Crow/docs/guides/routes.md

Routes define what happens when your client connects to a certain URL.<br>

## Macro
`CROW_ROUTE(app, url)`<br>
Can be replaced with `#!cpp app.route<crow::black_magick::get_parameter_tag(url)>(url)` or `#!cpp app.route_dynamic(url)` if you're using VS2013 or want runtime url evaluation. Although this usage is **NOT** recommended.
## App
Which app class to assign the route to.
## Path (URL)
Which relative path is assigned to the route.<br>
Using `/hello` means the client will need to access `http://example.com/hello` in order to access the route.<br>
A path can have parameters, for example `/hello/<int>` will allow a client to input an int into the url which will be in the handler (something like `http://example.com/hello/42`).<br>
Parameters can be `<int>`, `<uint>`, `<double>`, `<string>`, or `<path>`.<br>
It's worth noting that the parameters also need to be defined in the handler, an example of using parameters would be to add 2 numbers based on input:
```cpp 
CROW_ROUTE(app, "/add/<int>/<int>")
([](int a, int b)
{
    return std::to_string(a+b);
});
```
you can see the first `<int>` is defined as `a` and the second as `b`. If you were to run this and call `http://example.com/add/1/2`, the result would be a page with `3`. Exciting!

## Methods
You can change the HTTP methods the route uses from just the default `GET` by using `method()`, your route macro should look like `CROW_ROUTE(app, "/add/<int>/<int>").methods(crow::HTTPMethod::GET, crow::HTTPMethod::PATCH)` or `CROW_ROUTE(app, "/add/<int>/<int>").methods("GET"_method, "PATCH"_method)`.

!!! note

    Crow handles `HEAD` and `OPTIONS` methods automatically. So adding those to your handler has no effect.

## Handler
Basically a piece of code that gets executed whenever the client calls the associated route, usually in the form of a [lambda expression](https://en.cppreference.com/w/cpp/language/lambda). It can be as simple as `#!cpp ([](){return "Hello World"})`.<br><br>

### Request
Handlers can also use information from the request by adding it as a parameter `#!cpp ([](const crow::request& req){...})`.<br><br>

You can also access the url parameters in the handler using `#!cpp req.url_params.get("param_name");`. If the parameter doesn't exist, `nullptr` is returned.<br><br>

For more information on `crow::request` go [here](../../reference/structcrow_1_1request.html).<br><br>

### Response
Crow also provides the ability to define a response in the parameters by using `#!cpp ([](crow::response& res){...})`.<br><br>

Please note that in order to return a response defined as a parameter you'll need to use `res.end();`.<br><br>

Alternatively, you can define the response in the body and return it (`#!cpp ([](){return crow::response()})`).<br>

For more information on `crow::response` go [here](../../reference/structcrow_1_1response.html).<br><br>

### Return statement
A `crow::response` is very strictly tied to a route. If you can have something in a response constructor, you can return it in a handler.<br><br>
The main return type is `std::string`, although you could also return a `crow::json::wvalue` or `crow::multipart::message` directly.<br><br>
For more information on the specific constructors for a `crow::response` go [here](../../reference/structcrow_1_1response.html).

## Returning custom classes
**Introduced in: `v0.3`**<br><br>
If you have your own class you want to return (without converting it to string and returning that), you can use the `crow::returnable` class.<br>
to use the returnable class, you only need your class to publicly extend `crow::returnable`, add a `dump()` method that returns your class as an `std::string`, and add a constructor that has a `Content-Type` header as a string argument.<br><br>

Your class should look like the following:
```cpp
class a : public crow::returnable 
{
    a() : returnable("text/plain"){};
    ...
    ...
    ...
    std::string dump() override
    {
        return this.as_string();
    }
}
```
<br><br>

## Response codes
**Introduced in: `master`**<br><br>

Instead of assigning a response code, you can use the `crow::status` enum, for example you can replace `crow::response(200)` with `crow::response(crow::status::OK)`

## Catchall routes
**Introduced in: `v0.3`**<br><br>
By default, any request that Crow can't find a route for will return a simple 404 response. You can change that to return a default route using the `CROW_CATCHALL_ROUTE(app)` macro. Defining it is identical to a normal route, even when it comes to the `const crow::request&` and `crow::response&` parameters being optional.
!!!note

    For versions higher than 0.3 (excluding patches), Catchall routes handle 404 and 405 responses. The default response will contain the code 404 or 405.