how to overload == operator to allow it to be used

问题:

I am trying to overload == operator to compare objects like below.

class A
{
    int a;
public:
    A(int x) { a = x; }
    bool operator==(const A& obRight)
    {
        if(a == obRight.a)
        {
            return true;
        }
        return false;
    }
};

int main()
{
    A ob(10), ob2(10), ob3(10);
    if(ob == ob2) // This equality comparison compiles fine.
        cout<<"Equal"<<endl;
     if(ob == ob2 == ob3) //This line doesn't compile as overloaded 
                          // == operator doesn't return object (returns bool)
           cout<<"Equal"<<endl;
}

As i described above, i am unable to do multiple object comparison in a single line
like if(ob == ob2 == ob3) using overloaded == operator through member function.

Should i overload using friend function ?

回答1:

No. You fundamentally misunderstood your operation.

if (ob == ob2 == ob3) =
if (ob == (ob2 == ob3)

Think about the types.

if (A == (A == A))
if (A == bool) // Oh dear, no == operator for bool!

You need to have

if ((ob == ob2) && (ob == ob3))
if ((A == A) && (A == A))
if (bool && bool) // fine

回答2:

As a rule you SOULD NOT DO THIS in real code.
As the usage is completely different from what other people are expecting. Unexpected things are non-intuitive, and non-intuitive makes the code hard to maintain (or understand) for somebody that is not familiar with the code base.

But as an academic exercise.

What you want is to get the operator == to return an object so that if it is used in another test it will do the test but if it is just left in a boolean context then it will auto convert to bool.

#include <iostream>
using namespace std;

template<typename T>
class Test
{
    public:
        Test(T const& v, bool s)
            :value(v)
            ,state(s)
    {}

    Test operator==(T const& rhs) const
    {
        return Test<T>(value, state && value == rhs);
    }
    operator bool() const
    {
        return state;
    }
    private:
        T const&    value;
        bool        state;
};

class A
{
    int a;
    public:
    A(int x) { a = x; }
    Test<A> operator==(const A& obRight) const
    {
        return Test<A>(*this, a == obRight.a);
    }
};

int main()
{
    A ob(10), ob2(10), ob3(14);
    if(ob == ob2) // This equality comparison compiles fine.
        cout<<"Equal"<<endl;
    if(ob == ob2 == ob3) 
        cout<<"Equal"<<endl;
}

回答3:

You can create a function like this

#include<stdarg.h>
template<class T>
bool equals(size_t num, T val,...)
{
    va_list arguments;

    va_start(arguments,num);

    for(size_t i = 0; i<num; ++i)
        if(val != va_arg(arguments, int))
        {
            va_end ( arguments );
            return false;       
        }
    va_end ( arguments );
    return true;
}

and use it

if(equals(3,ob1,ob2,ob3))
    //do some stuff here