Is doubling the capacity of a dynamic array necessary?

Question

When making automatically expanding arrays (like C++\'s std::vector) in C, it is often common (or at least common advice) to double the size of the array each time it is fil

Answer 1

Q: 'doubling the capacity of a dynamic array necessary"
A: No. One could grow only to the extent needed. But then you may truly copy data many times. It is a classic trade off between memory and processor time. A good growth algorithm takes into account what is known about the program's data needs and also not to over-think those needs. An exponential growth of 2x is a happy compromise.

But now to your claim "following code only does 1 copy".

The amount of copying with advanced memory allocators may not be what OP thinks. Getting the same address does not mean that the underlying memory mapping did not perform significant work. All sorts of activity go on under-the-hood.

For memory allocations that grow & shrink a lot over the life of the code, I like grow and shrink thresholds geometrically placed apart from each other.

const size_t Grow[]   = {1, 4, 16, 64, 256, 1024, 4096, ... };
const size_t Shrink[] = {0, 2,  8, 32, 128,  512, 2048, ... };

By using the grow thresholds while getting larger and shrink one while contracting, one avoid thrashing near a boundary. Sometimes a factor of 1.5 is used instead.