为什么boost :: bind坚持将`boost :: placeholders`放到全局名称空间中? [英] Why boost::bind insists pulling `boost::placeholders` into global namespace?
问题描述
以下代码可以轻松修复，但很烦人.
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
}
有一个宏 BOOST_BIND_NO_PLACEHOLDERS ，但是使用此宏也会带来一些弊端，例如导致 boost :: placeholders 从包含的编译单元中消失< boost/bind.hpp> ，但不包含< boost/bind/placeholders.hpp> .
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>.
名称冲突也与其他库一起出现，例如 boost :: mpl ，我不认为维护人员不知道问题所在，但我想知道为什么他们坚持不弃用和在< boost/bind.hpp> 中使用命名空间boost :: placeholders 删除.
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>.
推荐答案
新版本的boost中似乎已解决了该问题.
Looks like it has been fixed in newer versions of boost.
当包含 boost/bind.hpp 时，我们会收到以下消息:
When including boost/bind.hpp we get this message:
#pragma消息:不建议在全局名称空间中声明绑定占位符(_1，_2，...)的做法.请使用< boost/bind/bind.hpp>+使用命名空间boost :: placeholders，或定义BOOST_BIND_GLOBAL_PLACEHOLDERS来保留当前行为.
该解决方案在 https://www.boost中进行了描述.org/doc/libs/1_73_0/boost/bind.hpp
因此，好的做法"解决方法是代替
So the "good practice" fix is to instead of
#include< boost/bind.hpp> 这将boost :: placeholders放在了全局命名空间中
#include <boost/bind.hpp>
which puts the boost::placeholders in global namespace
做
#include< boost/bind/bind.hpp> 这不会将boost ::占位符放在全局名称空间中.然后直接使用限定名称，例如 boost :: placeholders :: _ 1 ，或在本地使用名称空间boost :: placeholders
#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
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