How do I create a list of lists?

Problem

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I try to create a list of lists like this: RWTPtrDlist<RWTPtrDlist> xyz; but the compiler responds with errors indicating problems with operator==.

 

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Cause

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Any type used to parameterize RWTPtrDlist (and RWTValDlist, RWTPtrSlist and RWTValSlist as well) must have an operator== defined: int T::operator == (const T&) const. The class RWTPtrDlist (and RWTValDlist, etc) does not implement the == operator, thus parameterizing RWTPtrDlist with another may cause compiler errors. Some compilers insist on having a matching definition for every declaration in a templatized class, even if the method or operator is never used. Other compilers will ignore the lack of implementation and compile the code successfully.

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Action

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The solution is to derive from the list class and provide an == operator:

 template  class Dlist :    public RWTPtrDlist 
{    
   public:       
   int operator == (const T& list) const       
   {   return 0;   } 
}; 

and then create the list of lists:

RWTPtrDlist< Dlist > xyz;

or

RWTValDlist< Dlist > xyz;

The return value from Dlist::operator== can be constant as it is above, or you can fully implement the test for equality if you wish.