Tell us about the 'enum class' (scoped enums) mechanism in C++. How does it differ from regular enumerations and why was it introduced? What common mistakes occur when transitioning from old enums to enum class?

Answer

In C++11, the 'scoped enums' mechanism was introduced — the keyword enum class.

Main differences from classic enums:

• The contents of an enum class have their own scope, preventing pollution of the global namespace.
• There is no implicit conversion of enumerators to int (explicit casting is required).
• The default type of enum class is int, but another type can be specified (for example, enum class MyE : uint8_t { ... };).

Why is it needed: To enhance type safety, prevent naming conflicts, erroneous comparisons, and provide explicit control over storage.

Example:

enum Color { Red, Green };
Color c = Red;        // Regular enum, Red is globally visible
enum class State { Off, On };
State s = State::On;  // Need to specify State::On

You cannot write:

enum class State { Off, On };
int x = State::On; // Error! Only with a cast
int y = static_cast<int>(State::Off); // OK

Trick Question

Why can’t you perform a logical comparison between values of different enum classes, even if they have the same names and underlying types?

Answer:

Type safety: each enum class is a separate type, even if the underlying type is the same and the enumerators match in name. The compiler will treat them as different types and will not allow implicit comparisons.

Examples of real errors due to unawareness of the nuances of the topic.

---

Story

When transitioning from enum to enum class in a machine control project, they forgot to add explicit casting when logging values. Logs began to show "foreign" values (did not recognize State::On as a number), complicating debugging.

---

Story

In a REST service, after migrating from a regular enum to an enum class, the code compared values of different enumerations from different enum classes. The compiler did not allow this, and automated tests stopped building.

---

Story

In an auditing system, developers confused the old and new syntax — sometimes accidentally declaring enum class, but using enumerators without qualification, resulting in the wrong code compiling or linkage errors due to name collisions.