使用std :: copy将元素插入向量

Question

In my hand-write vector I needed to implement insert function, so I did it like this:

void insert(size_t index, const T& x) { 
    std::copy(begin() + index, end() - 1, begin() + index + 1);
    new(storage_ + index) T(x); // "storage_" is my vector's inner array
}

And this works absolutely correct on test like this:

myvector<int> v;
for (int i = 0; i < 10; i++)
    v.push_back(i + 1);
v.insert(5, 0); // insert(size_t position, const T& element)

This outputs:

1 2 3 4 5 0 6 7 8 9 10

(this means insert worked fine)

But I absolutely don't understand why this code works the way it is because on cplusplus.com ( link ) I found that std::copy works like this:

template<class InputIterator, class OutputIterator>
OutputIterator copy (InputIterator first, InputIterator last, OutputIterator result)
{
    while (first!=last) {
        *result = *first;
        ++result; ++first;
    }
return result;
}

But doesn't it mean, that if I call

std::copy(begin() + index, end() - 1, begin() + index + 1);

It will replace 7 with 6 when it executes *result = *first; and after ++result; ++first; ++result; ++first; it will replace 8 with 6 and so on.

In myvector iterators are defined like this:

typedef T* iterator;

and my begin / end is:

iterator begin() {
    return storage_ + 0;
}

iterator end() {
    return storage_ + size_;
}

Answer 1

What you're doing there does not conform to the requirements of std::copy , which include that result NOT be within the range [begin,end) . So anything's possible; you've broken the rules.

Anything is possible, that is... including it working correctly. It sounds like your particular standard library implementation has decided to be merciful, by noticing that you're violating the requirement and instead delegating to std::copy_backward . It's allowed to do this because one possible type of undefined behavior is "stuff still works".