p1099R0
Using Enum

1. Motivation

Consider an enum class:

enum class rgba_color_channel { red, green, blue, alpha };

Currently, a switch using this enum looks as follows:

std::string_view to_string(rgba_color_channel channel) {
  switch (channel) {
    case rgba_color_channel::red:   return "red";
    case rgba_color_channel::green: return "green";
    case rgba_color_channel::blue:  return "blue";
    case rgba_color_channel::alpha: return "alpha";
  }
}

The necessary repetition of the class enum name reduces legibility by introducing noise in contexts where the enum class is obvious.

To eliminate the noise penalty for introducing long (but descriptive) enum class names, this paper proposes that the statement

using enum rgba_color_channel;

introduce the enum member identifiers into the local scope so that they may be referred to unqualified.

Furthermore, the syntax

using rgba_color_channel::red;

should bring the identifier red into the local scope, so that it may be used unqualified.

The above example would then probably be written as

std::string_view to_string(rgba_color_channel channel) {
  switch (my_channel) {
    using enum rgba_color_channel;
    case red:   return "red";
    case green: return "green";
    case blue:  return "blue";
    case alpha: return "alpha";
  }
}

2. Rationale

2.1. Consistency

enum classes are not classes - they seem to be closer to namespaces consisting of static constexpr inline variables. The familiar using syntax that works for namespaces should therefore apply to them as well.

2.2. Better Identifiers

The introduction of this feature would allow better naming of enums. Currently, enums are named with as short an identifier as possible, often to the point of absurdity, when they are reduced to completely nondescriptive abbreviations that only hint at their proper meaning (just what does zfqc::add_op mean?)

With this feature, identifiers become available to unqualified lookup in local contexts where their source is obvious, giving control of lookup style back to the user of the enum, instead of baking name semantics into the type of the enum.

2.3. Evidence of Need

3. Proposal

4. using enum IDENTIFIER

We propose the addition of a new using enum statement:

using enum IDENTIFIER;

The above statement introduces the members of enum IDENTIFIER into the local namespace, thus enabling lookup without qualification. As usual, a name lookup ambiguity makes the program ill-formed (diagnostic required).

5. using ENUM_ID::IDENTIFIER

We propose to allow the syntax of

using ENUM_ID::IDENTIFIER

to introduce the IDENTIFIER into the local namespace, aliasing ENUM_ID::IDENTIFIER.

6. Acknowledgements

The authors would like to thank Marcel Ebmer and Lisa Lipincott for early feedback.