Re: Comma operator in for loop

Victor Bazarov <v.bazarov@comcast.invalid>
Fri, 24 Sep 2010 09:30:04 -0400
On 9/24/2010 9:11 AM, 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:


using namespace std;

int main() {
      set<unsigned> s;
      set<unsigned>::iterator i;
      for (i=s.begin(),++i;i!=s.end();++i)
      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

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)
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'

The set iterator is not of the random-access kind, you cannot add a
number to it.

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