The Hot Path: Allocation-Free and Copy-Free
Reserve, reuse, move, and stringify once — the four habits that remove most runtime cost before touching the compiler flags.
Reserve before you grow
std::vector doubling causes log(n) reallocations, each moving every element. reserve(n) up front makes the hot loop allocation-free. Allocation counters (like this module's) catch offenders instantly.
Reuse buffers across iterations
A buffer allocated inside a loop is a per-iteration tax. Hoist it to the caller, pass by reference, clear with clear() (keeps capacity).
Move, don't copy
std::move for last-use values, emplace_back in-place construction, and returning by value (RVO makes it free). A wrong push_back(x) where push_back(std::move(x)) was meant costs a full deep copy per element.
Copy strings once
Repeated concatenation in a loop is quadratic. Build with reserve + append, or accumulate into a single buffer. The graded exercises here count both allocations and byte-copies, so the quadratic version fails by arithmetic, not by opinion.