Allow extensible returning in handlers

[The-EDev]

we have JSON mode, basically sets the content-type header to application/json, just make the same thing for multipart, instead of converting to string and returning that, something like:

    CROW_ROUTE(app,"/multipart")
    ([](const crow::request& req){
        crow::multipart::message msg(req);
        return msg;
    });

[The-EDev]

One solution for this issue + support for all other types would be to introduce a returnable class containing primarily a dump() and set_mode(crow::response res) methods, the dump() method would simply put the class' content into the response body, and the set_mode(crow::response res) would set the headers (specifically content-type) to the appropriate ones, then any user/developer can create a wrapper for any library they might want to return directly.

[The-EDev]

so I was trying out yesterday's solution. it caused a circular dependency which I should've anticipated..

So I changed the method to just a content type variable, which for some reason didn't work. So I changed it to a method that returned the content type, which worked, here's the code.

struct returnable
{
    virtual const std::string dump() = 0;
    virtual const std::string content_type() = 0;
};

Implementation for content_type() in multipart::message:
const std::string content_type() override {return "multiplart/form-data";}

The child class would add an implementation to these, allowing it to be returned from a handler.
What I'm not sure of is if there's a better way to go about doing this.

[The-EDev]

@mrozigor as I'm not as familiar with C++ as I would like to be, what do you think of this approach?, does it make sense? is there a better way to go about it?

[mrozigor]

This looks good - I think it is good to explicitly 'mark' class as returnable.

[NobodyXu]

@The-EDev IMHO it is better to use class for virtual base class and use struct for c-like aggregate.

It makes the code more readable and less confusing to new comers.

[The-EDev]

Well I'm mainly following the existing code, as far as I understand, the main difference between struct and class is that one is public by default and the other is private

[NobodyXu]

But to new contributers, this code at the first glance look like a C-like structure (aggregate), not a virtual base class since there isn't a constructor or destructor.

Personally I think it is better to use class so that the reader immediately knows that this isn't a C-like structure.