safe_bool.hpp

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#ifndef SAFE_BOOL_HPP
#define SAFE_BOOL_HPP SAFE_BOOL_HPP

/**
   Taken from http://www.artima.com/cppsource/safebool3.html
   See there for more information.
*/
class safe_bool_base {
  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=void> class safe_bool : public safe_bool_base {
public:
    operator bool_type() const {
      return (static_cast<const T*>(this))->boolean_test()
          ? &safe_bool<T>::this_type_does_not_support_comparisons 
          : 0;
    }
protected:
    ~safe_bool() {}
};

template<> class safe_bool<void> : public safe_bool_base {
 public:
    operator bool_type() const {
      return boolean_test()==true ? 
          &safe_bool<void>::this_type_does_not_support_comparisons 
          : 0;
    }
 protected:
    virtual bool boolean_test() const=0;
    virtual ~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 // SAFE_BOOL_HPP

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