Question

I’m having a std::vector with elements of some class ClassA. Additionally I want to create an index using a std::map<key,ClassA*> which maps some key value to pointers to elements contained in the vector.

Is there any guarantee that these pointers remain valid (and point to the same object) when elements are added at the end of the vector (not inserted). I.e, would the following code be correct:

std::vector<ClassA> storage;
std::map<int, ClassA*> map;

for (int i=0; i<10000; ++i) {
  storage.push_back(ClassA());
  map.insert(std::make_pair(storage.back().getKey(), &(storage.back()));
}
// map contains only valid pointers to the 'correct' elements of storage

How is the situation, if I use std::list instead of std::vector?

Answer 1

Vectors – No. Because the capacity of vectors never shrinks, it is guaranteed that references, pointers, and iterators remain valid even when elements are deleted or changed, provided they refer to a position before the manipulated elements. However, insertions may invalidate references, pointers, and iterators.

Lists – Yes, inserting and deleting elements does not invalidate pointers, references, and iterators to other elements