crowsite/libs/crow/docs/guides/middleware.md

Middleware is used for altering and inspecting requests before and after calling the handler. In Crow it's very similar to middleware in other web frameworks.

All middleware is registered in the Crow application

crow::App<FirstMW, SecondMW, ThirdMW> app;

and is called in this specified order.

Any middleware requires the following 3 members:

• A context struct for storing request local data.
• A before_handle method, which is called before the handler.
• A after_handle method, which is called after the handler.

!!! warning

As soon as `response.end()` is called, no other handlers and middleware is run, except for after_handlers of already visited middleware.

Example

A middleware that can be used to guard admin handlers

struct AdminAreaGuard
{
    struct context
    {};

    void before_handle(crow::request& req, crow::response& res, context& ctx)
    {
        if (req.remote_ip_address != ADMIN_IP)
        {
            res.code = 403;
            res.end();
        }
    }

    void after_handle(crow::request& req, crow::response& res, context& ctx)
    {}
};

before_handle and after_handle

There are two possible signatures for before_handle and after_handle

1. if you only need to access this middleware's context.

   void before_handle(request& req, response& res, context& ctx)
   

2. To get access to other middlewares context

   template <typename AllContext>
   void before_handle(request& req, response& res, context& ctx, AllContext& all_ctx) 
   {
       auto other_ctx = all_ctx.template get<OtherMiddleware>();
   }
   

Local middleware

By default, every middleware is called for each request. If you want to enable middleware for specific handlers or blueprints, you have to extend it from crow::ILocalMiddleware

struct LocalMiddleware : crow::ILocalMiddleware 
{

After this, you can enable it for specific handlers

CROW_ROUTE(app, "/with_middleware")
.CROW_MIDDLEWARES(app, LocalMiddleware)
([]() {
    return "Hello world!";
});

or blueprints

Blueprint bp("with_middleware");
bp.CROW_MIDDLEWARES(app, FistLocalMiddleware, SecondLocalMiddleware);

!!! warning

Local and global middleware are called separately. First all global middleware is run, then all enabled local middleware for the current handler is run. In both cases middleware is called strongly
in the order listed in the Crow application.