c++和boost 1.71 - error:引用' _1 '是不明确的

Question

我使用的是websocketcpp和boost 1.71库。该代码用于boost 1.58，但在升级两个库后，它将无法编译。c++编译器为g++ (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0，代码如下:

using boost::property_tree::ptree;
using boost::property_tree::read_json;
using boost::property_tree::write_json;
using websocketpp::connection_hdl;
using websocketpp::lib::placeholders::_1;
using websocketpp::lib::placeholders::_2;
using websocketpp::lib::bind;
//.......
{
srv.init_asio();
srv.set_reuse_addr(true);
srv.set_open_handler(bind(&WsServer::on_open, this, websocketpp::lib::placeholders::_1));
srv.set_close_handler(bind(&WsServer::on_close, this, websocketpp::lib::placeholders::_1));
srv.set_message_handler(bind(&WsServer::on_message, this, websocketpp::lib::placeholders::_1, websocketpp::lib::placeholders::_2));
}

我用#define BOOST_BIND_NO_PLACEHOLDERS排序。但是，这会给出以下错误:

/usr/include/boost/bind/bind.hpp:319:35: error: no match for call to ‘(boost::_mfi::mf1<void, boost::property_tree::json_parser::detail::standard_callbacks<boost::property_tree::basic_ptree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> > >, char>) (
boost::property_tree::json_parser::detail::standard_callbacks<boost::property_tree::basic_ptree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> > >&, std::_Placeholder<1>&)’
319 |         unwrapper<F>::unwrap(f, 0)(a[base_type::a1_], a[base_type::a2_]);
|         ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//...
/usr/include/boost/bind/mem_fn_template.hpp:176:25: note:   template argument deduction/substitution failed:
usr/include/boost/bind/mem_fn_template.hpp:184:7: note: candidate: ‘R boost::_mfi::mf1<R, T, A1>::operator()(T&, A1) const [with R = void; T = boost::property_tree::json_parser::detail::standard_callbacks<boost::property_tree::basic_ptree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> > >; A1 = char]’
184 |     R operator()(T & t, A1 a1) const
|       ^~~~~~~~
/usr/include/boost/bind/mem_fn_template.hpp:184:28: note:   no known conversion for argument 2 from ‘std::_Placeholder<1>’ to ‘char’
184 |     R operator()(T & t, A1 a1) const

Answer 1

就像大家说的那样，不要using namespace。

另外，使用最近的boost，你应该避免使用BOOST_BIND_GLOBAL_PLACEHOLDERS，而不是<boost/bind.hpp>，包括<boost/bind/bind.hpp>(默认情况下不再这样做)。

这是您看到的冲突的来源，因为property_tree使用boost bind(如错误消息所示)。

完全不相关:不要(ab)使用PropertyTree，如果它是一个JSON库。可以使用Boost JSON或其他JSON库。

---

同样，无关的是，您不需要所有的usings。您已经可以使用ADL来减少类型:

#include <boost/property_tree/json_parser.hpp>
#include <iostream>
int main()
{
boost::property_tree::ptree pt;
std::istringstream str(R"({"hello":"world"})");
read_json(str, pt);
write_json(std::cout, pt);
}

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