How do I size the destination for copy and copy_if?

Preallocate enough output space or use back_inserter; writing past the end is undefined behavior.

Is overlapping copy safe?

Not always — use copy_backward or move_backward when shifting right in the same buffer; understand source/destination overlap.

When use move algorithms?

When ownership transfer is OK and sources may be left moved-from; keep copy when values must stay valid.

Performance concerns?

Reserve capacity, avoid redundant copies, and profile before replacing std::copy with raw memcpy except on trivial types.

C++ Copy Algorithms: std::copy, copy_if, copy_n & move

2026년 3월 12일 · 16분 읽기 · 수정 2026년 3월 30일 Beginner tutorial

이 글의 핵심

Range-based copy utilities: filtering, overlap, move for performance, and iterator pitfalls.

What are copy algorithms?

Copy algorithms move data from a source range to a destination: copy, copy_if, copy_n, move, remove_copy, and copy_backward for overlapping ranges.

Why use them? Less boilerplate than hand loops, clearer intent, and optimized library implementations.

Variants

copy: full [first, last)
copy_if: predicate selects elements
copy_n: first n from a start iterator
copy_backward: safe right shift in overlapping buffers
move / move_backward: move semantics

copy vs copy_if vs copy_n

Algorithm	Input	Selection	Destination size
`copy`	`[first, last)`	all	`last - first` or grow via back_inserter
`copy_if`	`[first, last)`	pred true	up to `last - first`
`copy_n`	from `first`, count `n`	first n	at least `n` or back_inserter

copy_n with fewer than n readable elements → UB — validate input length for streams.

copy_backward (overlapping ranges)

std::copy proceeds forward. If source and destination overlap and the block shifts right, unread source can be overwritten → UB. Use copy_backward(first, last, d_last) where d_last is the end of the destination range (reverse copy).

move_backward is the move analogue.

Iterator invalidation

Appending with back_inserter may reallocate vector and invalidate iterators. reserve reduces reallocations. Thread safety: shared containers need synchronization.

Performance vs memcpy

Trivially copyable types may compile to memmove-like code; non-trivial types (std::string, containers) require proper copy/move — never memcpy those.

Output iterators

back_inserter, front_inserter (lists/deques), inserter, ostream_iterator — choose by container and insertion policy.

Common pitfalls

Destination too small — UB; use resize, reserve + back_inserter, or count.
Overlap — use copy_backward / move_backward when shifting right.
After std::move algorithm — sources are valid but unspecified; clear or reassign before reuse.
copy vs move — large strings/vectors: move when source disposable.

FAQ

Q1: copy — full range copy.
Q2: copy_if — filter while copying.
Q3: move — cheap transfer for rvalue-like use.
Q4: remove_copy vs remove — copy-out filter vs mutating remove-erase idiom.
Q5: Overlap — copy_backward / temporary buffer.
Q6: Resources — Effective STL, C++ Primer, cppreference — copy.

One-line summary: Use STL copy algorithms for safe range copies; prefer move for large movable types when appropriate.

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Keywords

std::copy, copy_if, copy_n, copy_backward, std::move, remove_copy, back_inserter, STL