STL: Stores references or values?

Question

Hi,

I've always been a bit confused about how STL containers (vector, list, map...) store values. Do they store references to the values I pass in, or do they copy/copy construct +store the values themselves?

For example,

int i;
vector<int> vec;
vec.push_back(i);
// does &(vec[0]) == &i;

and

class abc;
abc inst;
vector<abc> vec;
vec.push_back(inst);
// does &(vec[0]) == &inst;

Thanks

Answer 1

STL Containers copy-construct and store values that you pass in. If you want to store objects in a container without copying them, I would suggest storing a pointer to the object in the container:

class abc;
abc inst;
vector<abc *> vec;
vec.push_back(&inst);

This is the most logical way to implement the container classes to prevent accidentally storing references to variables on defunct stack frames. Consider:

class Widget {
public:
    void AddToVector(int i) {
        v.push_back(i);
    }
private:
    vector<int> v;
};

Storing a reference to i would be dangerous as you would be referencing the memory location of a local variable after returning from the method in which it was defined.

Answer 2

That depends on your type. If it's a simple value type, and cheap to copy, then storing values is probably the answer. On the other hand, if it's a reference type, or expensive to copy, you'd better store a smart pointer (not auto_ptr, since its special copy semantics prevent it from being stored in a container. Go for a shared_ptr). With a plain pointer you're risking memory leakage and access to freed memory, while with references you're risking the latter. A smart pointer avoids both.