On Thu, Apr 22, 2021 at 4:02 PM Thomas Neumann via Std-Proposals <
std-proposals_at_[hidden]> wrote:

>
> Basically I would like to integrate a kind of borrow checker into C++
> for data types that opt-in to that mechanism. For these types the
> compiler enforces the invariant that you can have either one mutable
> reference or arbitrary many immutable references to an object.
>
> A straw man example could look like this:
>
> class MyIntVector [[borrowchecker]] {
> ...
> void push_back(int element);
> [[borrows]] const_iterator begin() const;
> };
>
> void foo(MyIntVector&);
>
> void bar() {
> MyIntVector v;
> v.push_back(1);
>

Consider
    auto it = std::as_const(v).end(); // let's just assume this is OK, and
borrows the object
    v.insert(it, 42); // insert 42 at the end of the vector
    // here it'd be OK to say that `it` is "invalidated", but notice that
from the compiler's POV `it` hasn't been destroyed

or equivalently,
    v.insert(v.end(), 42);

Your proposal will have to deal with this kind of code.
This is a "corner case," but it's pretty far from the actual corner of the
room. :)

–Arthur