C++ Virtual Method with Arguments List

Question

I'm working on a HTTP Implementation in C++ for education purposes and I have the following classes.

Header: Represents a Pool of HeaderFields

HeaderField: Represents one HeaderField (Name-Value Pair)

Because many headers have multiple related values in their value (such as "Content-Type: text/plain; charset=utf-8" contains the actual content-type beside the charset wich is an additional information) I thought of my HeaderField-Class as a base-class so I can have a ContentType-Class for example which derivers from HeaderField and can only handle Content-Type-Fields. Because I can't afford to maintain all possible Header-Fields in the HTTP-Protocol I need to make this dynamic. This means, that I want to register classes which can handle some type of HeaderField and if such a HeaderField is parsed, it should be a polymorphic Object of the registered class.

I found some solution for storing a classtype here.

The problem is in the allocate()-method: I thought it would be nice to have the Type- and TypeImpl-Class as universal classes and not just for this one purpose. Therefor I need to have an Argument-List in the allocate()-Method because some constructor-functions might need some arguments to be passed in...

This is what i tried:

class Type {
public:
    virtual ~Type() {};

    template<typename... Args>
    virtual void* allocate(Args... args)const = 0;
    ...
}

template<typename T>
class TypeImpl :public Type {
public:
    template<typename... Args>
    void* allocate(Args... args)const {
        return new T(args...);
    }
    ...
}

This throws a Compile-Error C2898 in VS2015. Does anyone have a workarround for this?

PS: I'm not sure whether or not va_list can be a solution since I only found examples of extracting single arguments from it.

offtopic: for education purposes I would not name a class `Header` to avoid confusion with a header — 463035818_is_not_an_ai, Oct 06 '17 at 11:49
When asking questions about build-errors, please copy-paste (as text) the *full* and *complete* error message into the question body, including any possible informational notes. Then mark the error(s) in the code you show, with e.g. a comment. — Some programmer dude, Oct 06 '17 at 11:50
...wait a sec.. you are talking about educating yourself? If yes, then forget my last comment — 463035818_is_not_an_ai, Oct 06 '17 at 11:50
Sorry, member function templates cannot be `virtual`. `allocate` will need to be less generic. — aschepler, Oct 06 '17 at 11:52
@tobi303 yeah u might be right im also working with namespaces but i left that out for the sake of simplicity :D — Rockettomatoo, Oct 06 '17 at 11:55
@jaskmar no i actually dont. The reason for the base class is the following: `vector types; types.push_back(TypeImpl); types.push_back(TypeImpl); ...` — Rockettomatoo, Oct 06 '17 at 12:03

C++ Virtual Method with Arguments List

0 Answers0