c++ - Why comparing three variables together with == evaluates to false?

Question

The output of the following program is "they are not equal", but I'd expect "they are equal" as the three compared variables (x,y, and z) are equal. Why?

#include <iostream>

int main()
{
    int y, x, z;
    y = 3;
    x = 3;
    z = 3;

    if (x == y == z)
    {
        std::cout << "they are equal
";
    }
    else
    {
        std::cout << "they are not equal
";
    }
}

Answer 1

This is because of the way expression and types are evaluated.

Let's evaluate the leftmost ==

x == y ...

This evaluates to true. Let's rewrite the expression:

//  x == y
if (true   == z) {
    // ...
}

The true is a boolean value. A boolean value cannot be directly compared to an int. A conversion from the boolean to an integer must occur, and the result is 1 (yes, true == 1). Let's rewrite the expression to its equivalent value:

//  true
if (1    == z) {
    //    ^--- that's false
}

But z isn't equal to 1. That expression is false!

Instead, you should separate both boolean expressions:

if (x == y && y == z) {
    // ...
}