Why int r = (int, int, int); doesn't give error and r has the value of the last integer?

Question

r has always the last integer between the parenthesis.

What is the output of the following program?

#include <iostream>
using namespace std;
main() {
    int i = 1, j = 2, k = 3, r;
    r = (i, j, k);
    cout << r <<endl;
}

Possible answers:

A - 1
B - 2
C - 3
D - Compile Error

What I want to know is why does this happen.

[Comma operator](https://en.wikipedia.org/wiki/Comma_operator) — BoBTFish, Mar 07 '16 at 21:04
@juanchopanza So then it's not a duplicate, at least not of that question. — David Schwartz, Mar 07 '16 at 21:05
The short answer to the question in title: because this is how the `C` language was designed to work (and `C++` inherited this behaviour). — axiac, Mar 07 '16 at 21:05
Your title shows a declaration `int r = (int, int, int);` (assuming the three `int`s refer to expressions rather than the type name), but the code in your question uses an assignment `r = (i, j, k)`. The answer happens to be the same in both cases, but it's best to be precise and consistent. — Keith Thompson, Mar 07 '16 at 21:06
Perhaps you should peruse this list of resources for learning C++ http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list?lq=1 — Rob K, Mar 07 '16 at 21:10
@BoBTFish I think it's not his code. It looks like a question from somewhere. — HolyBlackCat, Mar 07 '16 at 21:18
Note to @thiagowfx you should have fixed the indentation as well. — user4581301, Mar 07 '16 at 21:27
Possible duplicate of [How does the Comma Operator work](http://stackoverflow.com/questions/54142/how-does-the-comma-operator-work) — jww, Mar 07 '16 at 21:32

score 4 · Accepted Answer · answered Mar 07 '16 at 21:04

4

Therefore r will be given the value of k in this statement

r = (i, j, k);

answered Mar 07 '16 at 21:04

Ed Heal

1 Answers1