Ælex Ælex - 3 months ago 10
C++ Question

getting template type from lambda auto

I'm in a bit of a pickle, following up my previous question and using a code similar to the one I posted here.


  1. I use a variadic template function which accepts variadic objects

  2. It packs them into a tuple

  3. Iterates them using the
    visitor
    idiom

  4. Binds for each object a callback



Instead of the original minified example shown below:

template <typename... Args>
void make_classes(Args... args)
{
auto t = std::tuple<Args...>(args...);
unsigned int size = std::tuple_size<decltype(t)>::value;
auto execute = [](auto & obj){ obj.operator()(); };

for (int i = 0; i < size; i++) {
visit_at(t, i, execute);
}
}


I am trying to understand how I can deduce the template type of the
auto
lambda, so that I can bind it:

template <typename... Args>
void make_classes(Args... args)
{
auto t = std::tuple<Args...>(args...);
unsigned int size = std::tuple_size<decltype(t)>::value;
auto execute = [](auto & obj){
// obtain type of obj as T?
auto callback = std::bind(&T::deserialise, obj, std::placeholders::_1);
// do something else here using this callback.
};

for (int i = 0; i < size; i++) {
visit_at(t, i, execute);
}
}


There's a catch: the parameter objects are non-copyable (although I could change that), but I would like to know if/how the above could work by deducing the template type packed in the tuple as obtained by the visitor.

If I can't deduce the type inside the lambda, can I somehow store it within the tuple (e.g.,: type & object) in order to later extract it?

Answer

Just use another lambda:

auto callback = [&obj](auto& x){
    obj.deserialise(x);
};

std::bind is rarely useful. (If you really want to copy obj, you can drop the leading &.)


Moreover, you don't actually need a tuple...

template <class F, class... Args>
void for_each_arg(F&& f, Args&&... args) {
    using swallow = int[];
    (void)swallow{0,
        (void(f(std::forward<Args>(args))), 0)...
    };
}

template <typename... Args>
void make_classes(Args... args)
{
    for_each_arg([](auto& obj){
        auto callback = [&obj](auto& x) { obj.deserialise(x); };
        // do something with callback
    }, args...);
}