CODEWITHSUNDEEP
c++shared-ptr
prestokeys
prestokeys

Reputation: 4849

enable_shared_from_this. Why the crash?

Could someone explain why the following crashes? I'm using enable_shared_from_this so that bob does not get deleted.

class Person : public std::enable_shared_from_this<Person> {
    private:
        std::string name;
    public:
        Person (const std::string& name) : name(name) {}
        std::string getName() const {return name;}
        void introduce() const;
};

void displayName (std::shared_ptr<const Person> person) {
    std::cout << "Name is " << person->getName() << "." << std::endl;
}

void Person::introduce() const {
    displayName (this->shared_from_this());
}

int main() {
    Person* bob = new Person ("Bob");
    bob->introduce();  // Crash here.  Why?
}

Upvotes: 3

Views: 2912

Answers (2)

Horstling
Horstling

Reputation: 2151

One of the preconditions of shared_from_this is that the object (this) has to be already owned by some shared_ptr. It then returns a shared_ptr that shares ownership with the already existing shared_ptr.

Because your object is not owned by any shared_ptr when calling shared_from_this, you are invoking undefined behaviour (it crashes).

Try this instead:

int main() {
    auto bob = std::make_shared<Person>("Bob");
    bob->introduce();
}

Upvotes: 7

clcto
clcto

Reputation: 9648

The documentation states:

Before calling shared_from_this, there should be at least one std::shared_ptr<T> p that owns *this.

So if we change main() to:

std::shared_ptr<Person> bob( new Person ("Bob") );
bob->introduce();

there will be no issue since there is already a shared_ptr that owns *this

Upvotes: 4

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