C ++中的逗号运算符

Question

Below is the code.

#include<iostream>

using namespace std;

class x {
    int a;
public :
    x(int t=2):a(t) {}
    void print (){
        cout <<"value is "<<a;
    }
    x& operator,(x&a){
        return *this;
    }
};

int main(){
    x a(1),b(2),c(3),d(4);
    x t=(a,b,c,d);
    t.print();
    return 0;
}

output value is 1
please explain why the value is not 4 in this line xt=(a,b,c,d);

Answer 1

x t = (a,b,c,d);

No matter what order this expression is evaluated in, the left-most operand will always be returned because this in your x& operator , (x &instance) refers to the left operand while instance refers to the right.

It is thus returning a and you are getting a printed value of 1 .

If you didn't overload the comma operator, you may get 4 because an expression like (a, b, c) will return the right-most operand.