Can (a==1)&&(a==2)&&(a==3) evaluate to true? (and can it be useful?)

Question

Inspired by another question regarding java-script language. Can the expression

 (a==1)&&(a==2)&&(a==3)

evaluate to true i

Answer 1

Could be somewhat useful.

#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

struct Foo {
    std::vector<int> v = {1,2,3};
};

bool operator==(const Foo& foo, int i) {
    return std::any_of(foo.v.begin(), foo.v.end(), [=](int v){ return v == i; });
}

int main() {
    Foo a;

    if (a==1 && a==2 && a==3)
        cout << "Really??" << endl;
    return 0;
}

Answer 2

Operator overloading and macros are trivial solutions to such a riddle.

And if so, can it actually be useful?

Well, with some imagination... One possible use case I can think of are unit tests or integration tests in which you want to make sure that your overloaded operator== for some class works correctly and you know for sure that it works incorrectly if it reports equality for different operands when it's not supposed to do that:

class A {
public:
    A(int i);
    bool operator==(int i) const;
    // ...
};

A a(1);

if ((a==1)&&(a==2)&&(a==3)) {
    // failed test
    // ...
}