How Can I Achieve Move Capture in C Lambdas (C 11 and C 14)?-C++-php.cn

How Can I Achieve Move Capture in C Lambdas (C 11 and C 14)?

Barbara Streisand

Release： 2024-12-17 20:56:12

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How Can I Achieve Move Capture in C Lambdas (C 11 and C 14)?

Move Capture in C 11 Lambda

Generalized Lambda Capture in C 14

C 14 introduces generalized lambda capture, enabling move capture. In C 14, code like the following will be valid:

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

// move the unique_ptr into the lambda
go.run( [ u = move(u) ] { do_something_with( u ); } );

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Note that if you need to move the object from the lambda to another function, the lambda must be marked as mutable:

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

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Workaround for Move Capture in C 11

In C 11, capturing by move can be achieved using a helper function make_rref that creates an rvalue reference to the desired value:

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

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

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

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Use the make_rref function to capture the value in the lambda:

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() );
}

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Note that this workaround makes the lambda copyable, which may cause problems when capturing non-copyable objects.

Emulating Generalized Lambda Capture in C 11

Another approach to emulate generalized lambda capture involves defining a capture function:

#include <utility>

template <typename T, typename F>
class capture_impl
{
    T x;
    F f;
public:
    // ... implementation
};

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

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Use capture to capture the value by move:

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

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This solution provides a cleaner approach that disables copying the lambda if the captured type is not copyable.

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