#ifndef _SAFE_BOOL_H_#define _SAFE_BOOL_H_// copied from the The Safe Bool Idiom article by Bjorn Karlsson// http://www.artima.com/cppsource/safebool3.htmlclass safe_bool_base {public: //protected typedef void (safe_bool_base::*bool_type)() const; void this_type_does_not_support_comparisons() const {} safe_bool_base() {} safe_bool_base(const safe_bool_base&) {} safe_bool_base& operator=(const safe_bool_base&) {return *this;} ~safe_bool_base() {}};template <typename T> class safe_bool : public safe_bool_base {public: operator bool_type() const   {   return (static_cast<const T*>(this))->boolean_test() ? &safe_bool_base::this_type_does_not_support_comparisons : 0;  }protected:  ~safe_bool() {}};  template <typename T, typename U> void operator==(const safe_bool<T>& lhs,const safe_bool<U>& rhs) { lhs.this_type_does_not_support_comparisons(); return false;}template <typename T,typename U> void operator!=(const safe_bool<T>& lhs,const safe_bool<U>& rhs) { lhs.this_type_does_not_support_comparisons(); return false;}  #endif