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Typesafe C++ Enum with ToString() and FromString()

, 13 Nov 2010
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An enum with macro + template magic, providing type safety and type info

Introduction

C++ enums lack some features that coders often need. For example, the value of the last member to be able to iterate through the possible values of an enum. This problem is usually solved by adding a "last" member to the enum:

enum MyEnum
{
  MyEnum_member1,
  MyEnum_member2,
  MyEnum_Last
};

Another thing that causes headaches is the conversion between an enum value and its string representation. Most people, especially beginners, end up with something like this to be able to perform the conversion:

static const char* ENUM_STRINGS[MyEnum_Last] =
{
  "MyEnum_member1",
  "MyEnum_member2",
};

The code attached to this article provides a solution to these problems. It has been tested with VS2008 and CygWin g++ 4.5.0.

Using the Code

Using the code is simple; just include the SmartEnum.h file in your project (maybe in your stdafx.h), and then declare your enums with the macros provided in SmartEnum.h, like this:

BEGIN_SMART_ENUM(MyEnum)
  ENUM_MEMBER(member1)
  ENUM_MEMBER(member2)
END_SMART_ENUM(MyEnum)

You can place this enum declaration anywhere, inside class declarations and namespaces as well as you can do with C++ enums. With the above enum declaration, the following code snippets demonstrate how this enum solves the problems that are present when using C++ enums.

Using your enum members:

 MyEnum e = MyEnum::member1;

Iterating through the members of the enum:

for (MyEnum i=MyEnum::FIRST; i<MyEnum::LAST; ++i)
    printf("%d. MyEnum::%s\n", (int)i, i.ToString());

Now you also have some predefined constants and methods in the MyEnum "namespace", these are:

MyEnum::FIRST
MyEnum::LAST == MyEnum::COUNT == MyEnum::INVALID

You also have some useful methods:

static const char* MyEnum::ToString(const MyEnum& e);
       const char* MyEnum::ToString() const;
static MyEnum MyEnum::FromString(const char* s);
static bool MyEnum::IsValid(const MyEnum& e);
       bool MyEnum::IsValid() const; 
// pre- and postfix -- and ++ operators required to iterate

Other useful methods can be added that are required by your project (e.g.: Serialize).

A cool thing is that this enum is automatically initialized to MyEnum::INVALID so as to make it easier to avoid the common error of leaving an enum uninitialized.

When a program has many enums with similar member names, programmers can easily make the mistake of comparing two enum values of different types. This results in syntactically valid but buggy C++ code that actually compiles, maybe with a warning message. A C++ enum can also be compared with C++ integers. A smart enum submitted in this article can normally be compared only to the same type; you have to cast it to a primitive integral type if you want to do otherwise.

Disadvantages

The solution has some limitations; let's go through them:

  • The value of the enum is stored in a primitive C++ type (char) that makes debugging more difficult. To overcome this, I put in a (const char*) that always points to the name of the member when compiled in debug mode, thus making the size of the enum different in Debug/Release builds.
  • The underlying template metaprogram limits the number of enum members to 32.
  • These macros generate code that compile much slower than a normal C++ enum.
  • Currently, FromString() is O(n); it could be made O(log(n)). Doing so would bring on thread safety issues, so for now, I've chosen this simpler solution.
  • The members cannot have custom values as with regular C++ enums.

History

  • 13 November, 2010: Initial release

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

pasztorpisti
Software Developer (Senior) CRYTEK
Hungary Hungary
No Biography provided

Comments and Discussions

 
GeneralMy vote of 5 PinmemberHarrison H15-Dec-10 4:44 
I like the idea, but I think it'd be even cooler if the initializer for the enums would allow for a string representation that's different than the exact wording of the variable declaration. Your metaprogramming flies WAY over my head!
GeneralRe: My vote of 5 Pinmemberpasztorpisti15-Dec-10 12:13 
GeneralMy vote of 4 PinmemberJoxemi17-Nov-10 23:17 
NewsEnum to String and Vice Versa in C++ PinmemberFrancisXavier17-Nov-10 19:00 
GeneralRe: Enum to String and Vice Versa in C++ Pinmemberpasztorpisti18-Nov-10 0:58 
GeneralRe: Enum to String and Vice Versa in C++ PinmemberFrancisXavier22-Nov-10 1:34 
GeneralRe: Enum to String and Vice Versa in C++ Pinmemberpasztorpisti22-Nov-10 1:55 
GeneralMy vote of 2 Pinmemberbobyx8216-Nov-10 2:02 
GeneralYour choice of 'constant' names PinmemberStefan6315-Nov-10 22:27 
GeneralRe: Your choice of 'constant' names Pinmemberpasztorpisti16-Nov-10 0:58 
GeneralMy vote of 5 PinmemberKirill Kovalev14-Nov-10 21:05 

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