Can I use const in vectors to allow adding elements, but not modifications to the already added?

Question

Well, in C++0x you can...

In C++03, there is a paragraph 23.1[lib.containers.requirements]/3, which says

The type of objects stored in these components must meet the requirements of CopyConstructible types (20.1.3), and the additional requirements of Assignable types.

This is what's currently preventing you from using const int as a type argument to std::vector.

However, in C++0x, this paragraph is missing, instead, T is required to be Destructible and additional requirements on T are specified per-expression, e.g. v = u on std::vector is only valid if T is MoveConstructible and MoveAssignable.

If I interpret those requirements correctly, it should be possible to instantiate std::vector<const int>, you'll just be missing some of its functionality (which I guess is what you wanted). You can fill it by passing a pair of iterators to the constructor. I think emplace_back() should work as well, though I failed to find explicit requirements on T for it.

You still won't be able to sort the vector in-place though.

Types that you put in a standard container have to be copyable and assignable. The reason that auto_ptr causes so much trouble is precisely because it doesn't follow normal copy and assignment semantics. Naturally, anything that's const is not going to be assignable. So, you can't stick const anything in a standard container. And if the element isn't const, then you are going to be able to change it.

The closest solution that I believe is possible would be to use an indirection of some kind. So, you could have a pointer to const or you could have an object which holds the value that you want but the value can't be changed within the object (like you'd get with Integer in Java).

Having the element at a particular index be unchangeable goes against how the standard containers work. You might be able to construct your own which work that way, but the standard ones don't. And none which are based on arrays will work regardless unless you can manage to fit their initialization into the {a, b, c} initialization syntax since once an array of const has been created, you can't change it. So, a vector class isn't likely to work with const elements no matter what you do.

Having const in a container without some sort of indirection just doesn't work very well. You're basically asking to make the entire container const - which you could do if you copy to it from an already initialized container, but you can't really have a container - certainly not a standard container - which contains constants without some sort of indirection.

EDIT: If what you're looking to do is to mostly leave a container unchanged but still be able to change it in certain places in the code, then using a const ref in most places and then giving the code that needs to be able to change the container direct access or a non-const ref would make that possible.

So, use const vector<int>& in most places, and then either vector<int>& where you need to change the container, or give that portion of the code direct access to the container. That way, it's mostly unchangeable, but you can change it when you want to.

On the other hand, if you want to be able to pretty much always be able to change what's in the container but not change specific elements, then I'd suggest putting a wrapper class around the container. In the case of vector, wrap it and make the subscript operator return a const ref instead of a non-const ref - either that or a copy. So, assuming that you created a templatized version, your subscript operator would look something like this:

const T& operator[](size_t i) const
{
    return _container[i];
}

That way, you can update the container itself, but you can't change it's individual elements. And as long as you declare all of the functions inline, it shouldn't be much of a performance hit (if any at all) to have the wrapper.

You can't create a vector of const ints, and it'd be pretty useless even if you could. If i remove the second int, then everything from there on is shifted down one -- read: modified -- making it impossible to guarantee that v[5] has the same value on two different occasions.

Add to that, a const can't be assigned to after it's declared, short of casting away the constness. And if you wanna do that, why are you using const in the first place?

You're going to need to write your own class. You could certainly use std::vector as your internal implementation. Then just implement the const interface and those few non-const functions you need.

Although this doesn't meet all of your requirements (being able to sort), try a constant vector:

int values[] = {1, 3, 5, 2, 4, 6};
const std::vector<int> IDs(values, values + sizeof(values));

Although, you may want to use a std::list. With the list, the values don't need to change, only the links to them. Sorting is accomplished by changing the order of the links.

You may have to expend some brain power and write your own. :-(

I would have all my const objects in a standard array.
Then use a vector of pointers into the array.
A small utility class just to help you not have to de-reference the objects and hay presto.

#include <vector>
#include <algorithm>
#include <iterator>
#include <iostream>


class XPointer
{
    public:
        XPointer(int const& data)
            : m_data(&data)
        {}

    operator int const&() const
    {
        return *m_data;
    }

    private:
        int const*  m_data;

};

int const               data[]    =  { 15, 17, 22, 100, 3, 4};

std::vector<XPointer>   sorted(data,data+6);


int main()
{
    std::sort(sorted.begin(), sorted.end());
    std::copy(sorted.begin(), sorted.end(), std::ostream_iterator<int>(std::cout, ", "));
    int x   = sorted[1];
}