运算符重载C ++（x == y == z）

Question

I have encountered a strange problem, that I don't know how to solve in overloading. Im trying too overload operator == . The easy example would be:

class bla{
public:
int a;
void bla(int a):a(a){}//costructor
bool operator==(const bla& ob)
     return (a==ob.a);//chk if equal
};
void main(){
bla A,B;  
if (A==B)
   flag=1;//just an example...

this is easy and works great, i'm trying to handle the case of:

   if (A==B==C==D)

so i need to return type object and now type bool. i have tried to add another function :

bla &bla:: operator==(const bool answer){//as a member function
if (answer)
    return *this;

but it doesn't seem to help. have any suggestions? thx Stas

Answer 1

This is a horrible idea and you should never do it.

Here is how you do it.

#include <type_traits>

template<typename T, typename U>
struct decay_equiv
  : std::is_same<
    typename std::decay<T>::type,
    typename std::decay<U>::type
  >::type {};

template<typename T>
struct comparator {
  bool res;
  T passalong;
  explicit operator bool() { return res; }
  typename std::enable_if<
    !decay_equiv<T, comparator<T> >::value,
    comparator<T>
  >::type operator==(T const& rhs);
  comparator<T> operator==(comparator<T> const& rhs);
};

template<typename T>
typename std::enable_if<
  !decay_equiv<T, comparator<T>>::value,
  comparator<T>
>::type comparator<T>::operator==(T const& rhs) {
  if (!res) {
    return {res, rhs};
  }
  return {(passalong == rhs).res, rhs};
}

template<typename T>
comparator<T> comparator<T>::operator==(comparator<T> const& rhs) {
  if (!res || !rhs.res) {
    return {res, rhs};
  }
  return {(passalong == rhs.passalong).res, rhs.passalong};
}

struct bla {
  int a;
  comparator<bla> operator==(bla const& rhs);
  comparator<bla> operator==(comparator<bla> const& rhs);
};

comparator<bla> bla::operator==(bla const& rhs) {
  return {a == rhs.a, rhs};
}

comparator<bla> bla::operator==(comparator<bla> const& rhs) {
  if (!rhs.res) {
    return rhs;
  }
  return {a == rhs.passalong.a, rhs.passalong};
}

int main() {
  bla a = {0},b = {0},d = {0};
  if (a==b==d)
    return 0;
  return -1;
}

This code assumes C++11, but can be written in C++98 with only very minor changes.

Answer 2

i have managed to find a different approach and it seems to work great. thx to u all for your help and suggestions.

class bla{
public:
int a;
bool f;
void bla(int a):a(a){f = true;}//costructor
operator bool(){return f};
bla& operator==(const bla &ob){//chk if equal
    if (f)
       f=(a==ob.a);
     return *this;
}
};
void main(){
bla A(4),B(4),C(4),D(4);  
if (A==B==C==D)
   flag=1;//just an example...