Question

For example if I do something like this:

vector<int> myvector;
myvector.push_back(100);
int * ptr = &(myvector[0]);
myvector.clear();
myvector.push_back(10);

Will ptr still be valid? Or could it now be pointing to garbage?

Answer 1

23.2.3 §4 says:

a.clear() […] invalidates all references, pointers, and iterators referring to the elements of a and may invalidate the past-the-end iterator.

Since there is no such thing as “un-invalidation”, using ptr after clear results in undefined behavior.

On a side note, the parenthesis in &(myvector[0]) are not needed. Postfix operators always have higher precedence than prefix operators in C++, so writing &myvector[0] is just fine.