enum class instead of unscoped enum – EasyHack

Since C++11 when enum class (also named scoped enum) is introduced, it is preferred to plain enum which is inherited from C programming languages. The task here is to convert the old enum instances to enum class.

Rationale

enum class has many benefits when compared to plain enum, as it provides better type safety among other things. Implicit conversion to integers, lack of ability to define the underlying data type and compatibility issues were some of the problems with plain enum that enum class solved in C++11. Although since then enum has improved and one can specify underlying type in the scoped enumerations.

Plain enums pollute namespace, and you have to pick names that are too long, and have to carry the context inside their names. For example: INETMSG_RFC822_BEGIN inside enum _ImplINetRFC822MessageHeaderState. With an enum class, it is simply written as HeaderState::BEGIN. When placed inside a file/class/namespace that makes it relevant, it is much easier to use: it is more readable, and causes no issues for other identifiers with possible similar names.,

See this change:

• 593f08303 convert enum _ImplINetRFC822MessageHeaderState to enum class

You can read more about that in:

• Why is enum class considered safer to use than plain enum?

Finding Instances

You may find some of the instances with:

$ git grep -w enum *.cxx *.hxx|grep -v "enum class"

When you count it with wc -l, it shows something more than 2k instances.

Examples Commits

You can see some of the previous conversions here, which is around 1k changes:

$ git log --oneline -i -E --grep="convert enum|scoped enum"

This is a good, but lengthy example of such a conversion:

• 9072c5c855 convert SbxFlagsBits to scoped enum

Implementation

First of all, please choose good names for the new enum class and values. For example, you may convert APPLICATION_WINDOW_TITLE into Application::WindowTitle. Therefore, do not use the old names as they were.

Converting enum to enum class is not always straightforward. You should try to understand the code using the enum, and then try to replace it with enum class. You may need to add extra state/values for situations where 0 or -1 or some default value was used. There are cases where a numerical value is used for different conflicting purposes, and then you have to do some sort of conflict resolution to separate those cases.

You may end up modifying more and more files, and a few static_casts where they are absolutely necessary because you are interpreting some integer value read from input. These are the places where you should check the values yourself in the code. You have to make sure that the numerical value is appropriate before casting it to the enum class.

If you want to do bitwise operations, you should use o3tl::typed_flags, for example:

enum class FileViewFlags
{
    None = 0x00,
    MultiSelection = 0x02,
    ShowType = 0x04,
    ShowNone = 0x20,
};

template<> struct o3tl::typed_flags : o3tl::is_typed_flags<FileViewFlags, 0x26> {}

Then, you may use it like this:

    if (nFlags & FileViewFlags::MULTISELECTION)
        mxTreeView->set_selection_mode(SelectionMode::Multiple);

Please note that 0x26 is the mask, and is calculated by applying OR over all possible values. All the values must be non-negative.

Final Notes

This is a simple development task for LibreOffice also known as EasyHack, which is filed in Bugzilla as tdf#168771. These small tasks are defined to help newcomers to LibreOffice development community to improve their skills with LibreOffice coding.

You may find other instances related to C++ here:

• TDF Wiki: List of EasyHacks