Re: Comma operator in for loop

From:

Vladimir Jovic <vladaspams@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Fri, 24 Sep 2010 15:30:43 +0200

Message-ID:

<i7i965$md4$1@news.albasani.net>

Ralf Goertz wrote:

Vladimir Jovic wrote:

Victor Bazarov wrote:

On 9/24/2010 5:13 AM, Ralf Goertz wrote:

I guess I don't really understand the concept of commas as sequence
points but now I am in a position where I might have use for the comma
in a statement other than a declaration. Is the following legal:

#include<iostream>
#include<set>

using namespace std;

int main() {
     set<unsigned> s;
     s.insert(42);
     s.insert(84);
     set<unsigned>::iterator i;
     for (i=s.begin(),++i;i!=s.end();++i)
         cout<<*i<<endl;
     return 0;
}

It compiles and does what I want, but it could still be undefined
behaviour. So is it okay to use ",++i" before the first ";" of the
for-loop, or is it saver to check for i!=s.begin() in the *body* of the
loop?

It seems fine, and if it does what you need, keep it. If you just
needed to skip the first element, I'd probably do

i = ++(s.begin());

I would do this:
for (i=s.begin() + 1;i!=s.end();++i)
cout<<*i<<endl;
but it's not much different from your solution.

That was my first idea, but it doesn't compile:

error: no match for 'operator+' in 's.std::set<_Key, _Compare, _Alloc>:
:begin [with _Key = unsigned int, _Compare = std::less<unsigned int>, _Alloc = s
td::allocator<unsigned int>]() + 1'

Yes, off course. set::begin() returns bidirectional iterator, which do
not support arithmetic operations + and - :(