Why is `return std::move(x)` usually wrong?
A small question that reliably separates people who have read about move semantics from people who have measured them.
Exam
Answer these to pass this module.
Start here rather than at the reading. Whatever you miss tells you which part of Learn you actually need — and anything you get right is a part you can skip.
How does `return std::move(local);` differ from `return local;`?
When is returning std::move(x) actually the right thing?
In C++17, elision of a prvalue returned by value is:
Explain mode
Why is `return std::move(x)` usually wrong?
Explain what the compiler would have done if you had written `return x;` instead.
Your own words, two or three sentences, nothing looked up. The rubric unlocks once you submit.
Graded against a set rubric. Three a day on a free account. Need more? Ask on our Facebook page — it's free.
Learn
RVO and NRVO, C++17 mandatory copy elision for prvalues, and the implicit-move rule when returning a local by value.
Ask an AI to teach you this
I'm preparing for a quant developer interview on "Why is `return std::move(x)` usually wrong?" Concepts I need to own: RVO and NRVO, C++17 mandatory copy elision for prvalues, and the implicit-move rule when returning a local by value. Run this as interview practice, not a lecture. One concept at a time. Wait for my answer before moving on. Per concept: 1. Teach it. Lead with the one-line core claim, then a few short paragraphs. Assume I write C++ but have never had this explained properly. No jargon without defining it. 2. "The 60-second version" — what a strong candidate would SAY out loud. First person, spoken register, no bullets, no headers. That's what I'm rehearsing. 3. Ask me one question that exposes whether I only think I understood it. When I answer, reply in exactly this shape: Verdict: correct / partly correct / wrong — one line on what's wrong. What you missed: only if something's missing. 2-3 sentences. Vocabulary: only if I used a wrong or vague term. Give the right one. Example answer: the model spoken answer, 30 seconds max, first person. More info: define any term I probably don't know, one line each. Follow-up probe: the one question an interviewer asks someone who gave that model answer. Skip any section that doesn't apply. Don't pad. If I'm wrong, say wrong — don't soften it. Do not write my implementation for me. If I ask, refuse and ask what I'd try instead — I need to defend this code with the editor closed.
Do
Ship
Practice with AI
Use these to make the AI your examiner, not your author. If the AI wrote the code, this node doesn't count — you can't defend a design you didn't make.
- P8 — grade my explanation against the rubric, and name what I left out
- P5 — explain the slowdown at the hardware level