Classes for writing HTTP clients in C++

• Download demo project - 6.75 Kb
• Download source - 2.79 Kb

Introduction

Today, more and more developers want to write distributed transactional applications for the enterprise and leverage the speed, security, and reliability of server-side technology. Java has become the choice of thousands of developers for writing scalable programs (usually as servlets or JSP) that reside on web servers and execute business methods for clients. (We won't be talking about EJBs here !!)

These clients are usually HTML forms or Java applets that run within a web browser. What if your legacy C++ application wants to go thin, and hand-over all hard jobs like running business logic, accessing database etc to a Java servlet? How are you going to handle communication over HTTP, URL encoding variables and all that ? The classes we are going to discuss in this article will show you how easy it is to write HTTP clients in C++, even easier than writing a Java Applet for the same purpose !! You take my word for that.

Background

You must be familiar with OOPs, internet protocols etc. If you have developed a web application using Java, ASP or some other server side technology, then you will find this article easy to follow. To run the demo code, you must have access to a web server that support Java, I have used Tomcat. If your web server do not support Java, write the server-script in what ever language your web server can interpret and change the server-script's name in the demo code.

Using the code

This section is actually a walk through of the demo project.

First make the following include statements in the cpp file that use our HTTP classes:

// some standard headers

#include <string>

#include <vector>

#include <iostream>

using namespace std;
// windows headers

#include <windows.h>

#include <wininet.h>

// our header

#include "web.h"

using namespace openutils;

The program must also be linked with wininet.lib. In VC++ you can do this by going to Project->Settings->Link->Object/Library Modules and adding wininet.lib to the end of the default library names.

Next, we declare an object of the WebForm class.

WebForm wf;

You should tell WebForm the name of the web server that it should connect to:

wf.setHost("http://localhost:8080");

You can replace "localhost:8080" with any valid HTTP URL like:

"www.codeproject.com" or "208.45.33.44"

The next parameter that WebForm expect is the name of the script file that the web server should execute:

wf.setScriptFile("/banking/servlet/InterestServlet");

Please check out your web server's documentation for the path of your servlet files. On a Tomcat server, create the folder hierarchy "banking\WEB-INF\classes" in the "webapps" sub-folder and place the IntersetServlet.class file in that.

Now you are ready to add variables to be send to the servlet for processing. The "InterestServlet" expects three variables or parameters: name, rate and amt. These variables and their values can be added to the WebForm object by calling the putVariable() function.

wf.putVariable("name","James Dean");
wf.putVariable("rate","14");
wf.putVariable("amt","1200.89");

We can send an HTTP request to the servlet by calling the sendRequest() function.

wf.sendRequest();

The servlet will send back a response that will contain the calculated interest rate in HTML format. This response can be read into a buffer on the client side:

char response[101];            
if(wf.getResponse(response,100)) 
{
    cout << endl << response << endl;
}

We can also use the WebForm class to write simple clients that act like a web-browser. Here is a code snippet that download the home page of a well-known web site:

wf.setHost(http://www.microsoft.com);

wf.setScriptFile("/default.html"); 
// or wf.setScriptFile(""); 

wf.sendRequest();
char buff[10 * 1024];
if(wf.getResponse(buff,sizeof(buff))) 
{
    cout << buff << endl;
}
else 
{
    cout << "No response from server" << endl;
}

Notes

web.h defines a utility class for encoding a string in the HTTP encoding format. This class is used internally by WebForm. The usage of this class and the decoder class is demonstrated below:

string str = "AC-0099";
string str_enc = URLEncoder::encode(str); // AC%2d0099

string dec = URLDecoder::decode(str_enc); // AC-0099

Please remember to enclose all web.h function calls in a try-catch block that catch WebFormExceptions.

try {// web.h calls

} catch(WebFormException ex)
{ 
  cout << ex.getMessage(); 
}

Before running the demo, copy InterestServlet.class from the servlet folder to the appropriate script folder of your Java enabled web-server.

History

• Submitted to CodeProject : 10/20/2003

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How to handle file not found (HTTP Error 404) ?? AbinThomas 0:33 4 Jan '08   Hello, Thank you very much for sharing this wonderful peice of code. I am now using it to download certain files. But i need some help. If the requested file is not in the path, getResponse returns the html content of "HTTP Error 404 - File or directory not found". How can we check wheher the download was succesfull? Otherwise how to check if the file exists bfor attemting download ? Thanks abin. Sign In·View Thread·PermaLink Re: How to handle file not found (HTTP Error 404) ?? Vijay Mathew Pandyalakal 18:51 7 Jan '08   If the requested file is not found, the web server will send you a response like: HTTP/1.0 404 Not Found You need to check for the error code 404. I don't think HTTP supports querying or listing of files. Sign In·View Thread·PermaLink Thank you for the code + remark wakeboarderxxx1986 10:03 1 Nov '07   if(ch == 'a' || ch == 'b' || ch == 'c' || ch == 'd' || ch == 'e' || ch == 'f' || ch == 'g' || ch == 'h' || ch == 'i' || ch == 'j' || ch == 'k' || ch == 'l' || ch == 'm' || ch == 'n' || ch == 'o' || ch == 'p' || ch == 'q' || ch == 'r' || ch == 's' || ch == 't' || ch == 'u' || ch == 'v' || ch == 'w' || ch == 'x' || ch == 'y' || ch == 'z' || ch == '0' || ch == '1' || ch == '2' || ch == '3' || ch == '4' || ch == '5' || ch == '6' || ch == '7' || ch == '8' || ch == '9') This could have been written more efficiënt/beautifull using range checking i guess if(ch >= 'a' && ch <= 'z' || ch >= '0' && ch <= '9') Nice code and thank you!! Grtz Sign In·View Thread·PermaLink 2.00/5 (1 vote) Authentication in HTTP clients pelis 4:55 23 May '07   Hi Mathew, I have been testing your application and it works fine but the server responds with "401 Authorization Required" because I need to login in the system. Is this feature available in your application? Could you lead me how to do it? Because I don't know how to modify the HTTP headers! Actually, i have the code in delphi an i am trying to port it to C++, in delphi the key line is: IdHTTP.Request.ExtraHeaders.Add('Authorization: Basic '+ Base64Encode(Login.Text+':'+Password.Text)); Thank you very much in advance!!!! JRF Sign In·View Thread·PermaLink Re: Authentication in HTTP clients Vijay Mathew Pandyalakal 18:38 23 May '07   As of now, my HTTP library do not directly support user specified headers. You can easily add this feature by modifying the call to the win32 API function "InternetOpenUrl" in web.cpp. Additional headers can be given as the third argument of this function. In the current code it is left as NULL. See the MSDN documentation on InternetOpenUrl() for more information and modify web.cpp to suit your requirement. thanks, Vijay Sign In·View Thread·PermaLink [Linker error] undefined reference to `openutils::WebForm::WebForm()' lil_zee 6:05 27 Feb '07   hello, when i try to compile your demo or try to write something using your code i get errors like: [Linker error] undefined reference to `openutils::WebForm::WebForm()' I think it has something to do with the wininet.lib which i do not have... Sign In·View Thread·PermaLink Re: [Linker error] undefined reference to `openutils::WebForm::WebForm()' Vijay Mathew Pandyalakal 18:11 27 Feb '07   This seems to have nothing to do with wininet.lib Just make sure that the file 'web.cpp' is getting compiled and linked properley. Sign In·View Thread·PermaLink It does not work got all URLs Andyzyx 5:37 12 Feb '07   Hi, It looks cool. However, when I tried your program for some reason it does not seem to encode well for big sites[ say like http://www.yahoo.com ] and bails out with a "stack around the 'tmp' variable is corrupted" in encode(). Anything you would like to suggest to make this work for bigger sites as well. Thanks, Andy Sign In·View Thread·PermaLink Re: It does not work got all URLs Vijay Mathew Pandyalakal 19:56 12 Feb '07   Hi, Thanks for trying out my code. I tested it against big sites like yahoo and did not get the error you reported. But from your post I understand that the code will fail under certain cirtcumstances. Could you please post the code that led to the above error? Thanks, Vijay Sign In·View Thread·PermaLink To make shorter code.... frama 3:52 16 May '06   How about using strtol() & isalnum() ? Sign In·View Thread·PermaLink Small bugfix to the URL encoder SethR 21:35 10 Jul '05   If you want to properly encode all special characters you need leading 0's for the one digit ones. In URLEncoder::encode, change the line: sprintf(tmp,"%%%x",buff[i]); to: sprintf(tmp,"%%%02x",buff[i]); to fix it. -Seth Sign In·View Thread·PermaLink 5.00/5 (1 vote) Opening a URL in C program free_bird 15:29 26 Jun '04   Hello all, I am working with UNIX systems and write a program in C to open a Google page and do some search also. Can anyone please explain how to make such a connections. I tried readin about sockets but they didn't help me much. Thank you for your time. Second Thoughts are Ever Wiser - Euripides Sign In·View Thread·PermaLink Re: Opening a URL in C program Ravi Bhavnani 16:34 26 Jun '04   We use libCurl[^] for xplatform url access. Works very well. /ravi My new year's resolution: 2048 x 1536 Home | Articles | Freeware | Music ravib@ravib.com Sign In·View Thread·PermaLink 2.00/5 (1 vote) Opening a URL in C Program free_bird 15:23 26 Jun '04   Hello all, I am working with Unix based system. I have to write a program in C to open the google page and so some search on it. Can anyone please explain me as to how to do it? Thank you for your time. - Second Thoughts are Ever Wiser - Euripides Sign In·View Thread·PermaLink 2.00/5 (2 votes) curl -- a portable webform? goofy.j 21:52 28 Oct '03   First of all, I would like to say that Vijay Mathew Pandyalakal's WebForm is excellent because it is very simple and clear. Before tumbling on his code, I have setup an internal application webserver for calculating air trajectory and a program with simple GUI for users to submit requests to the server. I spent quite a length of time to figure out using cURL (http://curl.haxx.se) -- a portable library capable of FTP, HTTP, and etc. I post this info here for people working with other platforms. Jiye Zeng Sign In·View Thread·PermaLink 1.00/5 (1 vote) Nice, probably useful, but some remarks Patje 6:43 20 Oct '03   The class seems useful (although not directly for me at this moment), but I have some remarks: There is no way to specify how you want to pass the variables to the web site (post vs. get). Maybe it would be better if it would present itself as an input stream, so the ">>" operator could be used. Never, never, never, never force the developer to use "using namespace std". Without it, it won't compile, which means that your 'include file' is not written in a polite way. If you want to refer to std::string, just write std::string and never assume in an include file that there is a "using namespace std" somewhere. Luckily you didn't put the "using namespace std" in the include file itself because that would be really ugly. For the rest, I'll give you a 4, because the article shows how easy it is to use the Internet API, and for presenting a short piece of code (I like short examples). Enjoy life, this is not a rehearsal !!! My Articles: - Implementing a Subject/Observer pattern with templates - Different ways of writing class factories - AutoRunner: a template class to automatically run start- and cleanup-code in code blocks Sign In·View Thread·PermaLink Re: Nice, probably useful, but some remarks Jörgen Sigvardsson 10:12 20 Oct '03   Patje wrote: Never, never, never, never force the developer to use "using namespace std". As long as it's enclosed by a scope, there's no problem. -- Your life as it has been is over. From this time forward you will service us. Sign In·View Thread·PermaLink Re: Nice, probably useful, but some remarks Nemanja Trifunovic 20:17 20 Oct '03   Here, using namespace is in a header file, and that is bad. However, nice article. Sign In·View Thread·PermaLink Re: Nice, probably useful, but some remarks Vijay Mathew Pandyalakal 4:33 21 Oct '03   Thanks for your comments. I will take care of "std" in future ! Sign In·View Thread·PermaLink Unicode Uwe Keim 6:27 20 Oct '03   Your class isn't Unicode-awary in any way. I doubt a HTML client that can't handle Unicode is much usful in real world apps... -- - Free Windows-based CMS: www.zeta-software.de/enu/producer/freeware/download.html - See me: www.magerquark.de Sign In·View Thread·PermaLink 3.50/5 (2 votes) Re: Unicode John Crenshaw 10:24 17 Apr '07   The windows APIs for this are not Unicode ready, I know, I've used them in a Unicode project before. Unicode APIs exist, but it is not possible to get a connection that will allow you to perfom even basic read opperations using them (as I recall one of the required functions was ANSI only and you can't mix the ANSI and Unicode handles.) Anyway, if you need wide characters, the ANSI functions can deal with UTF-8 just fine. The internet doesn't really use Unicode anyway, it's all UTF-8. Sign In·View Thread·PermaLink 1.00/5 (1 vote) HTTPS? Ed K 6:05 20 Oct '03   Is there support for HTTPS? ed Regulation is the substitution of error for chance. Sign In·View Thread·PermaLink 1.80/5 (6 votes)

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