How to Implement Move Capture in C Lambdas?

Move Capture in Lambdas

Question:

How do we implement move capture, also known as rvalue references, in C 11 lambdas? For instance:

std::unique_ptr<int> myPointer(new int);

std::function<void(void)> example = [std::move(myPointer)] {
   *myPointer = 4;
};

Answer:

Generalized Lambda Capture in C 14

In C 14, generalized lambda capture allows for move capture. This code is now valid:

using namespace std;

auto u = make_unique<some_type>(some, parameters);  
go.run([u = move(u)] { do_something_with(u); }); 

To move objects from a lambda to another function, make the lambda mutable:

go.run([u = move(u)] mutable { do_something_with(std::move(u)); });

Workaround for Move Capture in C 11

A helper function, make_rref, can facilitate move capture. Its implementation is as follows:

#include <cassert>
#include <memory>
#include <utility>

template <typename T>
struct rref_impl {
    rref_impl() = delete;
    rref_impl(T&& x) : x{std::move(x)} {}
    rref_impl(rref_impl& other)
        : x{std::move(other.x)}, isCopied{true}
    {
        assert(other.isCopied == false);
    }
    rref_impl(rref_impl&& other)
        : x{std::move(other.x)}, isCopied{std::move(other.isCopied)}
    {
    }
    rref_impl& operator=(rref_impl other) = delete;
    T& operator&&() {
        return std::move(x);
    }

private:
    T x;
    bool isCopied = false;
};

template<typename T> rref_impl<T> make_rref(T&& x) {
    return rref_impl<T>{std::move(x)};
}

A test case for make_rref:

int main() {
    std::unique_ptr<int> p{new int(0)};
    auto rref = make_rref(std::move(p));
    auto lambda =
        [rref]() mutable -> std::unique_ptr<int> { return rref.move(); };
    assert(lambda());
    assert(!lambda());
}

Emulating Generalized Lambda Capture in C 11

Another workaround is provided by the capture() function:

#include <cassert>
#include <memory>

int main() {
    std::unique_ptr<int> p{new int(0)};
    auto lambda = capture(std::move(p),
        [](std::unique_ptr<int>& p) { return std::move(p); });
    assert(lambda());
    assert(!lambda());
}

capture is implemented as follows:

#include <utility>

template <typename T, typename F>
class capture_impl {
    T x;
    F f;
public:
    capture_impl(T&& x, F&& f)
        : x{std::forward<T>(x)}, f{std::forward<F>(f)} {}

    template <typename ...Ts> auto operator()(Ts&& ...args)
        -> decltype(f(x, std::forward<Ts>(args)...)) {
        return f(x, std::forward<Ts>(args)...);
    }

    template <typename ...Ts> auto operator()(Ts&& ...args) const
        -> decltype(f(x, std::forward<Ts>(args)...)) {
        return f(x, std::forward<Ts>(args)...);
    }
};

template <typename T, typename F>
capture_impl<T, F> capture(T&& x, F&& f) {
    return capture_impl<T, F>(
        std::forward<T>(x), std::forward<F>(f));
}

This solution prevents copying the lambda if the captured type is not copyable, avoiding runtime errors.

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