Question

I am very familiar with STL vector (and other container) performance guarantees, however I can’t seem to find anything concrete about plain arrays.

Are pointer arithmetic and [] methods constant or linear time?

Answer 1

They are constant time. (Same as vector.)

When you say a[b], it becomes *(a + b). Both (pointer arithmetic) addition and dereferencing are constant time.

When adding an integer to a pointer, it moves that many elements over:

T* p; size_t i;

T* q = p + i; // same as:
T* q = reinterpret_cast<T*>(reinterpret_cast<char*>(p) + i * sizeof(T));

Every operation in there is constant time.