[boost] nested BOOST_FOREACH and -Wshadow

Subject: [boost] nested BOOST_FOREACH and -Wshadow
From: Dustin Spicuzza (dustin_at_[hidden])
Date: 2009-02-06 16:24:24

Hey,

I really enjoy using BOOST_FOREACH on containers, saves a ton of typing.
However, one semi-annoying problem that I've ran into is that nesting
instances of BOOST_FOREACH causes the gcc argument -Wshadow to complain
quite a bit.

A useless example of this behavior is as follows:

#include <vector>
#include <iostream>
using namespace std;

#include <boost/foreach.hpp>

int main()
{
    vector < vector <int> > array;

    BOOST_FOREACH( vector<int>& i1, array)
        BOOST_FOREACH( int i2, i1)
            cout << i2;
}

And you get the following warnings:

g++ -Wshadow shadowtest.cpp -o shadowtest
shadowtest.cpp: In function ‘int main()’:
shadowtest.cpp:13: warning: declaration of ‘_foreach_col’ shadows a
previous local
shadowtest.cpp:12: warning: shadowed declaration is here
shadowtest.cpp:13: warning: declaration of ‘_foreach_cur’ shadows a
previous local
shadowtest.cpp:12: warning: shadowed declaration is here
shadowtest.cpp:13: warning: declaration of ‘_foreach_end’ shadows a
previous local
shadowtest.cpp:12: warning: shadowed declaration is here
shadowtest.cpp:13: warning: declaration of ‘_foreach_continue’ shadows a
previous local
shadowtest.cpp:12: warning: shadowed declaration is here

Now, I know that some people don't believe in getting rid of warnings
just for warnings sake, but the fix for this would be extremely trivial
and useful (given the amount of noise that it generates) -- just add a
new macro, BOOST_FOREACH_NESTED, which has a third parameter denoting
the level of nesting. That parameter would be token-pasted onto the end
of the variable name, and get rid of the warnings. One could define up
to N more macros of the form BOOST_FOREACH_NESTED_N that get rid of the
third parameter, by calling that macro appropriately. This wouldn't
break any existing code, and would allow those that choose to get rid of
the warnings.

There are a number of arguments against such a change: (a) -Wshadow is
gcc specific and isn't enabled by default, (b) If you don't like the
warning, don't nest BOOST_FOREACH statements, or don't use -Wshadow.
However, my feeling (and this is probably just a matter of personal
preference) is that given the triviality of the fix, theres no reason to
not fix it.

Granted, I could easily just make my own and stick it in a header file,
but it seems like it would make more sense to be in boost itself. I
could create a patch and post it if so desired.

Thanks!

Dustin

-- Innovation is just a problem away
-- 
Innovation is just a problem away