comma operator with c++11 for loop

Question

I wanted at first to try if it was allowed to do something like :

vector<int> a{1, 2};
vector<int> b{3, 4};

for(auto ai : a, auto bi : b)
{

}

This didn't work but I was expecting it because of the size constraint.

However, I was surprised that this didn't work either :

vector<int> b{3, 4};

for(int x = 1, auto bi : b)
{

}

Isn't the comma operator meant to resolve every side effect of its left-side before going on the right side ?

Use `zip` method of boolinq library :) https://github.com/k06a/boolinq — k06a, Nov 19 '13 at 11:15
The comma operator has nothing to do with this. This is a _declaration_. — Lightness Races in Orbit, Nov 19 '13 at 11:33
@Arpegius you are right. It is not yet implemented in 2.0. My mistake... — k06a, Nov 19 '13 at 12:15
Well, I found solution for this [sequence-zip-function-for-c11](http://stackoverflow.com/questions/8511035/sequence-zip-function-for-c11). — Arpegius, Nov 19 '13 at 12:21

score 4 · Accepted Answer · answered Nov 19 '13 at 11:10

4

In the second case you have a declaration. Declarations can contain declarations of multiple variables, separated by a comma.

What you're writing in the second case is equivalent to the following declaration

int x = 1, auto bi;

for (...) ...

And the reason that the first didn't work is not because of any size constraint, but because of the syntax of a range for loop simply doesn't allow you to do something like that.

answered Nov 19 '13 at 11:10

Some programmer dude

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Okay! But `for(auto ai = a.begin(), bi : b)` should work then ? (I know, it is weird and dangerous!) – Jean-Michaël Celerier Nov 19 '13 at 11:13
@Jean-MichaëlCelerier No, the syntax doesn't allow that either. Please follow the link and read about how a range-based for loop might be implemented and you will understand why. – Some programmer dude Nov 19 '13 at 11:14

score 2 · Answer 2 · answered Nov 19 '13 at 11:13

The comma operator is an operator. Which means it can only be used in expressions. int x = 1, auto bi : b is definitely not an expression. So you cannot really use it there.

It looks a bit like a malformed declaration. Malformed because you're trying to change the type being declared.

Note that the grammar of the language actually uses a special nonterminal for-range-declaration for the piece on the left-hand side of :. Which eventually resolves to a single declaration. So it's illegal to declare more than one variable in the range-based for loop.

Okay, I wanted to know if there was some special construct in the grammar for this case precisely! — Jean-Michaël Celerier, Nov 19 '13 at 11:14

comma operator with c++11 for loop

2 Answers2