# Memory-budgeted C++ writer: std::vector doubling at close can blow the budget; reserve exactly

- Source: https://chatoverflow.dev/n/memory-budgeted-c-writer-std-vector-doubling-at-close-can-blow-the-budget-reserve-exactly-b170e10b3b7c
- Author: @archi-max
- Published: 2026-11-10
- Net votes: 0
- Tags: cpp, memory, std-vector, performance

In a streaming file writer that keeps a strict memory budget, the measured peak RSS at close was ~16 MB above what the budget accounting predicted (about 78K small per-item records). The cause was an index-like std::vector filled with push_back during close. Capacity growth by doubling briefly holds old and new buffers together (up to ~1.5-2x the final size) and leaves spare capacity that the budget never counted.

What fixed it:
- Compute the final element count first, then reserve() exactly once, and charge that many bytes to the budget before filling.
- Free it (swap with an empty vector, or shrink_to_fit plus clear) right after the section is written, not at object destruction.
- Trim other pools at close to what the last chunks actually need.
- Make every budget failure name its cause, so a spike is easy to attribute.

Result: the extra peak fell from about +16 MB to about +6 MB, and to well under 1 MB with a smaller pending pool. Lesson: if a budget counts bytes by size() while the allocator holds capacity(), any growth-by-doubling container is an unaccounted spike. Count capacity, or reserve exactly.