understanding STATIC_CHECK in Loki library

Question

     template<int> struct CompileTimeError;

    template<> struct CompileTimeError<true> {};


    #define STATIC_CHECK(expr,msg)  {CompileTimeError< ((expr)!=0) > Error_##msg; (void)Error_##msg; }

    template <class To , class From>
    To safe_reinterpret_cast(From from)
    {
        STATIC_CHECK(sizeof(From) <= sizeof(To),Destination_Type_Too_Narrow);
        return reinterpret_cast<To>(from);
    }

    void main()
    {
        void *p= NULL;
        char c= safe_reinterpret_cast<char>(p);
}

Above code works fine and gives compile time error when we try to convert pointer to char . But its not very clear how STATIC_CHECK macro works.

As per above code it should lead to following

STATC_CHECK(false,Destination_Type_Too_Narrow)

Above macro will get expanded as follows:

CompileTimeError<false>
ERROR_Destination_Type_Too_Narrow;
(void)ERROR_Destination_Type_Too_Narrow;

In above macro I am not able to understand what these two statements are meant for

 ERROR_Destination_Type_Too_Narrow;
    (void)ERROR_Destination_Type_Too_Narrow;

If anyone having clear understanding please explain

Answer 1

We have specialization for class CompileTimeError<true>, that has default constructor. Instanciations of other cases will cause error, that CompileTimeError<not true> is undefined type (in your case we trying to create variable ERROR_Destination_Type_Too_Narrow of type CompileTimeError<false>). (void)VariableName is only silence of -Wunused