Guides: Flags

What are they?

Flags provide a type-safe way of storing OR-combinations of enum values. Flags<Enum> is a template class, where Enum is an user defined enum type that holds a list of the appropriate flag values.

The traditional C++ approach for storing OR-combinations of enum values is to use an int or uint variable. The inconvenience with this approach is that there's no type checking at all; any enum value can be OR'd with any other enum value and passed on to a function that takes an int.

The technique here I found in the Qt library. It provides type safe checking of enum arguments (you cannot mix enum definitions arbitrarily without causing a compiler error). Usually the flags are independant - you can have any combination of the enum values currently configured. However with some masking tricks (see QLabel's setAlignment method) it is possible to group subsets of the enum so that only one flag from each subset can be triggered at any one point in time.

Usage

CompilingLinking

Include the following at the top of any translation unit that requires compilation of signals and/or slots.

 #include <ycl/utilities.hpp>

 using YCL::Utilities::Flags;

Enum Declaration

Some care must be taken when defining the enum for use with this Flags class. The enum must classify each argument as an independant bit. For example,

 enum Setting
 {
     Fast   0x0001,
     Slow   0x0002,
     Red    0x0010,
     Blue   0x0020
 };

Alternatively, the shift operator can also be used.

Flag Declaration

It is easiest to declare the enum with a name corresponding to the type of flag it represents and then pluralise this with a convenient typedef when declaring the Flags class associated with it. For example,

 enum Setting
 {
      ... as above ....
 };

 typedef Flags<Setting> Settings;

functions

There immediately exists a helper function that allows you to pipe further flags to the current flag container.

 settings = settings|Fast|Slow;

A further convenience can be had if a simple friend piping operator is written alongside the definition of your enum:

 Flags<Setting> operator|(Setting flagOne, Setting flagTwo) { return Flags<Setting>(flagOne) | flagTwo; }

This then enables you to configure flags very simply with: Thanks to the helper (friend) functions, it is very simple to initialise a Flags variable.

     Settings settings(Fast|Slow)
     settings = Fast|Slow;

Note, it would be convenient if a very general template such as the piping operator above could be included with the library. To date however, the only method I know of is to define a very general template operator function. This actually works with g++, however msvc9 does not like it, and its probably for good reason. Such a template operator in the global namespace will inevitably clash with other friend operators and result in ambiguities.

Combinations

Combinations can be made illegal through customised accessors.

 class A
 {
     public:
         void configure(Settings s)
         {
             if ( ( s.testFlag(Gold) && s_.testFlag(Dark) ) ||
                  ( s_.testFlag(Gold) && s.testFlag(Dark) ) ||
                  ( s.testFlag(Gold) && s.testFlag(Dark) ) )
             {
                 cout << "This combination is not permitted." << endl;
                 return;
             } else {
                 s_ = s;
             }
         }
         Settings settings() { return s_; }

     private:
         Settings s_;
 };