Obscure C++ syntax

Question

Possible Duplicate:
C++ Comma Operator

About a year ago I noted some obscure syntax in a coding project I was working on:

table_value = table_index += 2, valueFromTable(table_index);

Does anyone recognise this?, it's like an assignment with an additional statement. This compiled in our entire suite of cross-platform compilers, so I'm pretty certain its valid C++ but I've never seen anything like it.

Any insight would be appreciated.

Gearoid

EDIT: heres some working code:

#include <iostream>
using namespace std ;

int valueFromTable(int a) { return a ; }

int main()
{
  int table_index = 0 ;
  int table_value = table_index += 2, valueFromTable(12);
  cout<<table_value<<endl;
  return 0 ;
}

Here is a fun interview question and answer: "What's wrong with int a[1]; a[1, 0] = -1;?" -> "The first index is out of bounds". — Johannes Schaub - litb, Nov 07 '10 at 14:23
Wow, thanks for the feedback y'all, I suppose I should've just searched for "comma c++", as google points to the answer immediately. — Gearoid Murphy, Nov 07 '10 at 14:25
litb, that's interesting but I'm confused. The -1 isn't actually assigned to the int at index 1 is it? So how can it be out of bounds? — CiscoIPPhone, Nov 07 '10 at 14:30
@litb: why not make the question even more weird: `a[1,0] = (-1,-2)` — Steve Jessop, Nov 07 '10 at 14:37

score 6 · Accepted Answer · answered Nov 07 '10 at 14:19

6

This is the Comma operator.
It's standard C and C++ but heavily frowned upon.

It evaluates both arguments, and returns the result of the second.

answered Nov 07 '10 at 14:19

SLaks

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Is this the same when used in `for` loops? `for (size_t i = 0; i < limit; ++i, ++some_other)` Just a qualification on the 'heavily frowned upon' part if so, ie. avoid when it's obfuscatory. – Steve Townsend Nov 07 '10 at 14:26
3

@Steve: "heavily frowned upon" doesn't mean "always a bad idea", it just means someone always thinks it's a bad idea. I call this the "passive-aggressive voice": "oh no, it's not *me* criticising *your* code. It's just that people don't like it, in general, when it happens". The kind of generalization that on Wikipedia attracts those little [Who?] tags ;-) – Steve Jessop Nov 07 '10 at 14:30

Obscure C++ syntax

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