Question

What’s the best alternative to BOOST_FOREACH when you want to iterate over std::set?

When I try and do what seems to be a reasonable method for iterating over a set I get compile errors. This code:

set<string> nameSet;
string aName;
BOOST_FOREACH(nameSet, aName) {
  cout << aName << endl;
}

generates this compile error with gcc 4.4.3:

error: no match for ‘operator=’ in ‘nameSet = boost::foreach_detail_::deref [with T = std::basic_string<char, std::char_traits<char>, std::allocator<char> >, C = mpl_::bool_<false>](((const boost::foreach_detail_::auto_any_base&)((const boost::foreach_detail_::auto_any_base*)_foreach_cur70)), 0u)’

According to this link the two aren’t compatible because std::set::iterator must allow mutation of the value it refers to.

I can think of a few ham fisted methods, but am wondering if there is a cleaner way to iterate over the set.

Answer 1

Works for me. Maybe you have the parameters to BOOST_FOREACH reversed.

#include <set>
#include <string>
#include <iostream>
#include <boost/foreach.hpp>

int main(int ac, char **av) {

  std::set<std::string> nameSet(av+1, av+ac);

  BOOST_FOREACH(const std::string& aName, nameSet) {
    std::cout << aName << "\n";
  }

  BOOST_FOREACH(std::string aName, nameSet) {
    std::cout << aName << "\n";
  }

  std::string aName;
  BOOST_FOREACH(aName, nameSet) {
    std::cout << aName << "\n";
  }
}

---

According to this link the two aren’t compatible because std::set::iterator must allow mutation of the value it refers to.

That’s not really what that link says. What you cannot do, and what that link says won’t work, is this:

BOOST_FOREACH(std::string& aName, nameSet) {
}

Because you cannot form a non-const reference from a const expression. (Namely, each the const members of a std::set.)