How to store unique_ptr in a queue

Question

I have such a code, where I try to store a std::unique_ptr<T> in a std::queue but it won't compile

#include "stdafx.h" #include <windows.h> #include <memory> #include <string> #include <iostream> #include <deque> using namespace std; class Foo { std::string _s; public: Foo(const std::string &s) : _s(s) { cout << "Foo - ctor"; } ~Foo() { cout << "Foo - dtor"; } void Say(const string &s) { cout << "I am " << _s << " and addtionaly " << s; } }; typedef std::pair<long, std::unique_ptr<Foo>> MyPairType; typedef std::deque<MyPairType> MyQueueType; void Func(const std::unique_ptr<Foo> &pf) { pf->Say("Func"); } void AddToQueue(MyQueueType &q, std::unique_ptr<Foo> &pF){ MyPairType p; ::GetSystemTimeAsFileTime((FILETIME*)&p.first); p.second = pF; // **Fails here** q.push_back(p); } int _tmain(int argc, _TCHAR* argv[]) { std::unique_ptr<Foo> pF(new Foo("Aliosa")); Func(pF); return 0; }

It says that I cannot assign in the method AddToQueue. I know this is possible to do with boost::shared_ptr but we are trying to get rid of boost dependency thus having such issue.

Any idea how to achieve the needed behavior? thx

Fantastic Mr Fox · Accepted Answer · 2015-01-06 06:18:08Z

This line:

p.second = pF;

is making a copy of a unique pointer (ie it is no longer unique). You can do the following:

MyPairType p; ::GetSystemTimeAsFileTime((FILETIME*)&p.first); p.second.swap(pF); q.push_back(p);

But remember that then pF will no longer reference the pointer address. If you want more references to the same address you need to use std::shared_ptr.

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