Erasing() an element in a vector doesn't work

Question

I have a vector. I need to delete the last 3 elements in it. Described this logic. The program crashes. What could be the mistake?

vector<float>::iterator d = X.end();
    for (size_t i = 1; i < 3; i++) {
        if (i == 1) X.erase(d);
        else X.erase(d - i);
    }

Answer 1

If there are at least 3 items in the vector, to delete the last 3 items is simple -- just call pop_back 3 times:

#include <vector>
#include <iostream>

int main() 
{
    std::vector<float> v = { 1, 2, 3, 4, 5 };
    for (int i = 0; i < 3 && !v.empty(); ++i)
       v.pop_back();

    for ( const auto &item : v ) std::cout << item << ' ';
        std::cout << '\n';
}

Output:

1 2

Answer 2

It is undefined behavior to pass the end() iterator to the 1-parameter erase() overload. Even if it weren't, erase() invalidates iterators that are "at and after" the specified element, making d invalid after the 1st loop iteration.

std::vector has a 2-parameter erase() overload that accepts a range of elements to remove. You don't need a manual loop at all:

if (X.size() >= 3)
    X.erase(X.end()-3, X.end());

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