How to register a controller with coroutines C++20

[capybaracplusplus]

Hi.

I'm currently experiencing an issue registering a controller with a return type - drogon::Taskdrogon::HttpResponsePtr

Example:

class LinkController : public drogon::HttpController<LinkController> {
public:
METHOD_LIST_BEGIN
ADD_METHOD_TO(LinkController::createShortLink, "/shorten", drogon::Post);
ADD_METHOD_TO(LinkController::redirectToOriginal, "/{hash}", drogon::Get, "hash");
METHOD_LIST_END

drogon::Task<drogon::HttpResponsePtr> createShortLink(const drogon::HttpRequestPtr &req);

drogon::Task<drogon::HttpResponsePtr> redirectToOriginal(const drogon::HttpRequestPtr &req,
const std::string &hash);
}

To:

error: no type named 'first_param_type' in 'struct drogon::internal::FunctionTraits<...>

I tried registering it manually through a method, passing it as a static method. I also tried a workaround using a lambda function that already calls my controllers, but none of this worked.

From the documentation I only found a code example

app.registerHandler("/num_users", 
[](HttpRequestPtr req, std::function<void(const HttpResponsePtr&)> callback) -> Task<> 
// Must mark the return type as a _resumable_ ^^^
{ 
auto sql = app().getDbClient(); 
try 
{ 
auto result = co_await sql->execSqlCoro("SELECT COUNT(*) FROM users;"); 
size_t num_users = result[0][0].as<size_t>(); 
auto resp = HttpResponse::newHttpResponse(); 
resp->setBody(std::to_string(num_users)); 
callback(resp); 
} 
catch(const DrogonDbException &err) 
{ 
// Exception works as sync interfaces.
auto resp = HttpResponse::newHttpResponse();
resp->setBody(err.base().what());
callback(resp);
}
// No need to return anything! This is a coroutine that yields `void`. Which is
// indicated by the return type of Task<void>
co_return; // If you want to (not required), use co_return
}

Link on doc - https://github.com/drogonframework/drogon/wiki/ENG-17-Coroutines
Link on my project - https://github.com/capybaracplusplus/cutr

This doesn't work for me. Please help me register these controllers correctly.
Thank you.

[an-tao]

@capybaracplusplus you should declare your methods like follows:

drogon::Task<drogon::HttpResponsePtr> createShortLink(drogon::HttpRequestPtr req);
drogon::Task<drogon::HttpResponsePtr> redirectToOriginal(drogon::HttpRequestPtr req,
                                                             std::string hash);

The reason is explained in the document you pasted:
Passing most parameters by value is a direct consequence of coroutines being asynchronous. It's impossible to track when a reference goes out of scope as the object may destruct while the coroutine is waiting. Or the reference may live on another thread. Thus, it may destruct while the coroutine is executing.