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c++templatesc-preprocessorpreprocessor-directive

C++ #elif directive is being discarded

I am trying to create a type declaration based on boundaries 

template<class B>
struct IntDecl {

enum {
    L = B::_l, U = B::_u
};
 #if (L >=0 && U <=255)
  typedef char Type;
 #elif (L>=0&&U<=65535) 
  typedef unsigned int Type;
 #endif

};

So, as you see here depending on the value of L and U the type will be defined. for example 

IntDecl< BOUND < 0, 65535 > >::Type i; // this should be a unsigned int 
IntDecl< BOUND < 0, 255 > >::Type i1;  // this should be a char

The problem is , both ( i and i1) are considered chars, in other words the #elif is being discarded. any help? why #elif is not executing?

Solution

  • The preprocessor pass happens before semantic analysis and enum is a semantic construct. You need to use templates to implement this, or make L and U macros that define preprocessor constants.