C++ Vector initial capacity

Question

I have some code which uses thousands of vectors each vector has only 4 entries, So I want to set the initial size of each vector to 4 so that I can optimize memory usage by

Answer 1

The capacity of a vector cannot be controlled by the constructors - there is no applicable overload.

The C++ standard doesn't give any guarantee about the capacity of a default-constructed vector vector bar;. However all well-known implementations use 0 as the default capacity. This is something you can rely on, as allocating memory at that point just doesn't make sense.

So I believe the answer to your question is: just use

vector bar;
bar.reserve(4);