Why can't I use reference types as the value type of a container type?

Question

For example, std::vector<int&> vec_int; this seems to be invalid in c++. Why is this invalid?

Answer 1

STL containers need to be able to construct objects with default constructor. You cannot do that with a reference. A reference is guaranteed to be valid, therefore it always has to be initialized with assignment.

You need to use a pointer instead.

Answer 2

Internally, a vector<T> uses an array to store a sequence of T objects. Since references aren't objects, there is no such thing as an array of references (see 8.3.2 §5 in the standard), thus reference types cannot be used to parameterize the vector template.

What you probably want is a vector of smart pointers such as std::vector<boost::shared_ptr<T> > or a dedicated pointer container such as boost::ptr_vector<T>.

Answer 3

Answer, as per chryspi request above. As commented in other responses, you cannot use references directly as references don't exist by themselves.

You can use references, however, but by using the boost::reference_wrapper<T> utility class:

    typedef boost::reference_wrapper<int> intref;
    std::vector<intref> v;
    int i;
    i = 9;

    v.push_back (boost::ref (i));  // store &i

    int& j = v[0];
    j = 10;
    //v[0].get() = 10;

    std::cout << i << std::endl; // prints 10

I put as an example how to modify directly the element v[0]. Note that it is somewhat tricky (you have to call the get() method) because you get a boost::reference_wrapper<T> instead of an actual reference.

For storing pointers safely with respect to memory, you can use boost::shared_ptr similarly.