How Can I Efficiently Capture Objects by Move in C Lambda Expressions?

Move Capture in Lambda Expressions

Introduction

In C , capturing by move, also known as rvalue reference, allows for the efficient transfer of ownership of an object to a lambda expression. This technique can be particularly useful in situations where the object's resources need to be managed explicitly within the lambda.

Generalized Lambda Capture in C 14

In C 14, generalized lambda capture provides an elegant solution for move capture. With this feature, it is possible to declare a lambda expression that captures a variable by value using move semantics:

using namespace std;

auto u = make_unique<some_type>(some, parameters);  

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

This approach moves the ownership of u into the lambda, ensuring that the resources are released appropriately. Additionally, if the object needs to be moved out of the lambda, the lambda must be declared as mutable.

Workaround for Move Capture in C 11

In C 11, move capture is not directly supported. However, there are techniques that can emulate this behavior. One approach is to use a helper function to create an rvalue reference (rref):

template <typename T>
struct rref_impl {
    rref_impl() = delete;
    rref_impl(T &&x) : x(std::move(x)) {}
    // ... (remaining implementation)
};

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

Using this helper function, it is possible to capture an object by move in a lambda:

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

Note that the lambda is declared as mutable to allow for modifying the captured rvalue reference.

Emulating Generalized Lambda Capture in C 11

Another approach to emulating generalized lambda capture in C 11 is to use a custom class to capture a variable and provide a way to move it out of the lambda:

template <typename T, typename F>
struct capture_impl {
    T x;
    F f;
    // ... (remaining implementation)
};

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 class can be used to capture a variable by move and provide a way to invoke the lambda:

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

Unlike the previous approach, this lambda is not copyable, if the captured type is not copyable. This prevents potential errors when copying the lambda.

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