C++ Boost http post request

Question

I have the read the Boost documentation and manage to run a get request: https://www.boost.org/doc/libs/1_77_0/libs/beast/doc/html/beast/quick_start/http_client.html

However I unable to figure out how to make a post request. I have tried the following stackoverflow posts but none of them work for me:

Boost ASIO HTTP client POST

HTTP POST request using boost::asio

Boost asio custom HTTP server reading HTTP post requests

BOOST ASIO POST HTTP REQUEST -- headers and body

Would someone be able to provide a full example with post similar to the first link?

Thank you

Here is what I tried:

#include <boost/beast/core.hpp>
#include <boost/beast/http.hpp>
#include <boost/beast/version.hpp>
#include <boost/asio/connect.hpp>
#include <boost/asio/ip/tcp.hpp>
#include <cstdlib>
#include <iostream>
#include <string>

namespace beast = boost::beast;     // from <boost/beast.hpp>
namespace http = beast::http;       // from <boost/beast/http.hpp>
namespace net = boost::asio;        // from <boost/asio.hpp>
using tcp = net::ip::tcp;           // from <boost/asio/ip/tcp.hpp>

// Performs an HTTP GET and prints the response
int main(int argc, char** argv)
{
  try
  {
    // Check command line arguments.
    if(argc != 4 && argc != 5)
    {
      std::cerr <<
        "Usage: http-client-sync <host> <port> <target> [<HTTP version: 1.0 or 1.1(default)>]\n" <<
          "Example:\n" <<
            "    http-client-sync www.example.com 80 /\n" <<
              "    http-client-sync www.example.com 80 / 1.0\n";
      return EXIT_FAILURE;
    }
    auto const host = argv[1];
    auto const port = argv[2];
    auto const target = argv[3];
    int version = argc == 5 && !std::strcmp("1.0", argv[4]) ? 10 : 11;
    
    // The io_context is required for all I/O
    net::io_context ioc;
    
    // These objects perform our I/O
    tcp::resolver resolver(ioc);
    beast::tcp_stream stream(ioc);
    
    // Look up the domain name
    auto const results = resolver.resolve(host, port);
    
    // Make the connection on the IP address we get from a lookup
    stream.connect(results);
    
    // Set up an HTTP GET request message
    http::request<http::string_body> req{http::verb::post, target, version};
    req.set(http::field::host, host);
    req.set(http::field::user_agent, BOOST_BEAST_VERSION_STRING);
    
    std::string body = "a=4&b=3";
    req.body() = body;
    req.set(http::field::content_type, "application/x-www-form-urlencoded;charset=UTF-8");
    
    // Send the HTTP request to the remote host
    http::write(stream, req);
    
    // This buffer is used for reading and must be persisted
    beast::flat_buffer buffer;
    
    // Declare a container to hold the response
    http::response<http::dynamic_body> res;
    
    // Receive the HTTP response
    http::read(stream, buffer, res);
    
    // Write the message to standard out
    std::cout << res << std::endl;
    
    // Gracefully close the socket
    beast::error_code ec;
    stream.socket().shutdown(tcp::socket::shutdown_both, ec);
    
    // not_connected happens sometimes
    // so don't bother reporting it.
    //
    if(ec && ec != beast::errc::not_connected)
      throw beast::system_error{ec};
    
    // If we get here then the connection is closed gracefully
  }
  catch(std::exception const& e)
  {
    std::cerr << "Error: " << e.what() << std::endl;
    return EXIT_FAILURE;
  }
  return EXIT_SUCCESS;
}

In command line I run:

$ ./http-client-sync.o localhost 8000 /sum 1.0
Error: end of stream

It should return [7]

$ curl --data "a=4&b=3" "http://localhost:8000/sum"
[7]

I created a web api as described here: https://www.rplumber.io/