在类内部的结构中定义枚举

Question

I have seen people writing code like this... whats the point of it..

class Test 
{
public:

    struct MethodEnum
    {
        enum Enum
        {
             Method1,
             Method2
        };
    };
};

Answer 1

In pre-C++11, the labels of enum are not scoped (which means the labels were accessible without any qualification with the enum type!). So to make them scoped , programmers wrote those code. But in C++11, it is not needed, as you can define scoped-enum, using enum class .

So in C++11, your code would look like this:

class Test 
{
public:
    enum class MethodEnum
    {
             Method1,
             Method2
    };
};

Answer 2

If there is more than one enumerated type defined in the same scope and two of those enumerated types have enumerations with the same name you get a conflict. This technique puts the names into separate scopes, possibly avoiding such a conflict by putting the names into separate scopes. That way, in member functions of Test you'd refer to the enumerators as MethodEnum::Method1 , etc.

In C++11 you can get scoped names with enum class .

Answer 3

Sometimes, you don't want the enumerators to pollute the surrounding scope; for example

enum Colour {
    Red,
    Yellow,
    Orange
};

enum Fruit {
    Apple,
    Physalis,
    Orange    // ERROR! already defined
};

Your example is the old-fashioned way to enclose them in a scope; these days, we have scoped enumerations ( enum class ) to do that more conveniently.

Answer 4

It sets logical (or semantic) linkage between MethodEnum and the enumeration. So for example you can write

switch ( SomeExpression )
{
   case Test::MethodEnum::Method1
      // do some method1
      break;

   case Test::MethodEnum::Method2
      / do some method2
      break;

   default:
      std::cout << "There is no such a method" << std::endl;
      break;
}