Working with std::unique_ptr and std::queue - c++

Question

Maybe it's my sinuses and that I fact that I just started learning about smart pointers today I'm trying to do the following:
Push to the queue
Get the element in the front
Pop the element (I think it will automatically deque once the address out of scope)
Here is the error
main.cpp:50:25: error: cannot convert ‘std::remove_reference&>::type’ {aka ‘std::unique_ptr’} to ‘std::unique_ptr*’ in assignment
50 | inputFrame = std::move(PacketQueue.front());
| ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
| |
| std::remove_reference<std::unique_ptr<MyObject::Packet>&>::type {aka std::unique_ptr<MyObject::Packet>}
Here is the code
#include <iostream>
#include <memory>
#include <queue>
using namespace std;
class MyObject
{
public:
struct Packet
{
uint8_t message;
uint8_t index;
};
void pushToQueue(void);
void FrontOfQueue(std::unique_ptr<Packet> *inputFrame);
private:
std::queue<std::unique_ptr<Packet>> PacketQueue;
};
void MyObject::pushToQueue(void)
{
Packet frame;
static int counter = 1;
frame.message = counter;
frame.index =counter;
counter++;
std::unique_ptr<Packet> passthru_ptr = std::make_unique<Packet>(std::move(frame));
PacketQueue.push(std::move(passthru_ptr));
cout<<"Pushed to queue\n" ;
}
void MyObject::FrontOfQueue(std::unique_ptr<Packet> *inputFrame)
{
inputFrame = std::move(PacketQueue.front());
}
int main()
{
cout<<"Hello World\n";
MyObject object;
object.pushToQueue();
object.pushToQueue();
{
// Scope
std::unique_ptr<MyObject::Packet> *frame;
object.FrontOfQueue(frame);
cout<< frame << endl;
}
{
// Scope
std::unique_ptr<MyObject::Packet> *frame2;
object.FrontOfQueue(frame2);
cout<< frame2 << endl;
}
return 0;
}
Link to the code (Online Compiler)

Answer 1

If I got your aim correctly, you definitely want
std::unique_ptr<MyObject::Packet> MyObject::FrontOfQueue()
{
auto rv = std::move(PacketQueue.front());
PacketQueue.pop();
return rv;
}
// ...
std::unique_ptr<MyObject::Packet> frame = object.FrontOfQueue();
Notice, no raw pointers are used.
I think it will automatically deque once the address out of scope.
This assumption is wrong. Nothing is dequeued until .pop() is called.

Answer 2

Here is my example with some extra logging to show whats going on.
includes an introduction of returning const references as well.
Live demo : https://onlinegdb.com/P2nFkdMy0
#include <iostream>
#include <memory>
#include <queue>
#include <string>
//-----------------------------------------------------------------------------
// do NOT use : using namespace std;
//-----------------------------------------------------------------------------
struct Packet
{
// moved to uint32_t for std::cout reasons.
// uint8_t is displayed as(special) characters
std::uint32_t index;
std::uint32_t message;
Packet() :
index{ next_index() },
message{ index }
{
std::cout << "created packet : " << index << "\n";
}
~Packet()
{
std::cout << "destroyed packet : " << index << "\n";
}
// small helper to not have to declare the static variable seperatly
static std::uint8_t next_index()
{
static int counter;
return counter++;
}
};
//-----------------------------------------------------------------------------
class MyObject
{
public:
void push_packet();
std::unique_ptr<Packet> pop_packet();
// this function returns a const reference (observation only)
// of the packet at the front of the queue
// while leaving the unique pointer on the queue (no moves needed
// packet will still be owned by the queue)
const Packet& front();
private:
std::queue<std::unique_ptr<Packet>> m_queue;
};
void MyObject::push_packet()
{
std::cout << "push_packet\n";
// push a packet
m_queue.push(std::make_unique<Packet>());
std::cout << "push_packet done...\n";
}
std::unique_ptr<Packet> MyObject::pop_packet()
{
std::unique_ptr<Packet> packet = std::move(m_queue.front());
m_queue.pop();
return packet;
}
const Packet& MyObject::front()
{
return *m_queue.front();
}
//-----------------------------------------------------------------------------
int main()
{
const std::size_t n_packets = 3ul;
MyObject object;
for (std::size_t n = 0; n < n_packets; ++n)
{
std::cout << "pushing packet\n";
object.push_packet();
}
for (std::size_t n = 0; n < n_packets; ++n)
{
std::cout << "packet at front : ";
std::cout << object.front().index << "\n";
std::cout << "popping front\n";
auto packet_ptr = object.pop_packet();
std::cout << "popped packet : " << packet_ptr->index << "\n";
}
return 0;
}