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Difference "URL Dispatching and Mapping" ver. 6 versus ver. 7

Content:

## Introduction

Now we will learn how to connect different URLs to their associated functions easily.

When a client requests a specific URL from the HTTP server,

it divides it into several parts (CGI variables): `SCRIPT_NAME`, `PATH_INFO` and `QUERY_STRING`. For example

the URL `/foo/bar.php/test?x=10` is separated into:

- `SCRIPT_NAME` = `/foo/bar.php`

- `PATH_INFO` = `/test`

- `QUERY_STRING` = `x=10`

CppCMS web applications are not distributed into several

scripts, but rather they're written using a single FastCGI application that runs on a specific "script" url, like "/myapp". So generally URLs would look like `/myapp/page/10`, where

`SCRIPT_NAME` = "/myapp" would remain constant and `PATH_INFO` = `/page/10` is changed according to the user's needs.

So the mapping between the URL and the application's member functions is done by matching a regular expression against the `PATH_INFO` part of the URL, even though this behaviour can be changed by using the `mount_point` class.

This job is done by `cppcms::url_dispatcher` class.

This job is done by the `cppcms::url_dispatcher` class.

The opposite of the dispatching is "mapping" when the names of the application parts are converted to a URL that can be

given to the user.

The opposite of the dispatching is "mapping" where the names of the application parts are converted to a URLs that can be given to the user.

## Code

### Mapping

Lets rewrite our [hello class](/wikipp/en/page/cppcms_1x_tut_hello):

Since we will be routing URLs, let's include the `url_dispatcher.h` and `url_mapper.h` header file.  Your include list should look like:

#include <cppcms/application.h>

#include <cppcms/service.h>

#include <cppcms/http_response.h>

#include <cppcms/url_dispatcher.h>

#include <cppcms/url_mapper.h>

#include <cppcms/applications_pool.h>

#include <iostream>

#include <stdlib.h>

Now in our constructor, let's map some URLs to member functions:

hello(cppcms::service &srv) :

cppcms::application(srv)

{

dispatcher().assign("/number/(\\d+)",&hello::number,this,1);

mapper().assign("number","/number/{1}");

dispatcher().assign("/smile",&hello::smile,this);

mapper().assign("smile","/smile");

dispatcher().assign("",&hello::welcome,this);

mapper().assign("");

mapper().root("/hello");

}

In the fourth line we connect the regular expression `/number/(\d+)` to the member function `number` of `this`

instance that receives `std::string` as parameter,

and 1st captured subexpression is passed to it (note the parameter 1 that represents the 1st subexpression as a parameter)

In this case the subexpression is nonempty string that contains digits.

In the 5th line we create a name for this part called

"number" and we provide a URL formatting pattern `/number/{1}` - the opposite of regular expression where `{1}` is a placeholder for the first parameter for formatting the appropriate URL.

In next lines 7-8 connect member function `smile`

to its URL and create a mapping. Because this URL

does not have any parameters it does not have

any placeholders.

The the line 10 we connect the empty URL to the

`welcome` member function and in the line 11 we define default empty mapping patter for the URL. Note, because

we pass only one parameter to the `assign()` function,

we define the default URL for the application.

In the line 13 we define the "root" of the all URLs for the

`url_mapper` class, in our case it should be identical the

the `SCRIPT_NAME` CGI variable.

### Description

So when the web server receives requests that match

a script name `/hello` it forwards them to the application:

- `/hello` - `welcome` function would be called

- `/hello/smile` - `smile` function would be called

- `/hello/number/10` - `number` function would be called and receive a string "10" as parameter.

Because the "routing table" between the URL and the

application member functions is done we don't need

to override the `cppcms::application::main` member

function as it does the all dispatching for us.

### Actions

The "number" member function.

void number(std::string num)

{

int no = atoi(num.c_str());

response().out() << "The number is " << no << "<br/>\n";

response().out() << "<a href='" << url("/") << "'>Go back</a>";

}

Note, we using `url("/")` as abstract URL for the default

application entry point - the `/` part describes the root

application (our single application) and the fact that

nothing follows it shows that the default URL should be

given - the mapping the twe `welcome` member functions

In the same way we create the `smile` member function:

void smile()

{

response().out() << ":-) <br/>\n";

response().out() << "<a href='" << url("/") << "'>Go back</a>";

}

Now we would write out `welcome` action:

void welcome()

{

response().out() <<

"<h1> Wellcome To Page with links </h1>\n"

"<a href='" << url("/number",1) << "'>1</a><br>\n"

"<a href='" << url("/number",15) << "'>15</a><br>\n"

"<a href='" << url("/smile") << "' >:-)</a><br>\n";

}

Note, we show three URLs to the different application

parts: "number" - and we would pass a parameter `1` to the `cppcms::application::url` function that would be rendered instead of the placeholder `{1}`, we do the same for

number 15 and pass a URL to "smile" part without parameters.

Lets describe in the example the actions that are taken for when we call `url("/number",15)`:

- First for "/" we go to the topmost application, in out case to `this` one.

- Next we match "number" against the known pattern "number" that was mapped to `"/number/{1}"`.

- We fetch the pattern and use the parameter that is passed to `url` function and convert: `/number/{1}` to `/number/15`.

- Finally we perpend the root URL path "/hello" and get our final URL `/hello/number/15`.

Note: the parameters to `url` family of functions can be anything that can be written to `std::ostream` - i.e. any object that defined operator `<<` with standard C++ streams.

## Running The Example

Recompile it:

c++ hello.cpp -lcppcms -o hello

And now for the exciting part, start your application:

./hello -c config.js

And hit the URL: <http://localhost:8080/hello>