C++ - Unresolved external symbol error [duplicate]

I've been searching around and have come to a stop with this thing.

I basically get these errors here:

1>Client.obj : error LNK2019: unresolved external symbol "class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __cdecl HTTP_POST_REQUEST(char *,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >)" (?HTTP_POST_REQUEST@@YA?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@PADV12@1@Z) referenced in function "public: virtual void __thiscall Client::Send(void)" (?Send@Client@@UAEXXZ)
1>G:\Development\C++\Client\Debug\Client.exe : fatal error LNK1120: 1 unresolved externals

To my knowledge I think it's that Client.obj is not getting compiled with the program (don't quote me on this)? But I have read that it might be some linking problems or something like that. To me it seems that I've done everythign right (but clearly I haven't).

Client.cpp

#include <iostream>
#include <string>
using namespace std;

#include "HttpUtils.h"
#include "Client.h"

Client::Client() {
    cout << "Works!" << endl;
}

void Client::Send() {
    string res = HTTP_POST_REQUEST("localhost", "/test.php", "data1=data&data2=data");
    cout << res << endl;
}

Client.h

#pragma once

#include <string>
using namespace std;

#ifndef CLIENT__H
#define CLIENT__H

class Client {

public:
    Client();

    virtual void Send();
};
#endif

main.cpp

#include <iostream>
#include <string>
using namespace std;

#include "Client.h"

int main(int argc, char *argv[]) {
    Client mainClient;
    Client* client1 = &mainClient;

    client1->Send();
    cin.get();
}

Any and all help is great! Thanks!

EXTRA CODE

HttpUtils.cpp

#include <stdlib.h>
#include <winsock.h>
#include <sstream>
#pragma comment (lib, "wsock32.lib")

#include <iostream>
#include <string>
using namespace std;

#include "HttpUtils.h"

void die_with_error(char *errorMessage) {
    cerr << errorMessage << endl;
    cin.get();
    exit(1);
}

void die_with_wserror(char *errorMessage) {
    cerr << errorMessage << ": " << WSAGetLastError() << endl;
    cin.get();
    exit(1);
}

string HTTP_POST_REQUEST(char *hostname, string path, string data) {
    string request;
    string response;
    int resp_leng;

    char buffer[BUFFERSIZE];
    struct sockaddr_in serveraddr;
    int sock;

    WSADATA wsaData;
    hostent *remoteHost;
    int port = 80;

    stringstream ss;
    ss << data.length();

    stringstream request2;
    request2 << "POST " << path << " HTTP/1.1" << endl;
    request2 << "User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)" << endl;
    request2 << "Host: " << hostname << endl;
    request2 << "Content-Length: " << data.length() << endl;
    request2 << "Content-Type: application/x-www-form-urlencoded" << endl;
    request2 << "Accept-Language: en-uk" << endl;
    request2 << endl;
    request2 << data;
    request = request2.str();

    cout << request << endl << "###################################################" << endl << endl;

    //  Init WinSock.
    if (WSAStartup(MAKEWORD(2, 0), &wsaData) != 0)
        die_with_wserror("WSAStartup() failed");

    //  Open socket.
    if ((sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0)
        die_with_wserror("socket() failed");


    //  Convert hostname into IP
    remoteHost = gethostbyname(hostname);
    char *temp;
    if (remoteHost != NULL) {
        temp = remoteHost->h_addr_list[0];
    } else {
        temp = NULL;
        return "Error: Invalid hostname passed to HTTP_POST().";
    }


    //  Connect.
    memset(&serveraddr, 0, sizeof(serveraddr));
    serveraddr.sin_family = AF_INET;

    int i = 0;
    if (remoteHost->h_addrtype == AF_INET)
    {
        while (remoteHost->h_addr_list[i] != 0) {
            serveraddr.sin_addr.s_addr = *(u_long *)remoteHost->h_addr_list[i++];
        }
    }

    serveraddr.sin_port = htons((unsigned short)port);
    if (connect(sock, (struct sockaddr *) &serveraddr, sizeof(serveraddr)) < 0)
        die_with_wserror("connect() failed");

    //  Send request.
    if (send(sock, request.c_str(), request.length(), 0) != request.length())
        die_with_wserror("send() sent a different number of bytes than expected");

    //  Get response.
    response = "";
    resp_leng = BUFFERSIZE;
    while (resp_leng == BUFFERSIZE)
    {
        resp_leng = recv(sock, (char*)&buffer, BUFFERSIZE, 0);
        if (resp_leng>0)
            response += string(buffer).substr(0, resp_leng);
        //note: download lag is not handled in this code
    }

    //  Disconnect
    closesocket(sock);

    //  Cleanup
    WSACleanup();

    //response.erase(0, 184);
    return  response;
}

HttpUtils.h

#pragma once

#include <iostream>
#include <string>
using namespace std;

#ifndef HTTP_UTILS__H
#define HTTP_UTILS__H
#define BUFFERSIZE 1024
void die_with_error(char *errorMessage);
void die_with_wserror(char *errorMessage);

string HTTP_POST_REQUEST(char *hostname, string path, string data);
#endif