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Unrestricted © Siemens AG 2015 All rights reserved.Smarter decisions, better products. Move semantics && rvalue references, part 2 Bert Rodiers, Software.

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Presentation on theme: "Unrestricted © Siemens AG 2015 All rights reserved.Smarter decisions, better products. Move semantics && rvalue references, part 2 Bert Rodiers, Software."— Presentation transcript:

1 Unrestricted © Siemens AG 2015 All rights reserved.Smarter decisions, better products. Move semantics && rvalue references, part 2 Bert Rodiers, Software Architect Siemens Industry Software NV

2 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 2Siemens PLM Software Move semantics && rvalue references Part 2 1.Alternative implementations for move operations 2.Rvalue references and templates (different binding rules) 3.Forwarding references  Reference Collapsing rules 4.Perfect forwarding 5.std::forward 6.How to pass arguments 7.(Moving into C++14 lambda expressions) 8.(Emplace)

3 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 3Siemens PLM Software Implementing Move Semantics CMyIntVector& operator=(CMyIntVector&& vector) noexcept { assert(&vector != this); // Alternative: If-test // Clean-up memory delete[] data_; // Fill-in members size_ = vector.size_; data_ = vector.data_; // Make vector an “empty”-vector vector.size_ = 0; vector.data_ = nullptr; return *this; }

4 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 4Siemens PLM Software Alternative implementations for move operations 1.std::swap to exchange members 2.Call move assignment operator in move constructor 3.Pass by value in assignment operator 4.Copy/swap idiom

5 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 5Siemens PLM Software Alternatives - std::swap to exchange members Advantages: Cleaner and easier code Issues: More expensive Delays destruction (less deterministic) Side effects (such as unlock) Originally introduced by Howard Hinnant doesn’t use the pattern anymore CMyIntVector& operator=(CMyIntVector&& vector) noexcept { using std::swap; swap(data_, vector.data_); swap(size_, vector.size_); return *this; }

6 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 6Siemens PLM Software Alternatives - Call move assignment operator in move constructor Source: MSDN Howard Hinnant: “Move operations are created for performance” OriginalThis implementation push_back of CMyIntVector 846 ms939 ms push_back of CIntPair 105 ms136 ms CMyIntVector(CMyIntVector&& other) noexcept : data_(nullptr), size_(0) { *this = std::move(other); }

7 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 7Siemens PLM Software Alternatives - pass by value in assignment operator operator= will move in implementation Advantage: Define one less Strong exception safety guarantee (if move doesn’t throw) Issues: What with noexcept? Performance (see later) MyString(const MyString&); MyString(MyString&&) noexcept; MyString& operator=(MyString);

8 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 8Siemens PLM Software Alternatives - Copy/swap idiom Combination of 2 alternatives: Swap functions call often grouped in separate swap function Advantage: Define one less Strong exception safety guarantee Implementation relatively trivial Issues: What with noexcept? Performance (lvalues + rvalues) Delays destruction MyString& operator=(MyString string) { std::swap(string_, string.string_); // swap every member variable return *this; }

9 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 9Siemens PLM Software Copy/swap idiom Normal assignment operator Copy/swap idiom std::string longstring("Answer to The Ultimate Question of Life, the Universe, and Everything"); std::string shortstring("42"); longstring = shortstring; // no reallocation necessary shortstring = longstring; // reallocation necessary std::string longstring("Answer to The Ultimate Question of Life, the Universe, and Everything"); std::string shortstring("42"); longstring = shortstring; // reallocation necessary shortstring = longstring; // reallocation necessary

10 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 10Siemens PLM Software Slide by Howard Hinnant (ACCU 2014)

11 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 11Siemens PLM Software Move semantics – Optimal implementation class MyString { public: MyString(const char* string) : string_(string) {} private: std::string string_; }; Rule of three five zero!

12 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 12Siemens PLM Software Rvalue references and deduced types Rules different with deduced types Example template void f(T&& arg); void g(int&& arg); f(4); g(4);

13 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 13Siemens PLM Software Rvalue references and deduced types Rules different with deduced types Example template void f(T&& arg); void g(int&& arg); int val = 4; f(val); g(val); //error C2664: 'void g(int &&)' : cannot convert argument 1 from // 'int' to 'int &&'

14 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 14Siemens PLM Software Rvalue references and deduced types Rules different with deduced types Example template void f(T&& arg); void g(int&& arg); int val = 4; f(val); g(std::move(val));

15 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 15Siemens PLM Software Rvalue references and deduced types Rules different with deduced types Example template void f(T&& arg); void g(int&& arg); const int val = 4; f(val); g(val); //error C2664: 'void g(int &&)' : cannot convert argument 1 from // ‘const int' to 'int &&'

16 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 16Siemens PLM Software Rvalue references and deduced types Rules different with deduced types Example template void f(T&& arg); void g(int&& arg); const int val = 4; f(val); g(std::move(val)); //error C2664: 'void g(int &&)' : cannot convert argument 1 from // ‘const int' to 'int &&'

17 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 17Siemens PLM Software Rvalue references and deduced types Rules different with deduced types Templates, auto T&& binds with everything (lvalues + rvalues) Two views for T&& 1.Still Rvalue references With reference-collapsing rules 2.T&& something completely different Forwarding reference (universal reference)

18 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 18Siemens PLM Software Forwarding reference (universal reference) T&& binds with everything (lvalues + rvalues) Restriction T has to have a specific form std::vector && const T&& => not a forwarding reference => forwarding reference? template struct C { void f(T&& arg); };

19 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 19Siemens PLM Software Forwarding reference: type deduction rules Lvalue => lvalue reference (T already has this type) Different rules from normal template functions Rvalue => rvalue reference Const not removed template void f(T&& arg); const int const_val = 4; f(const_val); // argument of function: const int& int value = 42; f(value);// argument of function: int& f(g()); // argument of function: int&& f(std::move(const_val));// argument of function: const int&&

20 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 20Siemens PLM Software Forwarding reference: why? make_unique without forwarding references template unique_ptr make_unique(const Type1& arg1, const Type2& arg2) { return (unique_ptr (new T(arg1, arg2))); } template unique_ptr make_unique(Type1&& arg1, Type2&& arg2) { return (unique_ptr (new T(std::move(arg1), std::move(arg2))); } template unique_ptr make_unique(const Type1& arg1, Type2&& arg2) { return (unique_ptr (new T(arg1, std::move(arg2))); } …

21 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 21Siemens PLM Software Forwarding reference: why? Perfect forwarding Example template unique_ptr make_unique(Types&&... args) { return (unique_ptr (new T(std::forward (args)...))); } template unique_ptr make_unique(Type1&& arg1, Type2&& arg2) { return (unique_ptr (new T(std::forward (arg1), std::forward (arg2)))); }

22 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 22Siemens PLM Software std::forward T not deduced from argument Forward converts type back to rvalue (reference) Remember: if has name => lvalue When to use std::forward? Forwarding reference Forward: only cast to rvalue reference if originally rvalue std::forward (arg1) Rvalue reference (not forwarding reference) Move: unconditionally cast std::move(arg1)

23 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 23Siemens PLM Software Perfect forwarding not so perfect Most common failure: braced initializer lists void f(const std::vector &); template void g(T&& arg) { f(std::forward (arg)); } f({ 1, 2 }); // Compiles g({ 1, 2 }); // Doesn't compile auto values = { 1, 2 }; g(values); // Compiles

24 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 24Siemens PLM Software Reference-collapsing rules Second Vision: T&&: Still Rvalue references Needs Different rules for type deduction reference-collapsing rules Two types involved: Declared template argument (for example T&&) Type of passed argument (for example (int&)) Different rules for type deduction (lvalue  rvalue) template void f(T&& arg); int value = 42; f(value); // T deduced to int& f(42); // T deduced to int

25 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 25Siemens PLM Software Reference-collapsing rules template void f(T&& arg); int value = 42; f(value); // T deduced to int& f(42); // T deduced to int f(value); T = int& arg: T&& == int& && => int& f(42); T = int arg: T&& == int&& => int&&

26 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 26Siemens PLM Software Reference-collapsing rules Lvalue + lvalue reference => lvalue reference Lvalue + rvalue reference => lvalue reference Rvalue + rvalue reference => rvalue reference Occurs: 1.Template instantiation 2.Auto variables 3.Typedef + alias declaration 4.Decltype template T&& forward(typename remove_reference ::type& arg) noexcept { return (static_cast (arg)); }

27 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 27Siemens PLM Software How to pass arguments Different alternatives 1.void setName(const std::string& name) { name_ = name; } Lvalue: 1 copy 1 Rvalue: 1 copy 1 2.void setName(const std::string& name) { name_ = name; } void setName(std::string&& name) noexcept { name_ = std::move(name); } Lvalue: 1 copy 1 Rvalue: 1 move 3.void setName(std::string name) { name_ = std::move(name); } Lvalue: 1 copy 2 + 1 move (Note: SSO) Rvalue: 2 moves

28 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 28Siemens PLM Software How to pass arguments 4. template void setName(T&& name) { name_ = std::forward (name); } Lvalue: 1 copy 1 Rvalue: 1 move 5. template typename std::enable_if ::value, void>::type setName(T&& name) { name_ = std::forward (name); } Lvalue: 1 copy 1 Rvalue: 1 move

29 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 29Siemens PLM Software How to pass arguments Solution 1 (const&) Performance not optimal for rvalues Solution 2 (const& + &&) Performance optimal More code to maintain Solution 3 (value) (Possibly huge) Pessimization for lvalues, small pessimization for rvalues What with noexcept? Solution 4 (T&& + forward) Error message not @function call, but deep in implementation code Accepts a.setName(42) (Explicit constructors) What with noexcept? Solution 5 (T&& + enable_if + forward) Complex Error messages not obvious (“Failed to specialize function template”) What with noexcept?

30 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 30Siemens PLM Software CppCon2015

31 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 31Siemens PLM Software C++ Core Guidelines

32 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 32Siemens PLM Software GSL: Guidelines Support Library

33 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 33Siemens PLM Software How to pass arguments Cheap to copy (e.g. int) Cheap to move (vector, string) or Moderate cost to move or Don’t know Expensive to move Out X f()f(X&) In/Out f(X&) In f(X)f(const X&) In & retain “copy”

34 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 34Siemens PLM Software How to pass arguments - advanced Cheap to copy (e.g. int) Cheap to move (vector, string) or Moderate cost to move or Don’t know Expensive to move Out X f()f(X&) In/Out f(X&) In f(X)f(const X&) In & retain “copy” f(X) f(const X&) + f(X&&) & move f(const X&) In & move from f(X&&)

35 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 35Siemens PLM Software Summary 1.Alternative implementations for move operations Rule of zero 2.Rvalue references and templates (different binding rules) 3.Forwarding references  Reference Collapsing rules 4.Perfect forwarding 5.std::forward 6.How to pass arguments 7.(Moving into C++14 lambda expressions) 8.(Emplace)

36 Restricted © Siemens AG 2015 All rights reserved.Smarter decisions, better products. Questions?

37 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 37Siemens PLM Software Side-topic: moving into lambdas Doesn’t compile! template std::unique_ptr generate(T arg); class A { public: void consume(std::unique_ptr, int); }; std::function f(std::shared_ptr & a) { std::unique_ptr value = generate(42); return [value, a](int arg) mutable { return a->consume(std::move(value), arg); }; }

38 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 38Siemens PLM Software Side-topic: moving into lambdas – second try Compiles… … but undefined behavior std::function f(std::shared_ptr & a) { std::unique_ptr value = generate(42); return[&value, a](int arg) mutable { return a->consume(std::move(value), arg); }; }

39 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 39Siemens PLM Software Side-topic: moving into lambdas – third try  Generalized lambda captures By reference: [&a = a] or [&a = b] By value: [c = d] or [c = c] // std::function f(std::shared_ptr & a) auto f(std::shared_ptr & a) { std::unique_ptr value = generate(42); return[value = std::move(value), a](int arg) mutable { return a->consume(std::move(value), arg); }; }

40 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 40Siemens PLM Software Side-topic: emplace_back Remember MyString with move operations (push_back in for loop: 932 ms*) * Different machine for (int i = 0; i < count; ++i) { vector.push_back(MyString("To be, or not to be, that is the question: Whether…")); } for (int i = 0; i < count; ++i) { vector.emplace_back("To be, or not to be, that is the question: Whether…"); }

41 2015-12-03 Unrestricted © Siemens AG 2015 All rights reserved. Page 41Siemens PLM Software Side-topic: emplace_back Remember MyString with move operations (push_back in for loop: 932 ms*) MyString using emplace_back: 901 ms* MyString constructed in-place No copy/move Can take more arguments For loop with std::vector > std::vector > vector; vector.push_back(std::make_tuple("test", 42, 0.)); vector.emplace_back("test", 42, 0.); push_backemplace_back Without reserve 682 ms579 ms With reserve 277 ms146 ms

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