Assigning a shared_ptr to another after calling get

Question

shared_ptr<std::string> shared_ptr1 = std::make_shared<std::string>("Foo");
shared_ptr<std::string> shared_ptr2 = std::make_shared<std::string>("Bar"); 

std::string* normal_ptr = shared_ptr1.get(); 
shared_ptr1 = shared_ptr2;

Now, will the first string "Foo" be garbage collected after the "shared_ptr1 = shared_ptr2" assignment? According to this , "Foo" isn't garbage collected. But I just want to make sure that what I am encountering isn't an undefined behavior.

Thank you!

We don't really know exactly what you're encountering. But I suspect it has to do with the pointer normal_ptr becoming invalid after the assignment shared_ptr1 = shared_ptr2? — Some programmer dude
– Some programmer dude, Commented May 21, 2019 at 17:24
@molbdnilo std::shared_ptr can be seen as C++'s way of supporting reference counting. Reference counting is a valid form of garbage collection. I believe the view point that std::shared_ptr is a form of garbage collection to be fair. — François Andrieux
– François Andrieux, Commented May 21, 2019 at 17:25
You are reading in deleted memory. If you run your code with valgrind, you'll see the errors. — Guillaume Racicot
– Guillaume Racicot, Commented May 21, 2019 at 17:26
You can’t deduce anything from the code you linked; it has undefined behaviour. — molbdnilo
– molbdnilo, Commented May 21, 2019 at 17:35
@FrançoisAndrieux shared_ptr is a shared object, with a lifetime as long as its longest-lived owner. Reference-counting is the (rather obvious) implementation mechanism supporting shared ownership, not the other way around. — molbdnilo
– molbdnilo, Commented May 21, 2019 at 17:45

NathanOliver · Accepted Answer · 2019-05-21 17:25:07Z

4

Now, will the first string "Foo" be garbage collected after the "shared_ptr1 = shared_ptr2" assignment?*

If you mean will the string that was allocated by std::make_shared<std::string>("Foo") be destroyed, then yes.

That means that normal_ptr, after you do shared_ptr1 = shared_ptr2; points to an object that no longer exists.

answered May 21, 2019 at 17:25

NathanOliver

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Comments

Guillaume Racicot · Accepted Answer · 2019-05-21 17:26:50Z

I would not use the phrase "garbage collected". After the mentioned assignment, the object will be simply deleted because no shared_ptr points to it. You can test it with the following code:

struct S{
    std::string s;
    S(std::string s){
        this->s = s;
    }
    ~S(){
        std::cout << "~S() with string " << s << std::endl;   
    }
};

int main()
{
    std::shared_ptr<S> shared_ptr1 = std::make_shared<S>("Foo");
    std::shared_ptr<S> shared_ptr2 = std::make_shared<S>("Bar"); 

    S* normal_ptr = shared_ptr1.get(); 
    std::cout << "after get()" << std::endl;
    shared_ptr1 = shared_ptr2;
    std::cout << "end of main" << std::endl;
}

which gives

after get()
~S() with string Foo
end of main
~S() with string Bar

which means that after the assignment, normal_ptr points to non-existing object.

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