How to create a variadic generic lambda?

Question

Since C++14 we can use generic lambdas:

auto generic_lambda = [] (auto param) {};

This basically means that its call operator is templated based on the parameters marked as auto.

The question is how to create a lambda that can accept a variadic number of parameters similarly to how a variadic function template would work ? If this is not possible what is the closest thing that could be used the same way ? How would you store it ? Is it possible in a std::function ?

Answer 1

I am not sure what your intention is but instead of storing it in a std::function you can use the lambda itself to capture the params. This is an example discussed on the boost mailing list. It is used in the boost::hana implementation

auto list = [](auto ...xs) {
    return [=](auto access) { return access(xs...); };
};

auto head = [](auto xs) {
    return xs([](auto first, auto ...rest) { return first; });
};

auto tail = [](auto xs) {
    return xs([](auto first, auto ...rest) { return list(rest...); });
};

auto length = [](auto xs) {
    return xs([](auto ...z) { return sizeof...(z); });
};

// etc...
// then use it like

auto three = length(list(1, '2', "3"));