CODEWITHSUNDEEP
c++thisweak-ptr
Nathaniel G.M.
Nathaniel G.M.

Reputation: 433

How do you assign weak_ptr by passing the "this" keyword?

In my program Groups will have shared pointers to Subjects; and Subjects will have weak pointers to their Groups. I want the Group to have a join() function that assigns the Subject's weak pointer to itself. Below is the minimal code for what I've tried. How do I fix the join() function?

#include <iostream>
#include <string>
#include <memory>

class Party;

class Subject
{
public:
    std::weak_ptr<Party> MyParty;
};

class Party
{
public:
    std::string Name;

    void join(std::shared_ptr<Subject> subject)
    {
        subject->MyParty = std::make_shared<Party>(*this); // <---- PROBLEM
    }
};

int main()
{
    auto& BlueParty = std::make_shared<Party>();
    BlueParty->Name = "Blue Party";

    auto& Jane = std::make_shared<Subject>();

    BlueParty->join(Jane);

    if (auto ptr = Jane->MyParty.lock())
    { 
        std::cout << "I am in " << ptr->Name << std::endl; 
    }

    else { std::cout << "I have no party." << std::endl; }

    return 0;
}

The program prints out "I have no party". If the assignment were successful, it should have printed out "I am in Blue Party".

Upvotes: 1

Views: 912

Answers (1)

Miles Budnek
Miles Budnek

Reputation: 30619

The line subject->MyParty = std::make_shared<Party>(*this); creates a new Party object that is a copy of *this and is managed by a temporary std::shared_ptr. subject->MyParty gets assigned from that temporary shared_ptr, but weak_ptrs don't keep the objects they point to alive. As soon as that statement completes, the temporary shared_ptr returned by make_shared is destroyed and takes the Party object it was managing with it. subject->MyParty now doesn't point to anything.

The solution is to use std::enable_shared_from_this:

class Party : public std::enable_shared_from_this<Party>
{
public:
    std::string Name;

    void join(std::shared_ptr<Subject> subject)
    {
        subject->MyParty = shared_from_this();
    }
};

Example

To use shared_from_this, the object must be owned by a std::shared_ptr. It's generally a good idea, in such a case, to mark the class's constructors private and use a factory function that returns a shared_ptr to a new instance so that objects of that type that aren't managed by a shared_ptr can't be accidentally created:

class Party : public std::enable_shared_from_this<Party>
{
public:
    std::string Name;

    static std::shared_ptr<Party> create()
    {
        return std::shared_ptr<Party>{new Party()};
    }

    void join(std::shared_ptr<Subject> subject)
    {
        subject->MyParty = shared_from_this();
    }
private:
    Party() = default;
    Party(const Party&) = delete;
};

Example

Sadly, this makes std::make_shared harder to use. For a bit more info on that issue see this question.

Upvotes: 6

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