Launching std::thread using shared_ptr

Question

When you construct a new thread the supplied function object is copied into the storage belonging to the newly created thread. I want to execute an object method in a new thread. The object should not be copied. So I pass shared_ptr of the object to std::thread constructor. How can I launch a new thread with std::shared_ptr() object? For example

class Foo {
public: 
    void operator()() {       
        // do something
    }    
};

int main() {
    std::shared_ptr<Foo> foo_ptr(new Foo);

    // I want to launch a foo_ptr() in a new thread
    // Is this the correct way?
    std::thread myThread(&Foo::operator(), foo_ptr.get());  
    myThread.join();     
}

Include all error messages and observations in your question please, that's what the linked policy says. VTC your question because of lacking this now, good evening. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Sep 18, 2015 at 16:19
you are nitpicking on missing includes and small stuff. Just let it go.. — Alexey
– Alexey, Commented Sep 18, 2015 at 16:55
I'm your soup nazi here of course ;-). Please improve your question, to be complete and helpful for future research. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Sep 18, 2015 at 17:06
:) I'm no longer engaging in this. I believe the question is clear as is. If you feel like the question should be closed it's fine. If you want to improve it by editing, it's fine too. — Alexey
– Alexey, Commented Sep 18, 2015 at 17:18
Look I've edited a number of other user's questions to improve their quality. Even though my code was a sketch, it's very minimal and clear. This site has helpful community in general. I don't support the way you handle this — Alexey
– Alexey, Commented Sep 18, 2015 at 17:30

Slava · Accepted Answer · 2015-09-18 16:14:36Z

11

You overcomplicate the issue, just pass std::shared_ptr itself, std::bind and std::thread know how to deal with it:

std::thread myThread( &Foo::operator(), foo_ptr );

This way std::thread instance will share ownership and that would guarantee object would not be destroyed before myThread

answered Sep 18, 2015 at 16:14

Slava

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2 Comments

Gizmo Over a year ago

Would that keep a reference to the shared_ptr when detach() is called on thread and the thread object is destroyed?

Gizmo Over a year ago

I tested it in MSVC: seems it does keep the shared_ptr refcount alive after detach. ideone.com/S9ZLyG

typ1232 · Accepted Answer · 2015-09-18 16:16:27Z

5

You have a lot of errors in you sketch code. I think a more readable way would be to use a lambda that captures the sptr by value and then deferences it to call the function.

#include <thread>
#include <memory>
#include <cstdio>

class Foo
{
public:
    void operator()()
    {
        printf("do something\n");
    }
};

int main()
{
    auto foo_ptr = std::make_shared<Foo>();

    std::thread myThread([foo_ptr] { (*foo_ptr)(); });
    myThread.join();
}

It is not perfectly clear what you want to do. If you want to launch a new thread that runs a member function without copying the class instance the syntax would be one of those:

Foo foo;
std::thread th1(&Foo::moo, &foo);
std::thread th2([&]{ foo.moo(); });

edited Sep 18, 2015 at 16:16

answered Sep 18, 2015 at 16:00

typ1232

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2 Comments

πάντα ῥεῖ Over a year ago

Isn't a std::make_shared missing also?

typ1232 Over a year ago

@πάνταῥεῖ Yes. The code actually worked when testing, because the member function does not use the this pointer.

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