Upgrade to Pro
— share decks privately, control downloads, hide ads and more …
Speaker Deck
Features
Speaker Deck
PRO
Sign in
Sign up for free
Search
Search
Effective Modern C++ 読書会 Item29, Item30
Search
Linda_pp
July 16, 2015
Technology
0
420
Effective Modern C++ 読書会 Item29, Item30
Item29: move に期待しすぎないようにしよう
Item30: perfect forwarding に失敗する場合を知ろう
Linda_pp
July 16, 2015
Tweet
Share
More Decks by Linda_pp
See All by Linda_pp
ripgrep をライブラリとして使う
rhysd
0
280
port-monolith-to-wasm-for-chrome-extension
rhysd
0
400
Fuzzing Rust Text Editor
rhysd
1
2.8k
Vim compiled to WebAssembly
rhysd
5
1.9k
about-neovim-0.4.0-floating-window
rhysd
3
2k
reply.vim
rhysd
0
1.1k
Vim ported to WebAssembly (VimConf 2018)
rhysd
4
3.1k
go-selfupdate-github で ツールを自己アップデートする
rhysd
5
4.1k
小さく始めて育てるコンパイラ
rhysd
13
4.7k
Other Decks in Technology
See All in Technology
疲弊しない!AWSセキュリティ統制の考え方 #devio_osakaday1
masahirokawahara
6
5.9k
o11y入門_外形監視を利用したWebアプリケーションへの最適なモニタリング_TechBrew
k5k
3
100
長期運用プロジェクトでのMySQLからTiDB移行の検証
colopl
2
660
Databricks:『生成AI World Cup』のご案内
databricksjapan
2
150
0→1開発における技術選定において一番大切なこと
bicstone
1
320
Apple Vision Pro trial session
akkeylab
0
120
Janus
bkuhlmann
1
490
最近たまに見かけるTiDBってなんだ? - Findy
pingcap0315
2
580
Autonomous Database Cloud 技術詳細 / adb-s_technical_detail_jp
oracle4engineer
PRO
14
35k
プロデザ! BY リクルート vol.18_リクルートのリサーチ実践組織「リサーチブーストコミュニティ」
recruitengineers
PRO
3
240
小さな開発会社がWebサービスを作る理由
polidog
PRO
1
150
LLM とプロンプトエンジニアリング/チューターをビルドする / LLM and Prompt Engineering and Building Tutors
ks91
PRO
0
220
Featured
See All Featured
Creatively Recalculating Your Daily Design Routine
revolveconf
209
11k
JavaScript: Past, Present, and Future - NDC Porto 2020
reverentgeek
39
4.4k
The Art of Programming - Codeland 2020
erikaheidi
41
12k
The World Runs on Bad Software
bkeepers
PRO
61
6.7k
Building Applications with DynamoDB
mza
88
5.6k
Unsuck your backbone
ammeep
662
57k
Agile that works and the tools we love
rasmusluckow
324
20k
The Straight Up "How To Draw Better" Workshop
denniskardys
227
130k
Pencils Down: Stop Designing & Start Developing
hursman
117
11k
Refactoring Trust on Your Teams (GOTO; Chicago 2020)
rmw
24
2.3k
Learning to Love Humans: Emotional Interface Design
aarron
266
39k
Understanding Cognitive Biases in Performance Measurement
bluesmoon
6
990
Transcript
Effective Modern C++ ಡॻձ Item29, Item 30 @Linda_pp @rhysd
Item 29: moveʹظ͗͢͠Δͳ ʢAssume that move operations are not present,
not cheap, and not used)
move • move C++11 ͷۄػೳͷͻͱͭ • C++03 ͷίʔυ C++11
ͰϏϧυ͠ͳ͓ͤ ͘ͳΔʂ • STL move ରԠ • ίϯςφͷίϐʔ͕ϙΠϯλ1ͭͷίϐʔʹͳΔ ͧʂ
࣮ࡍ move ͕ࢥͬͨ΄Ͳ༗ ༻Ͱͳ͍߹͕ͨ͘͞Μ͋Δ
move ૢ࡞͕ଘࡏ͠ͳ͍࣌ ҉ͷ move Λఆٛ͞ΕΔ͕݅ݫ͍͠ • copyૢ࡞ɼmoveૢ࡞ɼσετϥΫλ͕ Ϣʔβఆٛ͞Ε͍ͯͳ͍ (Item17)
• جఈΫϥε͕moveඇରԠͰͳ͍ (Item11)
move ૢ࡞ͷίετ͕͘ͳ ͍࣌ ΄ͱΜͲͷ STL ίϯςφͷ move → ώʔϓ ͷϙΠϯλ
ͷషΓସ͚͑ͩͰྑ͍ʢ O(1) ʣ DPOUBJOFS CV⒎FS DPOUBJOFS NPWF
move ૢ࡞ͷίετ͕͘ͳ ͍࣌ std::array → ֤ཁૉͷ move ͔͠Ͱ͖ͳ͍ʢ O(n) ʣ
TUEBSSBZ CV⒎FS NPWF TUEBSSBZ CV⒎FS
move ૢ࡞ͷίετ͕͘ͳ ͍࣌ std::string →Small String Optimization • খ͍͞จࣈྻ
stack ʹऔΔ • 64bit OS, libc++ ͳΒ 24 จࣈ·Ͱ • move std::array ͷΑ͏ʹ 0(n) ͔͔Δ
move ૢ࡞͕͑ͳ͍࣌ ྫ֎҆શͳૢ࡞Λఏڙ͢Δίϯςφʢྫɿ std::vector ͷιʔτʣ → ཁૉͷ move ͕ྫ֎Λ ͛Δͱ͖
copy ʹ fallback • ཁૉͷ move தʹྫ֎͕ൃੜ͢Δͱ vector ͷ move ͕ྃͰ͖ͳ͍
move ͕༗ޮͰͳ͍࣌·ͱΊ • move ૢ࡞͕ଘࡏ͠ͳ͍ͱ͖ → ҉ͷ move ݅ݫ͍͠ •
move ૢ࡞͕͘ͳ͍ͱ͖ → std::array • move ૢ࡞͕͑ͳ͍ͱ͖ → std::vector ͷཁૉ͕ྫ֎҆શ ʹ move Ͱ͖ͳ͍࣌ • lvalue ͷͱ͖ → Ұ෦ͷྫ֎ʢ҉ͷ moveʣΛআ͍ͯ lvalue move Ͱ͖ͳ͍
Things to Remember • ςϯϓϨʔτΛ͍ͬͯΔ࣌ͲΜͳܕ͕ͬ ͯ͘Δ͔Θ͔Βͳ͍ → move ʹ͍ͭͯอक తʹίʔυΛॻ͘
• ѻ͏ܕͷৄࡉ͕͔͍ͬͯΔ → ܕͷυΩϡϝ ϯτΛಡΜͰ move ͕༗༻Ͱ͋ΕͦΕʹґ ଘͨ͠ίʔυΛॻ͍ͯྑ͍
Item 30: perfect forwarding ʹࣦഊ͢Δ߹ΛΖ͏
Perfect Forwarding Universal Reference Λ͍ɼؔͷҾΛ (const-ness ؚΊͯʣผͷؔݺͼग़͠ʹ ͦͷ··సૹ͢Δ 1
// perfect forward する関数 fwd 2 template<class... Ts> 3 auto fwd(Ts&& ... args) 4 { 5 // 関数 f に引数をそのまま転送する 6 return f(std::foward<Ts>(args)...); 7 } 8 9 ... 10 11 // 引数によっては perfect forwarding が失敗する 12 fwd({expression});
Perfect Forwarding ͕͏·͘ ͍͔ͳ͍Τοδέʔε ɾ{} ʹΑΔॳظԽ ɾ0 NULL ΛψϧϙΠϯλͱͯ͠͏
ɾએݴ͚ͩͷ static const ·ͨ constexpr σʔλϝϯό ɾΦʔόʔϩʔυ͞Εͨؔ ɾؔςϯϓϨʔτ ɾϏοτϑΟʔϧυ
{} ʹΑΔॳظԽ 1 void f(const std::vector<int>&); 2 3 ... 4
5 // OK. std::vector<int> は {1, 2, 3} で初期化される 6 f( {1, 2, 3} ); 7 8 // コンパイルエラー! 9 fwd( {1, 2, 3} ); foo.cpp:15:5: error: no matching function for call to 'fwd' fwd({1, 2, 3}); ^~~ ! foo.cpp:8:6: note: candidate function not viable: requires 0 arguments, but 1 was provided void fwd(Args &&... args) ^ 1 error generated.
{} ʹΑΔॳظԽ • f Λݺͼग़ͨ͠߹ɼίϯύΠϥ f ͷҾ ͔Β {1,2,3} Λ
vector ʹ҉ม͢Εྑ ͍͜ͱ͕͔Δ • fwd Λͬͯؒతʹݺͼग़ͨ͠߹ɼfwd ͷҾ Universal Reference ͷͨΊɼ{1,2,3} ΛͲͷܕʹม͢Εྑ͍ͷ͔͔Βͳ͍
ͳͥ std::initializer_list<int> ʹ ͳΒͳ͍ͷ͔ • Ҿ͕ std::initializer_list ͱ໌ࣔ͞Ε͍ͯͳ͍ ݶΓɼinitializer_list ʹਪ͞Εͳ͍ʢnon-
deduced contextʣ • ҎԼͷ߹ auto ͷਪ OK 1 // auto が std::initializer_list<int> に推論される 2 auto il = {1, 2, 3}; 3 4 // OK.すでに推論済み 5 fwd(il);
0 NULL ΛψϧϙΠϯλʹ ͬͨ߹ 0 NULL ܕʹਪ͞Εͯ͠·͏ͨΊ ϙΠϯλʹมͰ͖ͳ͍ʢItem8ʣˠ
nullptr Λ͑OK 1 void f(int *) 2 {} 3 4 template<class... Args> 5 void fwd(Args &&... args) 6 { 7 return f(std::forward<Args>(args)...); 8 } 9 10 fwd(NULL);
એݴ͚ͩͷ static const ·ͨ constexpr σʔλϝϯό Undefined symbols for architecture
x86_64: "Widget::MinVals", referenced from: _main in foo-4cf4cf.o ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation) ! ※ clang 3.4 ͙Β͍·ͰίϯύΠϧ௨ͬͯ͠·͏ 1 struct Widget { 2 static constexpr int MinVals = 28; // 定義ではなく宣言 3 }; 4 5 void f(size_t val); 6 7 ... 8 9 f(Widget::MinVals); // OK 10 11 fwd(Widget::MinVals); // リンクエラー
એݴ͚ͩͷ static const ·ͨ constexpr σʔλϝϯό • f ͷ߹ MinVals
ͷ͕͠͞ΕΔͷͰ OK • fwd ͷ߹ɼUniversal Reference ࢀরͳͷ ͰɼMinVals ͷΞυϨε͕ඞཁʹͳΔ → Ϧϯ ΫΤϥʔ • ԼهͷΑ͏ʹ࣮ମΛఆ͓͚ٛͯ͠ OK const size_t Widget::MinVals;
Φʔόʔϩʔυ͞Εͨؔ foo.cpp:24:5: error: no matching function for call to 'fwd'
fwd(processVal); ^~~ foo.cpp:17:6: note: candidate function not viable: requires 0 arguments, but 1 was provided void fwd(Args &&... args) ^ 1 error generated. 1 int processVal(int val); 2 int processVal(int val, int priority); 3 4 void f(int (*fp)(int)); 5 6 ... 7 8 f(processVal); // OK 9 10 fwd(processVal); // コンパイルエラー!
Φʔόʔϩʔυ͞Εͨؔ • f ͷݺͼग़͠ͰҾͷܕ͔Β processVal ͷ ΦʔόʔϩʔυΛղܾͰ͖Δ • fwd ͷݺͼग़͠ͰҾ
Universal Reference ͷͨΊɼprocessVal ͷΦʔόʔ ϩʔυ͕ղܾͰ͖ͳ͍ • ԼهͷΑ͏ͳ߹ OK int (*p)(int) = processVal; // ͜͜ͰΦʔόʔϩʔυղܾࡁΈ fwd(p); // OK
ؔςϯϓϨʔτ Φʔόʔϩʔυͷ߹ͱಉ͡ 1 template<class T> 2 int processVal(T val);
3 4 void f(int (*fp)(int)); 5 6 ... 7 8 f(processVal); // OK 9 10 fwd(processVal); // T が解決できないため NG
ϏοτϑΟʔϧυ foo.cpp:25:9: error: non-const reference cannot bind to bit-field
'totalLength' fwd(h.totalLength); ^~~~~~~~~~~~~ foo.cpp:9:17: note: bit-field is declared here totalLength:16; ^ 1 error generated. 1 struct IPv4Header { 2 std::uint32_t version:4, 3 IHL:4, 4 DSCP:6, 5 ECN:2, 6 totalLength:16; 7 }; 8 9 void f(size_t s); 10 11 ... 12 13 IPv4Header h; 14 15 f(h.totalLength); // OK 16 fwd(h.totalLength); // コンパイルエラー!
ϏοτϑΟʔϧυ • ϏοτϑΟʔϧυͷΞΫηεඇconstࢀর → fwd ͷҾඇconstࢀরʹͳΔ → Ϗοτ ϑΟʔϧυඇconstࢀরʹଋറͰ͖ͳ͍ •
ಛఆͷϏοτϑΟʔϧυΛࢦ͢ϙΠϯλ͕ͭ ͘Εͳ͍ͷͱಉ͡ • ίϐʔΛͭ͘Ε OK 1 IPv4Header h; 2 3 auto length = static_cast<std::uint16_t>(totalLength); 4 5 fwd(length); // OK
·ͱΊ • Perfect Forwarding େͷ߹ࢥͬͨ௨ Γಈ͘ • ྫʹग़ͨΑ͏ͳكʹ͏·͍͔͘ͳ͍έʔε͕ ͋Δ •
ϫʔΫΞϥϯυΛ͓ͬͯ͘ͷ͕େࣄ
Things to Remember • Perfect Forwarding ࣦഊ͢Δ߹ • సૹؔͷҾͷܕͷਪʹࣦഊͨ࣌͠ •
సૹؔͷҾͷܕΛؒҧͬͨʢf ʹͤͳ͍ʣܕʹਪͯ͠͠·ͬͨ࣌ • ͦͷଞͷΤοδέʔε • {} ʹΑΔॳظԽ • 0 NULL ΛψϧϙΠϯλͱͯ͠͏ • એݴ͚ͩͷ static const ·ͨ constexpr σʔλϝϯό • Φʔόʔϩʔυ͞Εͨؔ • ؔςϯϓϨʔτ • ϏοτϑΟʔϧυ