template function parameter inheritance

Question

I have a dispatcher which can return any type, takes a command, and a FormData object. The idea is that I wish to inherit from FormData when passing around specific stuff.

struct FormData {};

struct Form : FormData {};

void login(const Form *f){}

enum Command
{
    LOGIN
};

template <typename T>
T dispatch(const Command command, const FormData *f)
{
    switch (command)
    {
    case LOGIN: login(f);
    }

    return T();
}

int main()
{
    Form f;

    dispatch<void>(LOGIN, &f);

    return 0;
}

I get an error saying cannot convert from Form to FormData. I take away the template, everything works fine (but I need the template)

Answer 1

Your FormData class is the base class, Form is derived, however your login function looks like

void login(const Form *f){}

But in your dispatch function, you are trying to pass a base-class pointer

T dispatch(const Command command, const FormData *f)
{
    switch (command)
    {
    case LOGIN: login(f);
    }

C++ simply won't let you do that. Form* can be implicitly converted to a FormData*, but not the other way around.

Perhaps you could add another template parameter to the dispatch function, and let that function figure out the concrete type at compile time:

struct FormData {};

struct Form : public FormData {};

void login(const Form *f){}

enum Command
{
    LOGIN
};    

template <typename T>
T dispatch(const Command command)
{
    return T();
}

template <typename T, typename FormDataType>
T dispatch(const Command command, const FormDataType *f)
{
    switch (command)
    {
    case LOGIN: login(f);
    }

    return dispatch(command);
}

int main()
{
    Form f;

    dispatch<void>(LOGIN, &f);
    dispatch<void>(LOGIN);
}