I want to use Main/Overpass API for retrieving data in c++ program.

This is a minimal working example in plain C/C++ with only minimal error checking and no response processing:

#include <iostream>
#include <string>
#include <vector>
#include <cstdio>
#include <arpa/inet.h>
#include <sys/socket.h>

const std::string host  = "81.19.81.199"; // IP of overpass.osm.rambler.ru
const int port          = 80;
const std::string query = "GET /cgi/interpreter?data=node%5Bname%3DGielgen%5D%3Bout%3B HTTP/1.1\r\n"
                          "Host: overpass.osm.rambler.ru\r\n"
                          "User-Agent: SteveC\r\n"
                          "Accept: */*\r\n"
                          "Connection: close\r\n"
                          "\r\n";

int main(int argc, char* argv[]) {
    int sock = socket(AF_INET, SOCK_STREAM, 0);
    if (sock == -1) {
        perror("error opening socket"); return -1;
    }

    struct sockaddr_in sin;
    sin.sin_port = htons(port);
    sin.sin_addr.s_addr = inet_addr(host.c_str());
    sin.sin_family = AF_INET;

    if (connect(sock, (struct sockaddr *)&sin, sizeof(sin)) == -1) {
        perror("error connecting to host"); return -1;
    }

    const int query_len = query.length() + 1; // trailing '\0'
    if (send(sock, query.c_str(), query_len, 0) != query_len) {
        perror("error sending query"); return -1;
    }

    const int buf_size = 1024 * 1024;
    while (true) {
        std::vector<char> buf(buf_size, '\0');
        const int recv_len = recv(sock, &buf[0], buf_size - 1, 0);

        if (recv_len == -1) {
            perror("error receiving response"); return -1;
        } else if (recv_len == 0) {
            std::cout << std::endl; break;
        } else {
            std::cout << &buf[0];
        }
    }

    return 0;
}

Example output:

HTTP/1.1 200 OK
Server: nginx/1.0.9
Date: Mon, 24 Dec 2012 11:45:34 GMT
Content-Type: application/osm3s+xml
Connection: close
Content-Length: 1893
Access-Control-Allow-Origin: *

<?xml version="1.0" encoding="UTF-8"?>
<osm version="0.6" generator="Overpass API">
<note>The data included in this document is from www.openstreetmap.org. The data is made available under ODbL.</note>
<meta osm_base="2012-12-24T11:44:01Z"/>

  <node id="371597317" lat="50.7412721" lon="7.1927120">
    <tag k="is_in" v="Bonn,Regierungsbezirk Köln,Nordrhein-Westfalen,Bundesrepublik Deutschland,Europe"/>
    <tag k="name" v="Gielgen"/>
    <tag k="place" v="suburb"/>
  </node>

[cropped]

</osm>

I strongly suggest using a third-party library for the network/HTTP code and you will probably also want to use a third-party library for parsing the XML/JSON/whatever response.

A much simpler option is to use a scripting language like Python, Ruby, or - if you like camels - Perl.