Why boost::bind insists pulling `boost::placeholders` into global namespace?

The following code can be fixed easily, but quite annoying.

#include <functional>
#include <boost/bind.hpp>
void foo() {
  using namespace std::placeholders;
  std::bind(_1, _2, _3); // ambiguous
}

There's a macro BOOST_BIND_NO_PLACEHOLDERS, but using this macro will also bring some drawbacks like causing boost::placeholders disappear from the compile unit included <boost/bind.hpp> but not included <boost/bind/placeholders.hpp>.

The name conflicts also comes with other libs like boost::mpl, I don't think the maintainers don't know the problem, but I want to know why they insist on not deprecating and deleting using namespace boost::placeholders in <boost/bind.hpp>.

Solution

Looks like it has been fixed in newer versions of boost.

When including boost/bind.hpp we get this message:

#pragma message: The practice of declaring the Bind placeholders (_1, _2, ...) in the global namespace is deprecated. Please use <boost/bind/bind.hpp> + using namespace boost::placeholders, or define BOOST_BIND_GLOBAL_PLACEHOLDERS to retain the current behavior.

The solution is described in https://www.boost.org/doc/libs/1_73_0/boost/bind.hpp

So the "good practice" fix is to instead of

#include <boost/bind.hpp> which puts the boost::placeholders in global namespace

#include <boost/bind/bind.hpp> which does not put the boost:: placeholders in global namespace. Then use the qualified names like boost::placeholders::_1 directly, or locally do using namespace boost::placeholders