CODEWITHSUNDEEP
c++lambdavariadic-templatesperfect-forwarding
RiaD
RiaD

Reputation: 47658

Move all arguments to lambda

I want to create lambda that will accept any number of arguments like:

template <typename... Args>
void f(Args... args) {
    auto l = [args...]() {
        g(args...);
    }
    // use l
}

Problem here is that it doesn't work with move-only types. If it was only 1 arg I would do smth like

void f(Arg arg) {
    auto l = [arg = std::move(arg)]() {
        g(move(arg));
    }
}

How to move all args to lambda?

Upvotes: 9

Views: 900

Answers (1)

Yakk - Adam Nevraumont
Yakk - Adam Nevraumont

Reputation: 275878

template <class... Args>
void f(Args... args) {
  auto l = [tup=std::make_tuple(std::move(args)...)] {
    std::apply([](auto&&...args){
      g(decltype(args)(args)...);
    }, tup );
  };
}

A bit icky.

Pack them into a tuple, then unpack tuple with std::apply. If you lack std::apply write yourself an equivalent one.

If you want to invoke g with rvalues, make outer lambda mutable, and move tuple into inner lambda.

Inner lambda can capture by default & if you want access to args of outer or the like.

We can even abstract this pattern a bit:

template<class F, class...Args>
auto forward_capture( F&& f, Args&&...args ) {
  return [
    f=std::forward<F>(f),
    tup=std::make_tuple(std::forward<Args>(args)...)
  ]{
    return std::apply( f, tup );
  };
}

use:

template <typename... Args>
void f(Args... args) {
  auto l = forward_capture(
    [](auto&&...args) {
      g(args...);
    },
    std::move(args)...
  );
  // use l
}

If you want the capture list first, we can do it:

template<class...Args>
auto forward_capture( Args&&...args ) {
  return [
    tup=std::make_tuple(std::forward<Args>(args)...)
  ](auto&& f)mutable{
    return [
      f=decltype(f)(f),
      tup=std::move(tup)
    ]{
      return std::apply( f, tup );
    };
  };
}

use:

template <typename... Args>
void f(Args... args) {
  auto l = forward_capture(std::move(args)...)(
    [](auto&&...args) {
      g(args...);
    }
  );
  // use l
}

which has the "advantage" that we have 3 nested lambdas.

Or more fun:

template<class...Args>
struct arrow_star {
    std::tuple<Args...> args;
    template<class F>
    auto operator->*(F&& f)&& {
      return [f=std::forward<F>(f),args=std::move(args)]()mutable{
        return std::experimental::apply( std::move(f), std::move(args) );
      };
    }
};
template<class...Args>
arrow_star<std::decay_t<Args>...> forward_capture( Args&&...args ) {
  return {std::make_tuple(std::forward<Args>(args)...)};
}
template<class...Args>
auto f(Args... args)
{
  return
    forward_capture( std::move(args)... )
    ->*
    [](auto&&...args){
      g(decltype(args)(args)...);
    };
}

live example.

Upvotes: 8

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