Question

I have a general macro:

#define mSwitch( Root, Case )  Root##_Case_##Case

#define mSpecialDisplay( what, Val )  mSwitch(mSpecialDisplay,what)(Val)
#define mSpecialDisplay_Case_Int(Val)    ...do stuff
#define mSpecialDisplay_Case_Float(Val)  ...do stuff
...more special cases

how do I guarantee that the variable Case is fully expanded before it gets pasted in mSwitch?

It works fine if mSwitch is passed a literal value, but if there are several layers of indirection, or intermediary operations, mSwitch ends up pasting one of those before they get fully expanded.

I’m using MSVC 2005.

Is there a simple way to make sure a parameter is fully expanded before pasting is done?

Thanks

---

well, it isn’t that hard to give an example maybe:

  #define mMDebugInfo( ... ) mMDebugExp( mMDebugInfo_( 0, __VA_ARGS__ ) )

  #define mMDebugInfo_( C, ... ) mMAritize( mMSwitch( mMDebugInfo, mMMetaTrait( Detect, __VA_ARGS__ ) ), (C, __VA_ARGS__) )

  #define mMDebugInfoRep( C, ... ) mMXP##C( mMDebugInfo_ )mMXP##C((mMIInc(C),__VA_ARGS__)) //(mMExpDo(mMGlue( mM, C)##DebugInfo_(mMIInc(C),__VA_ARGS__))

  #define mMDebugInfo1( C, ... ) mMAritize( mMSwitch( mMDebugInfo, mMMetaTrait( Detect, __VA_ARGS__ ) ), (mMIInc(C), __VA_ARGS__) )

  #define mMDebugInfo_Case_Nil(...) [Nil]

  #define mMDebugInfo_Case_CntArgs(C,I,...) 
mMDebugInfoRep(C,I),mMDebugInfoRep(C,__VA_ARGS__)
  #define mMDebugInfo_Case_PrnNull(C,I) [()]

  #define mMDebugInfo_Case_Prn(C,I)   ( mMDebugInfoRep(C,mMDPrn(I)) )

  #define mMDebugInfo_Case_ActFn(C,I) mMAritize( mMDebugInfo_Case_Fn, (C, I, mMTrait_Fn_mM##I) )

  #define mMDebugInfo_Case_PassFn(C,I) mMAritize( mMDebugInfo_Case_Fn, (C, mMTrait_Fn_mM##I) )

  #define mMDebugInfo_Case_Fn( C,Name, Reg, ArgCnt, PArgs ) [Name:ArgCnt]( mMAritize( mMSwitch( mMDebugInfo_Case_Fn, ArgCnt ), (C, mMDPrn( PArgs ) )) )

  #define mMDebugInfo_Case_Fn_Case_V(C, _1, ...) mMDebugInfoRep(C, _1), mMDebugInfoRep(C, __VA_ARGS__)

  #define mMDebugInfo_Case_Fn_Case_0(...) [Nil]

  #define mMDebugInfo_Case_Fn_Case_1(C, _1, ...) mMDebugInfoRep(C, _1)

  #define mMDebugInfo_Case_Fn_Case_2(C, _1, _2, ...) mMDebugInfoRep(C, _1), mMDebugInfoRep(C, _2)

  #define mMDebugInfo_Case_Fn_Case_3(C, _1, _2, _3, ...) mMDebugInfoRep(C, _1), mMDebugInfoRep(C, _2), mMDebugInfoRep(C, _3)  

  #define mMDebugInfo_Case_Fn_Case_4(C, _1, _2, _3, _4, ...) mMDebugInfoRep(C, _1), mMDebugInfoRep(C, _2), mMDebugInfoRep(C, _3), mMDebugInfoRep(C, _4)

  #define mMDebugInfo_Case_Int(C,I)   [Num:I]

  #define mMDebugInfo_Case_Digit(C,I) [Dig:I] 

  #define mMDebugInfo_Case_Bool(C,I)  [Bin:I]

  #define mMDebugInfo_Case_CCode(C,I) [CCd:I]

  #define mMDebugInfo_Case_UToken(C,I) [UT:I]

this is debug code that has no problems recursively parsing nested expressions like:

DebugInfo( BInt( BNot( IAdd(4,BNot(IAdd(6,7)) ) ) ) ); 
"

which yields:

"[BInt:1]( [BNot:1]( [IAdd:2]( [Dig:4], [BNot:1]( [IAdd:2]( [Dig:6], [Dig:7] ) ) ) ) )"

The macro functions in the example expression are in inactive form. The problem is happening when I activate the form – the parse chain for the individual arguments can get arbitrarily long and they aren’t getting resolved completely before they are getting used.

Answer 1

This is the usual idiom for this:

#define mSwitch(Root, Case) mSwitch_(Root, Case)
#define mSwitch_(Root, Case) Root##_Case_##Case

All of the arguments to a C preprocessor macro are fully expanded before the macro itself is expanded, unless the # or ## operator is applied to them; then they’re not expanded. So to get full expansion before ##, you pass the arguments through a wrapper macro that doesn’t use ##.