validate integer is some enum class item (C++11)

Question

i have some enum class

enum class Foo { A=1, B=18 , Z=42 };

i want to check if some integer can be converted into a Foo. What would be the ideal way to do this? this is for runtime check (the integer is not known yet at compile-time)

Obviously i can do this the hard way (write a function bool CheckEnum(Foo); with a big-ass switch returning true for all cases except the default one), but i was hoping a more elegant mechanism that avoided so much writing. MPL or Boost.Preprocessor would be a perfectly acceptable solution, but one of which i sadly know very little about

Answer 1

Ok i'm a little bit fed up with this issue (some of my enums are nearly 100 items) so i decided to tackle it with code generation, which might not be everyone's cup of tea, but i've realised that is really no such a big deal.

Basically i went for Python Cog, which allows me to embed python snippets inside comments in my .h and .cpp files and auto-generate code. I use it basically like a really smart, imperative macro system:

i added the following to Test.h

/*[[[cog
#----------- definitions

import cog

def createCategoryConstants( enumVar , bitShift ):
    categoryIndex = 0
    for cat in enumVar:
        cog.outl(' const unsigned int %s_op_mask = (%d << %d); ' %(cat[0] , categoryIndex , bitShift))
        categoryIndex += 1
    cog.outl('\n\n')

def createMultiCategoryEnum( enumVar , enumTypename ):
    cog.outl(' enum class %s { ' % enumTypename )
    categoryIndex = 0
    for i in enumVar:
        itemIndex = 0
        catName = 'NotExpected'
        remainingCategories = len(enumVar)- categoryIndex - 1
        for j in i:
            if (itemIndex == 0):
                catName = j
                itemIndex = 1
                continue
            enumItemIndex = 0
            for enumItem in j:
                remainingEnums = len(j) - enumItemIndex - 1
                currentLine = ' %s = %s_op_mask | %d ' %(enumItem, catName, enumItemIndex)
                if (remainingCategories != 0 or remainingEnums != 0):
                    currentLine += ' , '
                cog.outl(currentLine)
                enumItemIndex += 1
            itemIndex += 1
        cog.outl('') #empty line to separate categories
        categoryIndex += 1
    cog.outl(' };\n\n')

def createIndexFromEnumFunction( enumVar , enumTypename , functionName ):
    cog.outl('uint32_t %s(%s a) { \n switch (a)\n {' % (functionName , enumTypename) )
    absoluteIndex = 0
    for cat in enumVar:
        elemInCat = 0
        for i in cat:
          if elemInCat != 0:
             for enumItem in i:
               cog.outl('case %s:' % enumItem)
               cog.outl(' return %d; \n' % absoluteIndex)
               absoluteIndex += 1
          elemInCat += 1
    cog.outl(' } \n } \n\n ')


def createMultiEnum( enumVar , enumTypename ):
    createCategoryConstants( enumVar , 4)
    createMultiCategoryEnum( enumVar , enumTypename )
    createIndexFromEnumFunction( enumVar , enumTypename , 'FromOpToIndex' )

#------------- generation

multiEnum =[ ['CatA', ['A1', 'A2' , 'A3_foo']] , ['CatSuper8' , ['Z1_bla' , 'Z10' , 'Z11']] ]

createMultiEnum( multiEnum , 'multiFooEnum')

]]]*/
//[[[end]]]

Then i added cog invocation in my Makefile pre-build step:

.build-pre:
# Add your pre 'build' code here...
    python /usr/local/bin/cog.py -I../../../tools/cog/ -r *.h

And the results show up just below:

]]]*/
 const unsigned int CatA_op_mask = (0 << 4); 
 const unsigned int CatSuper8_op_mask = (1 << 4); 



 enum class multiFooEnum { 
 A1 = CatA_op_mask | 0  , 
 A2 = CatA_op_mask | 1  , 
 A3_foo = CatA_op_mask | 2  , 

 Z1_bla = CatSuper8_op_mask | 0  , 
 Z10 = CatSuper8_op_mask | 1  , 
 Z11 = CatSuper8_op_mask | 2 

 };


uint32_t FromOpToIndex(multiFooEnum a) { 
 switch (a)
 {
case A1:
 return 0; 

case A2:
 return 1; 

case A3_foo:
 return 2; 

case Z1_bla:
 return 3; 

case Z10:
 return 4; 

case Z11:
 return 5; 

 } 
 } 


//[[[end]]]

So, now my enum validation is about making sure the code generation (invoked at compile time) is done correctly

Answer 2

A solution to this problem is to ditch the enums, and replace it with some arrays which are created using XMACROs.

Answer 3

There is no "ideal" way to do it. All ways are going to have to involve some manual work.

You need to create a data structure that contains all of the acceptable values. Then search that data structure with the runtime value you need. A std::set or std::unordered_set would be adequate for this purpose.

Your main difficulty will be in maintaining that list, as it will need to be updated every time you change your enum class.