将 boost beast http 请求转换为字符串并写入 ostringstream

Question

我正在尝试使用 boost beast 进行 http 调用，并希望在写入套接字之前先编写它；我尝试使用 ostringstream 来获取 request 的值以在打印的日志中获取它并得到以下错误消息：

string verb = "POST";
using http::field;
http::request<http::string_body> request;
request.method_string(verb);
request.target(server_endpoint);
request.version(11);
request.set(field::host, hostname);
request.set(field::accept, "*/*");
request.set(field::authorization, authorization_token);
request.set(field::user_agent, client_name);
request.set(field::content_length, req_str.length());
request.set(field::content_type, "application/x-www-form-urlencoded");
request.set(field::connection, field::close);
request.body() = req_str;
request.prepare_payload();
fast_ostringstream oss;
oss <<"Request message" << request;
PBLOG_INFO(oss.str());
oss.clear();



HttpTracRequestHandler.cpp:78:26: error: invalid operands to binary expression ('framework::string_::fast_ostringstream' and
      'http::request<http::string_body>' (aka 'boost::beast::http::message<true, boost::beast::http::basic_string_body<char,
      std::char_traits<char>, std::allocator<char> >, boost::beast::http::basic_fields<std::allocator<char> > >'))
        oss <<"Request message" << request;
        ~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~
/BARXPBViewstore/GIT_Workspace/barx_rel/mq8_barx/barx-server/appl/include/FoxException/GenericException.h:133:19: note: candidate function
      [with TData = boost::beast::http::message<true, boost::beast::http::basic_string_body<char, std::char_traits<char>,
      std::allocator<char> >, boost::beast::http::basic_fields<std::allocator<char> > >] not viable: no known conversion from
      'framework::string_::fast_ostringstream' to 'GenericException &' for 1st argument
GenericException& operator<<(GenericException &e, const TData& data)
                  ^
/BARXPBViewstore/GIT_Workspace/boost/boost_1_70_0/boost/beast/http/impl/write.hpp:921:1: note: candidate function [with isRequest = true,
      Body = boost::beast::http::basic_string_body<char, std::char_traits<char>, std::allocator<char> >, Fields =
      boost::beast::http::basic_fields<std::allocator<char> >] not viable: no known conversion from 'framework::string_::fast_ostringstream'
      to 'std::ostream &' (aka 'basic_ostream<char> &') for 1st argument
operator<<(std::ostream& os,
^
/BARXPBViewstore/GIT_Workspace/boost/boost_1_70_0/boost/beast/http/write.hpp:721:1: note: candidate function [with isRequest = true, Fields
      = boost::beast::http::basic_fields<std::allocator<char> >] not viable: no known conversion from
      'framework::string_::fast_ostringstream' to 'std::ostream &' (aka 'basic_ostream<char> &') for 1st argument
operator<<(std::ostream& os,
^
/BARXPBViewstore/GIT_Workspace/boost/boost_1_70_0/boost/beast/http/impl/write.hpp:901:1: note: candidate function [with Fields =
      boost::beast::http::basic_fields<std::allocator<char> >] not viable: no known conversion from 'framework::string_::fast_ostringstream'
      to 'std::ostream &' (aka 'basic_ostream<char> &') for 1st argument

Answer 1

您的代码看起来很正常。事实上，它适用于标准字符串流：

Live On Coliru

#include <boost/beast/http.hpp>
#include <iostream>
#include <sstream>

namespace http = boost::beast::http;

#define PBLOG_INFO(a) do { std::clog << (a) << std::endl; } while(0);

int main() {
    std::string verb = "POST", req_str = "payload";
    using http::field;
    http::request<http::string_body> request;
    request.method_string(verb);
    request.target("server_endpoint");
    request.version(11);
    request.set(field::host, "hostname");
    request.set(field::accept, "*/*");
    request.set(field::authorization, "authorization_token");
    request.set(field::user_agent, "client_name");
    request.set(field::content_length, req_str.length());
    request.set(field::content_type, "application/x-www-form-urlencoded");
    request.set(field::connection, field::close);
    request.body() = req_str;
    request.prepare_payload();

    {
        std::ostringstream oss;
        oss <<"Request message " << request;
        PBLOG_INFO(oss.str());
        oss.clear();
    }
}

打印

Request message POST server_endpoint HTTP/1.1
Host: hostname
Accept: */*
Authorization: authorization_token
User-Agent: client_name
Content-Type: application/x-www-form-urlencoded
Connection: Close
Content-Length: 7
payload

有什么问题？

问题似乎是 Beast 的流式操作符重载不适用于 fast_ostringstream 的任何类型。这是设计使然：

  no known conversion from 'framework::string_::fast_ostringstream'
  to 'std::ostream &' (aka 'basic_ostream<char> &')

作者fast_ostringstream 显然是在作弊，因为它可能很快，但绝对不是ostream& :)。

我试图通过创建自己的作弊流来得到类似的错误：

struct my_cheat_stream {
    using Device = boost::iostreams::back_insert_device<std::string>;
    using Stream = boost::iostreams::stream<Device>;

    std::string buf;
    Stream _impl { buf };

    template <typename T> my_cheat_stream& operator<<(T&& rhs) {
        _impl << std::forward<T>(rhs);
        return *this;
    }

    std::string&&    str()&& { _impl.flush(); return std::move(buf); }
    std::string_view str()&  { _impl.flush(); return buf; }

    void clear() {
        buf.clear();
        _impl.clear(std::ios::failbit|std::ios::badbit);
    }
};

但是，it simply compiles，因为很明显，operator<< <T>() 只适用。也许你可以相应地修复你的fast_ostringstream，或者使用其他东西。

肮脏的解决方法：

你可以使用上面的反例作为杠杆：

template <typename Stream> struct wrap_non_std_stream {
    Stream& _wrapped;
    explicit wrap_non_std_stream(Stream& ref) : _wrapped(ref) {}

    template <typename T> auto& operator<<(T const& rhs) {
        _wrapped << rhs;
        return *this;
    }

    auto& operator<<(std::ostream&(*manip)(std::ostream&)) {
        _wrapped << manip;
        return *this;
    }
};

现在应该可以了

    fast_ostringstream oss;
    wrap_non_std_stream woss{oss};
    woss << "Request message " << request;

如果底层流支持，它也将支持操纵器：

    woss << std::endl << std::fixed << std::boolalpha << std::flush;