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

问题

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>.

回答1:

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

回答2:

You can use

#define BOOST_BIND_NO_PLACEHOLDERS

before including other Boost headers.

I don't know when this was introduced, only that it works in 1.67. Feel free to edit with more accurate information.

来源：https://stackoverflow.com/questions/53203970/why-boostbind-insists-pulling-boostplaceholders-into-global-namespace

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c++

boost

namespaces