2023-08-26 - handling enums in C++

handling enums in C++ is tricky. but there are good practices that can help you out. here are two from the set.

let's say we have a simple enum called Foo, with some values. we want to add a str() helper, that would covert it to sth more human-readable than a number. very common implementation would be:

enum class Foo { A, B, C };
 
// ...
 
auto str(Foo f)
{
  switch(f)
  {
    case Foo::A: return "A";
    case Foo::B: return "B";
    case Foo::C: return "C";
    default: return "<unknown>";
  }
}

looks good, all tests pass. however there is a potential for a problem – it's not future proof. let's now add Foo::D:

enum class Foo { A, B, C, D }; // notice Foo::D is added!
 
// ...
 
auto str(Foo f)
{
  switch(f)
  {
    case Foo::A: return "A";
    case Foo::B: return "B";
    case Foo::C: return "C";
    default: return "<unknown>";
  }
}

looks good, all tests pass… however now there _is_ a problem! new value is not handled and it will return an incorrect value at runtime!

there's a neat way of fixing this problem and making code future-proof. compile with -Wall -Werror (as you anyway should!) and make sure no switch has any default. eg.:

auto str(Foo f)
{
  switch(f)
  {
    case Foo::A: return "A";
    case Foo::B: return "B";
    case Foo::C: return "C";
  }
  return "<unknown>";
}

change looks insignificant, but now if we miss handling 1 value, we get a warning from a compiler and with -Werror it stops the build right away.

c.cpp: In function ‘auto str(Foo)’:
c.cpp:11:9: error: enumeration value ‘D’ not handled in switch [-Werror=switch]
   11 |   switch(f)
      |         ^
cc1plus: all warnings being treated as errors

bug is detected at compile time! nice… :)