Re: How to have two cases with the same value?

"James Kanze" <>
8 Jan 2007 23:32:16 -0500
Roy Smith wrote:

Section of ISO 14882 says, "No two of the case constants in
the same switch shall have the same value". Surprisingly, this is causing
me grief right now.

I'm using a function which is documented to return one of a set of symbolic
constants. The problem is, it's not guaranteed that all the symbols have
distinct values. Let's say FOO and BAR are two of the values.
Semantically, they mean the same thing, but they may or may not have the
same numeric value (and whether they do or not changes on different
versions of the library). Yes, this is brain-dead, but I didn't design the

I want to write something like:

int err = f();
switch (err) {
   case FOO:
   case BAR:

There are actually many cases, each with a pair of symbols which similarly
may or may not have the same value. For the curious, this is a Microsoft
implementation of an RFC, and the two sets of possible return values are
the RFC-specified symbols and Microsoft's own Winsock error codes.

The compiler complains that I've got two cases with the same value (in
violation of Is there any portable way to deal with this other
than a long chain of if/else's:

int err = f();
if (err == FOO || err == BAR) {
} else...

About the only thing I can think of is to use some sort of a
map. Something like:

    typedef void (*ErrHandler)() ;
    typedef std::map< ErrEnum, ErrHandler >
                        HandlerMap ;
    struct HanderMapInit
        int value ;
        ErrHandler handler ;
                            operator HandlerMap::value_type() const
            return HandlerMap::value_type( value, handler ) ;
    } ;

    HanderMapInit const initTable[] =
        { FOO, &doBlah },
        { BAR, &doBlah },
        // ...
    } ;
    HandlerMap const map( begin( initTable ), end( initTable) ) ;

    // ...

                        err = map.find( f() ) ;
    (*(err != map.end() ? *err : doDefault))() ;

Or you might stop with the initTable, and just use std::find.
Simpler, less memory, and no order of initialization problems.
Either way, it seems like a lot of work for very little, but I
don't know of any other solution off-hand. (Note that it
requires the handling for each case to be a separate function,

James Kanze (Gabi Software) email:
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