C++ STL vector Deep erase

Question

How to deep erase a vector?

Consider the following code.

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

using namespace std;

int main(){
    vector<int> v {1,2,3,4,5};
    for_each(begin(v),end(v),[&v](int& n){
        static auto i = (int)0;
        if(n == 2){
            v.erase ( begin(v) +2, end(v));
        }
        cout << n << "  having index " << i++ << endl;
    });
    v.erase ( begin(v) +2, end(v));
    cout << v.size() << endl << v[4] << endl;
}

Output is

1  having index 0
2  having index 1
3  having index 2
4  having index 3
5  having index 4
2
4

What I want is accessing v[i] for any i from 2 to 4 to be invalid and compiler to throw an error.

In simpler words how to deep erase a vector?

Answer 1

You're triggering undefined behavior and therefore your results cannot be trusted.

If you need to check for boundaries use std::vector::at

vector<int> v{ 1,2,3,4,5 };
v.erase(begin(v) + 2, end(v));
try {
  auto val = v.at(4);
} catch (std::out_of_range&) {
  cout << "out of range";
}

Unless you code your own facility or workaround, you can't have such compile-time checks with std::vector . More information and some suggestions here: https://stackoverflow.com/a/32660677/1938163

Answer 2

I do not have time right now to give examples, but the only way I can think to cleanly do this is to either make a custom class to wrap or subclass vector. You could then produce a custom [] and at operators which throw an error on certain indexes. You could even have a deletion method which adds indeces to this list of banned ones.

Now if you need the error at compile time this is harder. I think something might be possible using a constexpr access operator and some static_assert s but I am not confident exactly how off hand.