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

Question

My comments on this answer got me thinking about the issues of constness and sorting. I played around a bit and reduced my issues to the fact that this code:

#include <vector>

int main() {
    std::vector <const int> v;  
}

will not compile - you can't create a vector of const ints. Obviously, I should have known this (and intellectually I did), but I've never needed to create such a thing before. However, it seems like a useful construct to me, and I wonder if there is any way round this problem - I want to add things to a vector (or whatever), but they should not be changed once added.

There's probably some embarrassingly simple solution to this, but it's something I'd never considered before. I probably should not have mentioned sorting (I may ask another question about that, see this for the difficulties of asking questions). My real base use case is something like this:

vector <const int> v;     // ok (i.e. I want it to be OK)
v.push_back( 42 );        // ok
int n = v[0];             // ok
v[0] = 1;                 // not allowed

Answer 1

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.