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[C++] enum class

Traditional C++ enum had several issues. To solve these problems, C++11 introduced a new feature called enum class. In this article, I will examine the problems with the traditional enum and how they are solved with enum class.

First, traditional enum could not be forward-declared. The reason was that if the values in the enumerator were unknown, it was impossible to determine their size. However, enum class is treated as int if an underlying type is not specified, assigning values outside the range of an int will raise a compilation error. If you want to use values outside the range of an int, you need to specify the underlying type.

Another problem with traditional enum was that the scope of enumerator names was not limited. Let's see the following example.

Here, we try to represent the results of IO and Parse functions with enums. However, this code will not compile because the Error and Ok of IOResult conflict with those of ParseResult. To resolve this issue, you can change the enumerator names, or use namespaces.

However, with enum class, the names of enumerators are limited to the scope of the enum class, so there is no need for such verbose code.

Most importantly, the biggest problem with traditional enum was that they were weakly typed and could be implicitly converted to integer types. However, enum class do not allow implicit conversion to integer types. If you try to use an enum class as an int, you will encounter a compilation error. You need to use static_cast to explicitly cast it.

As explained above, traditional enum cannot be forward-declared, their enumerator names are not limited in scope, and they can be implicitly converted to integer types. For now, enum class are the correct approach in most case.

Note: This article is a translation of a Korean post written in 2015. If you want to read the original, please refer to this link.

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