CODEWITHSUNDEEP
c++c++11pointersvectordata-structures
Paul Renton
Paul Renton

Reputation: 2670

C++ std::vector<T*> if you hold a pointer to a pointer element, does it become invalidated upon resize?

Let us say I create std::vector<int*> myVec; and reserve 100 entries and populate the vector with values so that all 100 elements have valid pointers. I then cache a pointer to one of the elements, 

int * x = myVec[60];

Then, if I append another int * which triggers a resize along with a move due to heap fragmentation, does the previous pointer to a pointer become invalidated or does it point to the new location in memory?

If my memory servers me correct, if the example were to std::vector<int> myVecTwo with the same conditions as above and I stored a ptr like 

int * x = &myVecTwo[60];

and proceeded to append and resize, that pointer would be invalided.

So, my question is as follows. Would the pointer to the pointer become invalidated? I am no longer certain because of the new C++ std functionality of is_trivially_copyable and whether the std::vector makes use of such functionality or POD checks.

Would the pointer by invalidated in C++11?

Upvotes: 3

Views: 246

Answers (1)

songyuanyao
songyuanyao

Reputation: 172964

No.

As you showed, after the reallocation, the pointers to the elements of the vector, like int * x = &myVecTwo[60]; would be invalidated. After the reallocation, the elements themselves would be copied, but the object pointed by the element pointers won't be affected, then for int * x = myVec[60];, after the reallocation x is still pointing to the same object, which has nothing to do with the reallocation of the vector.

Upvotes: 5

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