Crow/docs/guides/routes.md

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

##Macro CROW_ROUTE(app, url)
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.
Using /hello means the client will need to access http://example.com/hello in order to access the route.
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).
Parameters can be <int>, <uint>, <double>, <string>, or <path>.
It's worth nothing 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:

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).

##Handler Basically a piece of code that gets executed whenever the client calls the associated route, usually in the form of a lambda expression. It can be as simple as #!cpp ([](){return "Hello World"}).

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

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.

For more information on crow::request go here.

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

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

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

For more information on crow::response go here.

###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.

The main return type is std::string. although you could also return a crow::json::wvalue or crow::multipart::message directly.

For more information on the specific constructors for a crow::response go here.

##Returning custom classes 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.
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.

your class should look like the following:

class a : public crow::returnable 
{
    a() : returnable("text/plain"){};

    ...
    ...
    ...

    std::string dump() override
    {
        return this.as_string();
    }
}