Static templated member function as function pointer

Question

I'm attempting to replace a series of static to non static member function pairs throughout my code, I can achieve this with a macro but I was hoping I could do so with a static function which takes the non static member function as a template argument, which then gets stored as a function pointer. See the following code:

struct widget_param
{
    void* ptr;
};

struct widget_data
{
    bool(*callback)(widget_param*);
};

template <class CLASSNAME>
class widget_base
{
protected:
    static CLASSNAME* get_instance(widget_param* param)
    {
        return static_cast<CLASSNAME*>(param->ptr);
    }

public:
    template <bool(CLASSNAME::*f)(widget_param*)>
    static bool static_to_nonstatic(widget_param* param)
    {
        return get_instance(param)->*f(param);
    }
};

class widget_derived : public widget_base<widget_derived>
{
public:
    // Attempting to replace this function
    static bool static_do_stuff(widget_param* param)
    {
        return get_instance(param)->do_stuff(param);
    }
    bool do_stuff(widget_param* param)
    {
        param;
        cout << "Success!";
        return true;
    }
};

int main() {
    widget_derived derived;
    //widget_data data{ widget_derived::static_do_stuff}; // Current approach
    widget_data data{ widget_derived::static_to_nonstatic<widget_derived::do_stuff> };

    widget_param param{ &derived };
    data.callback(&param);
    return 0;
}

I was expecting the template to evaluate to:

static bool static_to_nonstatic(widget_param* param);

Is it possible to do what I'm trying to achieve, without resorting to using a macro?

This just can't work. You need to store the this pointer somewhere, you can't solve it like this, at compile time. If you don't have C++ <functional>, probably you should just store a this pointer in struct widget_param — Gábor Buella
– Gábor Buella, Commented Mar 6, 2016 at 14:30
I see, I misunderstood the code. Or was too lazy to fully read it, sry. So param->ptr stores the this pointer. — Gábor Buella
– Gábor Buella, Commented Mar 6, 2016 at 14:52

Stew · Accepted Answer · 2016-03-06 14:35:33Z

1

Resolved, since the call is to a member function pointer - it needed to be surrounded by parenthesis, changed the template to the following and it worked:

template <bool(CLASSNAME::*f)(widget_param*)>
static bool static_to_nonstatic(widget_param* param)
{
    return (get_instance(param)->*f)(param);
}

answered Mar 6, 2016 at 14:35

Stew

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