Enum vs Strongly typed enum

I am a beginner in C++ programming.

Today I come across a new topic: strongly typed enum. I've researched it a bit but till now I am unable to find out why do we need this and what is the use of the same?

For example if we have:

enum xyz{a, b, c};
/*a = 0, b = 1, c = 2, (Typical C format)*/

Why do we need to write:

enum class xyz{a, b, c};

What are we trying to do here? My most important doubt is how to use it. Could you provide a small example, which will make me understand.

Solution 1:

OK, first example: old-style enums do not have their own scope:

enum Animals {Bear, Cat, Chicken};
enum Birds {Eagle, Duck, Chicken}; // error! Chicken has already been declared!

enum class Fruits { Apple, Pear, Orange };
enum class Colours { Blue, White, Orange }; // no problem!

Second, they implicitly convert to integral types, which can lead to strange behaviour:

bool b = Bear && Duck; // what?

Finally, you can specify the underlying integral type of C++11 enums:

enum class Foo : char { A, B, C};

Previously, the underlying type was not specified, which could cause compatibility problems between platforms. Edit It has been pointed out in comments that you can also specify the underlying integral type of an "old style" enum in C++11.

Solution 2:

There's a good article about enums at this IBM page, it's very detailed and well-written. Here are some important points in a nutshell:

The scoped enums solve most of the limitations incurred by regular enums: complete type safety, well-defined underlying type, scope issues, and forward declaration.

• You get type safety by disallowing all implicit conversions of scoped enums to other types.
• You get a new scope, and the enum is not anymore in the enclosing scope, saving itself from name conflicts.
• Scoped enums gives you the ability to specify the underlying type of the enumeration, and for scoped enums, it defaults to int if you choose not to specify it.
• Any enum with a fixed underlying type can be forward declared.

Solution 3:

Values of enum class is really of type enum class, not underlying_type as for C-enums.

enum xyz { a, b, c};
enum class xyz_c { d, f, e };

void f(xyz x)
{
}

void f_c(xyz_c x)
{
}

// OK.
f(0);
// OK for C++03 and C++11.
f(a);
// OK with C++11.
f(xyz::a);
// ERROR.
f_c(0);
// OK.
f_c(xyz_c::d);