Vectors and Pointers

Question

The main issue with vectors and pointers to their elements is that they can be reallocated in memory whenever a push_back is called, rendering the pointer invalid.

I am trying to implement a suffix trie, where I store a data structure node in a vector of nodes. I know that for a string of size n the number n(n+1)/2 is an upperbound for the number of nodes in the trie.

So will the code

std::string T = "Hello StackOverflow!";
std::vector<Node> nodes;

int n = T.length();
nodes.reserve(n*(n+1)/2);

guarantee that any pointers I create referring to elements of nodes will not be invalidated? ie will this guarantee that the vector is not reallocated?

---

Edit : I've implemented this and I keep getting the following error at runtime.

terminate called after throwing an instance of 'std::out_of_range'
what():  basic_string::at: __n (which is 0) >= this->size() (which is 0)
Aborted (core dumped)

Any ideas what could be causing this?

Answer 1

According to the standard ( N4140 ):

23.3.6.3 vector capacity
....

 void reserve(size_type n); 

....
After reserve() , capacity() is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value of capacity() otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument of reserve() .

and

23.3.6.5 vector modifiers
....

 void push_back(const T& x); void push_back(T&& x); 

Remarks: Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid.

Answer 2

You can be certain that your pointers will not be invalidated if you are careful. See std::vector::push_back . It says this about invalidation :

If the new size() is greater than capacity() then all iterators and references (including the past-the-end iterator) are invalidated. Otherwise only the past-the-end iterator is invalidated.

Simply make sure you do not push_back beyond capacity or call other methods that may invalidate. A list of methods that invalidate is available here in section 'Iterator invalidation'.