaws outweights by magnitudes the perceived advantages.

Furthermore, it smells as a clumsy attempt to simulate namespacing without using C++ namespaces.

On the pro-side, you isolate this code, and if private, make it unusable but from the "outside" class...

Nested enums

Pros: Everything.

Con: Nothing.

The fact is enum items will pollute the global scope:

// collision
enum Value { empty = 7, undefined, defined } ;
enum Glass { empty = 42, half, full } ;

// empty is from Value or Glass?

Ony by putting each enum in a different namespace/class will enable you to avoid this collision:

namespace Value { enum type { empty = 7, undefined, defined } ; }
namespace Glass { enum type { empty = 42, half, full } ; }

// Value::type e = Value::empty ;
// Glass::type f = Glass::empty ;

Note that C++0x defined the class enum:

enum class Value { empty, undefined, defined } ;
enum class Glass { empty, half, full } ;

// Value e = Value::empty ;
// Glass f = Glass::empty ;

exactly for this kind of problems.