CODEWITHSUNDEEP
c++shared-ptr
user2840470
user2840470

Reputation: 969

Properly create shared pointer for other objects

I am trying to use a shared pointer that will be shared to different objects throughout my program. I have noticed that when I create the pointer before giving it to the objects that need it, there is no data. It is probably something to do with how I am implementing the pointers. I have added a sample of what I am trying to do, and what is happening

int main() {
    // This would be the data I want to share. For simplicity
    // using a vector of ints
    std::vector<int> data;
    // Create the shared pointer
    // The objects that are getting access are not allowed to change the 
    // the data so thats why I put in const (could be wrong on what that is
    // actually doing)
    std::shared_ptr<const std::vector<int>> pointer_to_vector =
    std::make_shared<std::vector<int>>(data);
    // Add data to the object wanting to share
    data.push_back(1);
    data.push_back(2);
    data.push_back(3);
    data.push_back(4);
    // If I look at pointer_to_vector in debu at this point it shows no
    // data, size 0
}

If I create the shared pointer (pointer_to_vector) after I add the data (push_back) it works. However if I want to add data to the vector later in the program, the pointer will not be updated, so the objects dont have access to the new data. Is there something I am doing wrong, or fundamentally my understanding is wrong?

Upvotes: 0

Views: 334

Answers (2)

Toby Speight
Toby Speight

Reputation: 30709

This code:

std::vector<int> data;
auto pointer_to_vector = std::make_shared<std::vector<int>>(data);

creates a new object that is a copy of data and returns a shared pointer to it. You then add elements to data, but not to the new vector.

However, if you write:

auto pointer_to_vector = std::make_shared<std::vector<int>>();
auto& data = *pointer_to_vector;

then you have a newly-created vector, and data is a reference to that object. The latter looks closer to what you intended.

Upvotes: 0

πάντα ῥεῖ
πάντα ῥεῖ

Reputation: 1

data is unrelated to the instance created with std::make_shared<std::vector<int>>(data);.

What you really want is

std::shared_ptr<const std::vector<int>> pointer_to_vector =
std::make_shared<std::vector<int>>(data);
// Add data to the object wanting to share
pointer_to_vector->push_back(1);
pointer_to_vector->push_back(2);
pointer_to_vector->push_back(3);
pointer_to_vector->push_back(4);

The vector created with the std::make_shared<std::vector<int>>(data) takes a copy for initialization. The instances are still unrelated though.

Upvotes: 2

Related Questions