C++ std::move doesn't move anything: A deep dive into Value Categories

📝 Discussion Summary (Click to expand)

1. std::move Marks for Potential Move, Does Not Guarantee Ownership Transfer

Users emphasize std::move casts to rvalue but leaves actual move to callee.
"shmerl: I always understood move as moving ownership" vs. "vlovich123: Std move doesn’t move ownership. It simply casts into something that could have its ownership taken."
"masklinn: it's intended for transferring ownership, but what it actually does is mark the value as transferrable."

2. Moves Are Not Always Statically Knowable

Debate on runtime decisions in callees preventing static analysis.
"tsimionescu: It is absolutely knowable statically if ownership will be taken."
"masklinn: The function I posted only moves its parameter half the time, at random." (counterexample with random branch).
"knorker: the act of calling std::move... in no way invokes a move constructor."

3. Pitfalls: noexcept, Const, Moved-From State, NRVO

STL requires noexcept moves for efficiency; other gotchas abound.
"HarHarVeryFunny: containers just copy everything instead [without noexcept]."
"drysine: the language doesn't dictate what post-condition your class has for move-ctor... it could be 'don't touch this object after move' (and it's UB)."
"Fiveplus: return std::move(local_var)... is technically a pessimization beyond just breaking NRVO."

4. C++ Complexity, Poor Naming, Better in Rust

Critiques of footguns, design; suggestions like std::offer; Rust praised.
"drob518: so many foot guns and so much rampant complexity."
"masklinn: std::move looks and feels like a function, but it doesn't do what it says... Pin is a much better candidate [in Rust]."
"vouwfietsman: std::offer" (upvoted suggestion); "oezi: std::move is a cast."

🚀 Project Ideas

MoveSemantics Analyzer

Summary

A static analysis tool that scans C++ code for common std::move pitfalls, such as unnecessary moves breaking NRVO, const moves silently copying, and non-noexcept moves in containers.
Core value proposition: Prevents performance regressions and bugs by warning on "move doesn't move" cases with explanations and fixes.

Details

Key	Value
Target Audience	C++ developers using modern C++11+ features
Core Feature	Analyzes call sites, overload resolution, and post-move states; integrates with clang-tidy
Tech Stack	Clang AST tooling, LLVM lib, C++
Difficulty	Medium
Monetization	Hobby

Notes

HN users lament "std::move silently copies const values" (j1elo) and "it's non local - whether std::move even does anything is dependent on the signature" (vlovich123); this tool would catch them statically.
High utility for code reviews; sparks discussions on Godbolt integrations.

NoexceptMove Enforcer

Summary

Clang-tidy extension that automatically adds/fixes noexcept to move constructors/assignments where safe, and audits STL container usage for performance pitfalls.
Core value proposition: Ensures move optimizations in vectors/lists without manual checks, avoiding "thousands of expensive copy operations" (TFA).

Details

Key	Value
Target Audience	C++ library authors and performance-critical app devs
Core Feature	Detects throwing moves, suggests noexcept(false) overrides; runs as pre-commit hook
Tech Stack	Clang-tidy, C++20 concepts for analysis
Difficulty	Medium
Monetization	Revenue-ready: Freemium VSCode extension

Notes

Addresses "clang-tidy has a check... but not enabled by default" (krona) and "performance-noexcept-move-constructor" praises; users like juliangmp want default flags.
Practical for large codebases; HN would love sharing configs.

MoveViz Interactive Explorer

Summary

Web-based visualizer like Godbolt but specialized for move semantics: input code, see value categories, actual moves/copies, post-move states animated.
Core value proposition: Demystifies "move doesn't move" by showing assembly, category flows, and Rust comparisons side-by-side.

Details

Key	Value
Target Audience	C++ learners and educators frustrated by mental models
Core Feature	Live compilation with move traces, hypothetical "what if noexcept" previews
Tech Stack	Compiler Explorer fork, WebAssembly Clang, D3.js visuals
Difficulty	High
Monetization	Hobby

Notes

Quotes like "I never understood move semantics until I learned Rust" (bitbasher) and Godbolt links (knorker); perfect for "value categories confusing" (porise).
Viral potential on HN; great for tutorials/conferences.

StdOffer Renamer

Summary

IDE plugin/refactor tool that replaces std::move with configurable "std::offer" (or disown/rvalue), adding compile-time assertions if no move occurs.
Core value proposition: Reduces confusion from misleading name, enforces intent with optional errors on fallback to copy.

Details

Key	Value
Target Audience	Teams adopting modern C++; Rust migrants
Core Feature	Semantic rename preserving behavior; template-aware assertions via macros
Tech Stack	Clangd/LSP, tree-sitter for parsing
Difficulty	Low
Monetization	Revenue-ready: Subscription for teams

Notes

Inspired by naming debates: "std::offer" (vouwfietsman), "disown" (shmerl), "std::rvalue" (pseidemann); fixes "std::move is like putting a sign... but receiver decides" (masklinn).
HN discussion potential on better names; utility in grepping/migrating codebases.