CODEWITHSUNDEEP
c++c++11
chrise
chrise

Reputation: 4253

moving unique pointers between deques

I have a container class for some unique pointers to objects.

I now need to take one of them, do something with them in a different class and hand over to another class (and at some point recycle them and put them back to the container ).

Below is the outline of the code. However I am getting confused with the right way to move unique pointers with functions:

using uptrPod =  unique_ptr<Pod>;
using dqUptrPods = deque<uptrPods>;

class Container {
public:
    Container() :
        n_elements_( 500 )
    {
        for ( auto i = 0; i < n_elements_; i++ ) {
        free_pods_.push_back( uptrPod( new Pod()) );
    }

    const uptrPod&& getPod() {
        auto up_pod= std::move( free_pods_[0] );
        free_pods_.erase( free_pods_.begin() );
        return up_pod;
    }

    void returnPod( const uptrPod&& up_pod ) {
        free_pods_.push_back( up_pod );
    }

private:
    long n_elements_;
    dqUptrPods free_pods_;
};

class PodTracker {

    void addPod( const uptrPod&& up_pod ) { dq_pods_.pushback( up_pod ); }
    dqUptrPods dq_pods_;
};

class PodHandler {

    void movePod() {
        up_pod = std::move( container_->getPod() );

        /// do stuff with pod

        pod_tracker_->addPod( up_pod );
    }

    Container* container_;
    PodTracker* pod_tracker_;
};

I am getting the error:

cannot bind std::unique_ptr l value to const uptrPod&& { aka const std::unique_ptr &&

How can I hand around the pointer between the classes?

Upvotes: 1

Views: 455

Answers (1)

Vittorio Romeo
Vittorio Romeo

Reputation: 93324

Pass/return the std::unique_ptr instances by value, and explicitly use std::move:

uptrPod getPod() { 
    auto up_pod= std::move( free_pods_[0] ); 
    free_pods_.erase( free_pods_.begin() );
    return up_pod;
}


void returnPod( uptrPod up_pod ) { 
    free_pods_.push_back( std::move(up_pod) );        
}


class PodTracker{
    void addPod(uptrPod up_pod) { dq_pods_.push_back(std::move(up_pod)); }
    dqSptrPods dq_pods_;
};


void movePod() {

    // `std::move` is not necessary here, as `getPod()`
    // is already a temporary (rvalue)
    auto up_pod = container_->getPod();

    /// do stuff with pod

    pod_tracker_->addPod( std::move(up_pod) ) ;   
}

example on wandbox

Note that const rvalue references do not make much sense - an rvalue reference.

The main purpose of rvalue references is to allow us to move objects instead of copying them. And moving the state of an object implies modification. As a result, the canonical signatures for the move constructor and the move assignment operator both take its argument as a non-const rvalue reference.

(From: "What are const rvalue references good for?")

Upvotes: 4

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