C++0x :: Introduction to some amazing features

C++0x Introduction to some amazing features Christian S. Perone Porto Alegre - RS christian.perone@gmail.com 16 March 2011Christian S. Perone () C++0x 16 March 2011 1 / 65

Table of contents1 Introduction What is C++0x ?2 New proposals Type Inference Trailing return types Static Assertion Constant Expressions Variadic Templates Strongly-typed enums Initializer Lists Range-based for Naming the NULL Lambda Rvalue References3 Conclusion C++0x Feature Availability Links Christian S. Perone () C++0x 16 March 2011 2 / 65

Introduction What is C++0x ?Outline1 Introduction What is C++0x ?2 New proposals Type Inference Trailing return types Static Assertion Constant Expressions Variadic Templates Strongly-typed enums Initializer Lists Range-based for Naming the NULL Lambda Rvalue References3 Conclusion C++0x Feature Availability Links Christian S. Perone () C++0x 16 March 2011 3 / 65

Introduction What is C++0x ?What is C++0x ?What ?C++0x is a new standard for the next C++ generation. Intended to replace the current C++ standard (ISO/IEC 14882); Christian S. Perone () C++0x 16 March 2011 4 / 65

Introduction What is C++0x ?What is C++0x ?What ?C++0x is a new standard for the next C++ generation. Intended to replace the current C++ standard (ISO/IEC 14882); Simplify simple tasks; Christian S. Perone () C++0x 16 March 2011 4 / 65

Introduction What is C++0x ?What is C++0x ?What ?C++0x is a new standard for the next C++ generation. Intended to replace the current C++ standard (ISO/IEC 14882); Simplify simple tasks; Improve performance; Christian S. Perone () C++0x 16 March 2011 4 / 65

Introduction What is C++0x ?What is C++0x ?What ?C++0x is a new standard for the next C++ generation. Intended to replace the current C++ standard (ISO/IEC 14882); Simplify simple tasks; Improve performance; Improve Standard Library; Christian S. Perone () C++0x 16 March 2011 4 / 65

Introduction What is C++0x ?What is C++0x ?What ?C++0x is a new standard for the next C++ generation. Intended to replace the current C++ standard (ISO/IEC 14882); Simplify simple tasks; Improve performance; Improve Standard Library; Make C++ easier to teach and learn; Christian S. Perone () C++0x 16 March 2011 4 / 65

Introduction What is C++0x ?What is C++0x ?What ?C++0x is a new standard for the next C++ generation. Intended to replace the current C++ standard (ISO/IEC 14882); Simplify simple tasks; Improve performance; Improve Standard Library; Make C++ easier to teach and learn; Keep (the not so easy) compatibility constraints; Christian S. Perone () C++0x 16 March 2011 4 / 65

Introduction What is C++0x ?What is C++0x ?What ?C++0x is a new standard for the next C++ generation. Intended to replace the current C++ standard (ISO/IEC 14882); Simplify simple tasks; Improve performance; Improve Standard Library; Make C++ easier to teach and learn; Keep (the not so easy) compatibility constraints; Fit into the real world;As of March 2011, ISO C++ Standards Committee has voted the C++0x speciﬁcation to FDIS (Final Draft International Standard ) status, what makes it ready for review an approval by ISO. It can take 6 months to a year before the standard is oﬃcially published by ISO (expected to be released by the end of 2011). Christian S. Perone () C++0x 16 March 2011 4 / 65

New proposals Type InferenceOutline1 Introduction What is C++0x ?2 New proposals Type Inference Trailing return types Static Assertion Constant Expressions Variadic Templates Strongly-typed enums Initializer Lists Range-based for Naming the NULL Lambda Rvalue References3 Conclusion C++0x Feature Availability Links Christian S. Perone () C++0x 16 March 2011 5 / 65

New proposals Type Inference Type Inference: auto-typed Variables Since the advent of templates and template metaprogramming methods, some types became hard to use, some examples of this are:1 std::vector<std::string>::const_iterator it1 = my_vec.cbegin();23 std::map<std::list<double>, // endless types ! continue next line...4 std::set<std::vector<std::string> > >::iterator it2 = my_map.begin(); Consider now, the same example, this time using the auto-typed variables:1 auto it1 = my_vec.cbegin();2 auto it2 = my_map.begin(); Note that since auto uses the return type to deduce the variable type, new iterators were added into the STL, like the vector::cbegin(), which returns a constant iterator. Christian S. Perone () C++0x 16 March 2011 6 / 65

New proposals Type Inference Type Inference: auto-typed Variables More examples: 1 auto my_variable; // error: a symbol whose type contains 2 // ’auto’ must have an initializer 3 4 auto my_variable1 = 20 + 21; // int 5 auto my_variable2 = 100 / 3; // int 6 auto my_variable3 = 100 / 3.0; // double 7 auto my_array = new int[50]; // int* 8 9 // sqrt has overload for 3 types, and we have passed a double as argument,10 // this overload returns double, the type inference will be double.11 auto my_variable4 = sqrt(my_variable3); Special example:1 auto length = strlen("What is my platform ?"); Returns a 4-byte integer on a 32-bit compilation or 8-byte int on a 64-bit compilation. Christian S. Perone () C++0x 16 March 2011 7 / 65

New proposals Type Inference Type Inference: decltype decltype The C++0x decltype gives the type of the expression argument, without evaluating the expression, actually decltype only do a syntax check. Example:1 std::string my_text = "lalalala";2 decltype(my_text) another_text;34 decltype(my_text.length()) len_size;5 // decltype doesn’t executes the string::length() method, it6 // only uses the return type of the method to deduce the type Christian S. Perone () C++0x 16 March 2011 8 / 65

New proposals Trailing return typesOutline1 Introduction What is C++0x ?2 New proposals Type Inference Trailing return types Static Assertion Constant Expressions Variadic Templates Strongly-typed enums Initializer Lists Range-based for Naming the NULL Lambda Rvalue References3 Conclusion C++0x Feature Availability Links Christian S. Perone () C++0x 16 March 2011 9 / 65

New proposals Trailing return types Trailing return types Trailing return type is a new form to specify the return type of functions or templates. You’ll see that this form is specially useful for templates. Consider the follow examples:1 auto get_value() -> float2 { return 666.0; } In this new form, the auto keyword works just as a placeholder, the type speciﬁed in this form is now placed after the -> operator, which in our case is float. This form becomes more interesting when you use decltype to deﬁne the return type:1 auto get_value() -> decltype(666.0)2 { return 666.0; } Here the compiler will deduce (at compile time) what is the type of the constant 666.0 and use it as a return type of the function. Christian S. Perone () C++0x 16 March 2011 10 / 65

New proposals Trailing return types Trailing return types Now consider this template example:1 template <typename FirstType, typename SecondType>2 /*Unknown Type*/ AddThem(FirstType t1, SecondType t2)3 {4 return t1 + t2;5 } The function adds two values and then returns the sum. But what if we pass an int and a double as arguments ? What should be the return type ? And if we pass two other distinct objects from distinct classes ? Christian S. Perone () C++0x 16 March 2011 11 / 65

New proposals Trailing return types Trailing return types .. and here is why the trailing return type is useful:1 template <typename FirstType, typename SecondType>2 auto AddThem(FirstType t1, SecondType t2) -> decltype(t1 + t2)3 {4 return t1 + t2;5 } The compiler will deduce at compile time what is the return type for the expression t1+t2 and then use it as the return type of our template. Please note that the expression t1+t2 isn’t evaluated, decltype only perform a syntax checking of the expression, it doesn’t executes the expression. Maps to... Note that the representation of the type deﬁnition operator -> is exactly the same used in Haskell, and it’s called ”maps to” (→) operator in mathematics; we’ll see it later again in the deﬁnition of Lambda functions. Christian S. Perone () C++0x 16 March 2011 12 / 65

New proposals Static AssertionOutline1 Introduction What is C++0x ?2 New proposals Type Inference Trailing return types Static Assertion Constant Expressions Variadic Templates Strongly-typed enums Initializer Lists Range-based for Naming the NULL Lambda Rvalue References3 Conclusion C++0x Feature Availability Links Christian S. Perone () C++0x 16 March 2011 13 / 65

New proposals Static AssertionStatic Assertion The assertion problemWe currently support only two facilities to test assertions: assert macro; #error preprocessor directive;Citing the proposal (N1720 rev.3): To enforce a template parameter constraint, for example, it should be possible to write an assertion that will be tested when the associated template is instantiated. The assert macro tests assertions at runtime, which is far later than would be desired by the author of a template library, and which implies a runtime performance cost. The #error preprocessor directive is processed too early to be of much use in this regard. In particular, the #error directive is processed before templates are instantiated, and hence cannot be used to test assertions involving template arguments. Christian S. Perone () C++0x 16 March 2011 14 / 65

New proposals Static Assertion Static Assertion The assertion solution: static assert(constant-expression, string-literal); Examples: At namespace scope:1 static_assert(sizeof(long) >= 8, "64-bit code generation not"2 "enabled/supported."); At class scope:1 template <class CharT, class Traits = std::char_traits<CharT> >2 class basic_string {3 static_assert(tr1::is_pod<CharT>::value,4 "Template argument CharT must be a POD type in"5 "class template basic_string");6 // (... )7 }; Christian S. Perone () C++0x 16 March 2011 15 / 65

New proposals Static Assertion Static Assertion Double-checking design assumptions: 1 template <class T, bool> 2 class container_impl { 3 static_assert(!std::tr1::is_scalar<T>::value, 4 "Implementation error: this specialization intended" 5 "for non-scalars only"); 6 // (...) 7 }; 8 9 template <class T>10 class container_impl<T, true> {11 static_assert(std::tr1::is_scalar<T>::value,12 "Implementation error: this specialization"13 "intended for scalars only");14 // (...)15 }; Christian S. Perone () C++0x 16 March 2011 16 / 65

New proposals Constant ExpressionsOutline1 Introduction What is C++0x ?2 New proposals Type Inference Trailing return types Static Assertion Constant Expressions Variadic Templates Strongly-typed enums Initializer Lists Range-based for Naming the NULL Lambda Rvalue References3 Conclusion C++0x Feature Availability Links Christian S. Perone () C++0x 16 March 2011 17 / 65

New proposals Constant Expressions Constant Expressions Current C++ standard is very restrictive on the rules that deﬁne what is and what isn’t a constant expression. To ensure compile-time evaluation of constant expressions, developers are forced to use macros instead of inline functions. Example:1 const int var_a = INT_MAX;2 const int var_b = numeric_limits<int>::max();34 int array_a[var_a]; //fine5 int array_b[var_b]; //error: "constant expression required" Only the var a using the constant INT MAX is evaluated at compile-time in the example above. The compiler often optimizes calls and use compile-time evaluation, but it isn’t obliged to do that. We need a way to ensure that a deﬁned constant expression will always be evaluated at compile-time, otherwise we need to fall in an explicit error. Christian S. Perone () C++0x 16 March 2011 18 / 65

New proposals Constant Expressions Constant Expressions In the proposal N2235, a constant expression is deﬁned as: it returns a value (i.e., has non-void return type); its body consists of a single statement of the form return expr; it is declared with the keyword constexpr. Examples of valid constant expressions (citing the proposal): 1 constexpr int square(int x) 2 { return x * x; } // fine 3 4 constexpr long long_max() 5 { return 2147483647; } // fine 6 7 constexpr int abs(int x) 8 { return x < 0 ? -x : x; } // fine 910 constexpr void f(int x) // error: return type is void11 { /* ... */ } Christian S. Perone () C++0x 16 March 2011 19 / 65

New proposals Constant Expressions Constant Expressions Examples of invalid constant expressions (citing the proposal): 1 constexpr int next(int x) 2 { return ++x; } // error: use of increment 3 4 constexpr int g(int n) // error: body not just return expr 5 { 6 int r = n; 7 while (--n > 1) r *= n; 8 return r; 9 }1011 constexpr int twice(int x);12 enum { bufsz = twice(256) }; // error: twice() isnt (yet) defined1314 constexpr int fac(int x)15 { return x > 2 ? x * fac(x - 1) : 1; }16 // error: fac() not defined before use Christian S. Perone () C++0x 16 March 2011 20 / 65

New proposals Variadic TemplatesOutline1 Introduction What is C++0x ?2 New proposals Type Inference Trailing return types Static Assertion Constant Expressions Variadic Templates Strongly-typed enums Initializer Lists Range-based for Naming the NULL Lambda Rvalue References3 Conclusion C++0x Feature Availability Links Christian S. Perone () C++0x 16 March 2011 21 / 65

New proposals Variadic TemplatesVariadic Templates Variadic TemplatesVariadic templates are templates which accept an arbitrary number oftemplate arguments, providing for instance a clean implementation oftype-safe printf() and class templates such as tuples. Christian S. Perone () C++0x 16 March 2011 22 / 65

New proposals Variadic Templates Variadic Templates - Example Consider the follow example (from N2087 propostal): 1 template<typename T, typename... Args> 2 void printf(const char* s, const T& value, const Args&... args) 3 { 4 while (*s) 5 { 6 if (*s == ’%’ && *++s != ’%’) 7 { 8 std::cout << value; 9 return printf(++s, args...);10 }11 std::cout << *s++;12 }13 throw std::runtime_error("extra arguments provided to printf");14 } Christian S. Perone () C++0x 16 March 2011 23 / 65

New proposals Variadic Templates Variadic Templates - Example Continue... (from N2087 propostal): 1 void printf(const char* s) 2 { 3 while (*s) 4 { 5 if (*s == ’%’ && *++s != ’%’) 6 throw std::runtime_error("invalid format string:" 7 "missing arguments"); 8 std::cout << *s++; 9 }10 }1112 // Call13 printf("%s %s %d", std::string("a"), "b", 2); Christian S. Perone () C++0x 16 March 2011 24 / 65

New proposals Variadic Templates Variadic Templates - Tuples See how variadic templates are used by the <tuple> library:1 std::tuple<double, int> tup;2 double value_0 = std::get<0>(tup);3 std::get<1>(tup) = 666; Getting tuple information:1 typedef std::tuple<int,double,int> MyTuple;2 MyTuple tup(1, 2.0, 3);34 int tup_size = std::tuple_size<MyTuple>::value; // 3 = Tuple size5 std::tuple_element<0, MyTuple>::type value_0; // value_0 has the first el6 value_0 = std::get<0>(tup); // value_0 has now the firs Christian S. Perone () C++0x 16 March 2011 25 / 65

New proposals Strongly-typed enumsOutline1 Introduction What is C++0x ?2 New proposals Type Inference Trailing return types Static Assertion Constant Expressions Variadic Templates Strongly-typed enums Initializer Lists Range-based for Naming the NULL Lambda Rvalue References3 Conclusion C++0x Feature Availability Links Christian S. Perone () C++0x 16 March 2011 26 / 65

New proposals Strongly-typed enums Strongly-typed enums Some problems with current C++ enums standard (citing the proposal): They are not type-safe1 enum Color { ClrRed, ClrOrange, ClrYellow,2 ClrGreen, ClrBlue, ClrViolet };3 enum Alert { CndGreen, CndYellow, CndRed };4 Color c = ClrRed;5 Alert a = CndGreen;6 a = c; // error7 a = ClrYellow; // error8 bool armWeapons = ( a >= ClrYellow ); // ok; oops Scoping problems1 enum E1 { Red };2 enum E2 { Red }; // error Christian S. Perone () C++0x 16 March 2011 27 / 65

New proposals Strongly-typed enumsStrongly-typed enumsThe proposal N2347 suggests a new strongly-typed enum with the followproperties: 1 Declared using enum class 1 enum class E {E1, E2, E3 = 100, E4 /* = 101 */ }; 2 Conversions 1 enum class E {E1, E2, E3 = 100, E4 /* = 101 */ }; 2 void f(E e) { 3 if( e >= 100 ) // error: no E to int conversion 4 // (...) 5 } 6 int i = E::E2; // error: no E to int conversion Christian S. Perone () C++0x 16 March 2011 28 / 65

New proposals Strongly-typed enumsStrongly-typed enums 3 Underlying type (default is int ) 1 enum class E : unsigned long {E1 = 1, E2 = 2, Ebig = 0xFFFFFFF0U}; 4 Scoping 1 enum class E {E1, E2, E3 = 100, E4 /* = 101 */ }; 2 E e1 = E1; // error 3 E e2 = E::E2; // ok Christian S. Perone () C++0x 16 March 2011 29 / 65

New proposals Initializer ListsOutline1 Introduction What is C++0x ?2 New proposals Type Inference Trailing return types Static Assertion Constant Expressions Variadic Templates Strongly-typed enums Initializer Lists Range-based for Naming the NULL Lambda Rvalue References3 Conclusion C++0x Feature Availability Links Christian S. Perone () C++0x 16 March 2011 30 / 65

New proposals Initializer Lists Initializer Lists The main idea of initializer lists is to allow the use of the brace-enclosed lists of expressions in all contexts that allow initializers. Example (before initializer lists):1 std::vector<std::string> vec;2 vec.push_back("How do I...");3 vec.push_back("...hate to use push_back...");4 vec.push_back("...every time I need an item"); Example (after initializer lists):1 std::vector<std::string> vec = {"thank", "you", "god"};2 // or (equivalent)3 std::vector<std::string> vec {"thank", "you", "god"}; Christian S. Perone () C++0x 16 March 2011 31 / 65

New proposals Initializer Lists Initializer Lists Example as function argument:1 // Definition2 void pretty_function(std::string str,3 std::initializer_list<int> list)4 { /*...*/ };56 // Calling7 function_name("string", {1, 2, 3, 4}); Or:1 // Returning an initializer list2 std::initializer_list<int> pretty_function(void)3 {4 return {1, 2, 3, 4};5 } Christian S. Perone () C++0x 16 March 2011 32 / 65

New proposals Initializer Lists Initializer Lists Diﬀerence between the {} and the traditional initialization:1 // C and C++ implicitly truncates:23 int x = 2.5; // silent becomes x = 245 // In C++0x, the narrowing isn’t allowed:67 double x = 2; // ok8 int y {2.5}; // error, narrow type Christian S. Perone () C++0x 16 March 2011 33 / 65

New proposals Range-based forOutline1 Introduction What is C++0x ?2 New proposals Type Inference Trailing return types Static Assertion Constant Expressions Variadic Templates Strongly-typed enums Initializer Lists Range-based for Naming the NULL Lambda Rvalue References3 Conclusion C++0x Feature Availability Links Christian S. Perone () C++0x 16 March 2011 34 / 65

New proposals Range-based for Range-based for Range-based for was created to avoid the use of iterators in simple range loops, so we don’t need to care about iterators. The syntax of the new range-based for is: for(for-range-declaration : expression) statement Here is an example:1 for(auto item : item_list)2 std::cout << item; You could also use it to transform:1 std::vector<int> item_list {1,2,3};2 for(auto &v : item_list) v++;3 for(auto &v : item_list)4 std::cout << v << std::endl; Will output: 2 3 4. Christian S. Perone () C++0x 16 March 2011 35 / 65

New proposals Naming the NULLOutline1 Introduction What is C++0x ?2 New proposals Type Inference Trailing return types Static Assertion Constant Expressions Variadic Templates Strongly-typed enums Initializer Lists Range-based for Naming the NULL Lambda Rvalue References3 Conclusion C++0x Feature Availability Links Christian S. Perone () C++0x 16 March 2011 36 / 65

New proposals Naming the NULL nullptr - Naming the NULL The current C++ standard (C++03) deﬁnes a special rule in which the 0 can be at same time an integer constant and a null pointer constant. This rule results in the follow confusion, consider those both overloaded functions:1 void f(int);2 void f(char *); The call to f(0) will resolve to f(int). There is no way to call f(char*) using a null pointer argument without using an explicit cast like f((char*)0). To solve this, they deﬁned a new rule: 0 must be only an integer constant, and a new separate word was created to represent the null pointer: nullptr. Christian S. Perone () C++0x 16 March 2011 37 / 65

New proposals Naming the NULL nullptr - Naming the NULL Examples1 : 1 char* ch = nullptr; // ch has the null pointer value 2 char* ch2 = 0; // ch2 has the null pointer value 3 int n = nullptr; // error 4 int n2 = 0; // n2 is zero 5 6 if(ch == 0); // evaluates to true 7 if(ch == nullptr); // evaluates to true 8 if(ch); // evaluates to false 9 if(n2 == 0); // evaluates to true10 if(n2 == nullptr); // error11 if(nullptr); // error, no conversion to bool12 if(nullptr == 0); // error1314 // arithmetic15 nullptr = 0; // error, nullptr is not an lvalue16 nullptr + 2; // error 1 Examples cited as in the propostal. Christian S. Perone () C++0x 16 March 2011 38 / 65

New proposals LambdaOutline1 Introduction What is C++0x ?2 New proposals Type Inference Trailing return types Static Assertion Constant Expressions Variadic Templates Strongly-typed enums Initializer Lists Range-based for Naming the NULL Lambda Rvalue References3 Conclusion C++0x Feature Availability Links Christian S. Perone () C++0x 16 March 2011 39 / 65

New proposals Lambda Lambda - the rise of the Functional empire The λ calculus was introduced in 1930 by Alonzo Church and is important for Computer Science because it provides simple semantics for computation with functions. One important observation from the λ calculus is that functions need not to be named, they can be written in anonymous form. Example (without types):1 square_add(x, y) = x*x + y*y can be written as: (x, y ) → x ∗ x + y ∗ y . Christian S. Perone () C++0x 16 March 2011 40 / 65

New proposals Lambda Lambda - the rise of the Functional empire See for example, how λ takes its form in Python language:1 map(lambda x: x*2, [1,2,3]) ... which can be read as: apply this lambda function ((x) → x ∗ 2) into the list [1, 2, 3]. Christian S. Perone () C++0x 16 March 2011 41 / 65

New proposals LambdaLambda - Grammar Lambda Grammar in C++0x Figure: From http://www.codeproject.com/KB/cpp/cpp10.aspx Christian S. Perone () C++0x 16 March 2011 42 / 65

New proposals Lambda Lambda - Examples Simple example:1 // The same as λx → x ∗ 2.0:2 auto lambda_function =3 [](float x) { return x * 2.0; };45 float ret = lambda_function(2.0); // returns 4.0 Here we have declared a lambda function, assigned it into the lambda function variable, and then called it with the argument 2.0. Note that the compiler, in this case, was able to deduce the return type since it is a simple expression, but there are cases in which we need to do that explicitly. Christian S. Perone () C++0x 16 March 2011 43 / 65

New proposals LambdaLambda - Closures ClosureLambda expressions may look like simple functions, actually, they act likeunnamed functions, but they’re represented by a closure type. WhenLambda expressions are evaluated, they result in a closure object,containing the body of the Lambda expression as well the environmentwhere the Lambda function is declared. The closure type of the Lambdaexpression has a public inline function call operator implemented, whosethe parameters and return type are speciﬁed by the Lambda expression.Curiosity: ”The reason it is called a closure is that an expressioncontaining free variables is called an open expression, and by associatingto it the bindings of its free variables, you close it. - Ake Wikstrom (1987). Christian S. Perone () C++0x 16 March 2011 44 / 65

New proposals Lambda Lambda - Examples Note that in the ﬁrst example, we haven’t speciﬁed the trailing return type of the lambda:1 // error: cannot deduce lambda return type from a braced-init-list2 auto lambda_function =3 [](int x){ return {1,2}; };45 // ok !6 auto lambda_function =7 [](int x)->std::initializer_list<int> { return {1,2}; }; Since the return {1,2} (initializer list<int>) isn’t an expression, we need to specify the trailing return type explicitly. Christian S. Perone () C++0x 16 March 2011 45 / 65

New proposals LambdaLambda - CapturesUntil now, we haven’t used any variable of the upper-scope inside thelambda function, but you’ll need to do that in different flavors, and that’swhy we have different Capture modes: Capture modes [] Capture nothing. [=] Capture everything by value. [&] Capture everything by reference. [var] Capture only var by value. [&var] Capture only var by reference.According to the propostal, for each entity captured by copy, an unnamednon-static data member is declared in the closure type. Christian S. Perone () C++0x 16 March 2011 46 / 65

New proposals Lambda Lambda - Capture examples Example 1: Capturing nothing.1 auto f1 = [](int x) { return x * 2; }; Example 2: Capturing everything by value.1 int scope_value = 42;2 auto f2 =3 [=](int x)4 {5 scope_value++; // error here: scope_value is read-only6 return x * scope_value; // ok7 }; Christian S. Perone () C++0x 16 March 2011 47 / 65

New proposals Lambda Lambda - Capture examples Example 3: Capturing everything by reference.1 int scope_value = 42;2 auto f3 =3 [&](int x)4 {5 scope_value++; // ok6 return x * scope_value; // ok7 }; Christian S. Perone () C++0x 16 March 2011 48 / 65

New proposals Lambda Lambda - Capture examples Example 4: Capturing variable by value.1 int scope_value = 42;2 int scope_value2 = 0;34 auto f4 =5 [scope_value](float x)6 {7 int ret = scope_value2; // error here: scope_value2 not captured8 return x * scope_value; // ok9 }; Christian S. Perone () C++0x 16 March 2011 49 / 65

New proposals Lambda Lambda - Capture examples Example 5: Capturing variable by reference. 1 int scope_value = 42; 2 int scope_value2 = 0; 3 4 auto f5 = 5 [&scope_value](int x) 6 { 7 scope_value++; // ok 8 scope_value2++; // error: scope_value2 not captured 9 return scope_value * x; // ok10 }; Christian S. Perone () C++0x 16 March 2011 50 / 65

New proposals Lambda Lambda - Capture examples Example 6: Composite capture mode. 1 int a = 1; 2 int b = 2; 3 int c = 3; 4 5 auto f6 = 6 [=, &c](int x) // capture everything by value, but "c" by reference. 7 { 8 c = 2; // ok 9 a = 1; // error: "a" is read-only10 return scope_value * b; // ok11 }; Christian S. Perone () C++0x 16 March 2011 51 / 65

New proposals Lambda Lambda - Capture examples After the speciﬁcation of the arguments and before the speciﬁcation of the trailing return type, we can specify the keyword mutable. This keyword makes the variables mutable. Example:1 int a = 1;2 auto f7 =3 [=](int x) mutable // capture everything by value4 {5 a = 5; // ok6 return x * a; // ok7 }; Note that is ok to assign a value into the ”a” variable, but after calling this lambda function, you’ll note that the ”a” continues with value 1 instead of 5, this is because the variable inside the lambda scope is now a copy of the variable and not the variable itself or the reference. Christian S. Perone () C++0x 16 March 2011 52 / 65

New proposals Lambda Lambda - Stateful examples Stateful Example Consider the follow example:1 int value = 0;2 auto lambda_fun = [=] { return value; };3 value = 2;4 std::cout << lambda_fun() << std::endl; What would be the value show ? The answer is 0. Remember the closures ! Christian S. Perone () C++0x 16 March 2011 53 / 65

New proposals Lambda Lambda - Where Where are lambdas useful ? No more function pointers, now we have function objects (which are in fact the same type of a lambda function object):1 void call_callback(int arg1, function<void(int)> callback_function)2 {3 callback_function(arg1);4 } No more operator() overloading, many STL methods will be beneﬁted Lambdas:1 std::vector<int> vec {1,2,3,4};2 std::for_each(vec.begin(), vec.end(), [](int &v) { v*=v; }); Christian S. Perone () C++0x 16 March 2011 54 / 65

New proposals Rvalue ReferencesOutline1 Introduction What is C++0x ?2 New proposals Type Inference Trailing return types Static Assertion Constant Expressions Variadic Templates Strongly-typed enums Initializer Lists Range-based for Naming the NULL Lambda Rvalue References3 Conclusion C++0x Feature Availability Links Christian S. Perone () C++0x 16 March 2011 55 / 65

New proposals Rvalue ReferencesRvalue References Rvalue vs LvalueDefinition lvalue is an expression which identifies an object with extended life. It’s more intuitive to think in lvalues as the named “things” on the left side of expressions. rvalue is an expression which identifies an temporary object. This object goes away as soon as the expression ends, you can imagine rvalues as transient values. Christian S. Perone () C++0x 16 March 2011 56 / 65

New proposals Rvalue References Rvalue Example 1 T a; 2 // The reference1 is an lvalue reference 3 T& reference1 = a; 4 5 T b; 6 // The reference2 is an rvalue reference 7 T&& reference2 = b; 8 9 // The real difference10 T& reference3 = T(); // Error, since T& isn’t const11 T&& reference4 = T(); // Ok Ok, but why would I want this ? Christian S. Perone () C++0x 16 March 2011 57 / 65

New proposals Rvalue References Move Semantics Performance ! C++0x introduces the concept of Move Semantics. Copying is expansive, imagine for example two std::vectors, called v1 and v2; the follow statement:1 std::vector<int> v1 = load_big_vector();2 std::vector<int> v2;3 v2 = v1; The statement at line 3 will involve a function call, the memory pool allocation, and the copy of each item from v1 to v2. This is normal when we really need two copies of the same vector, but sometimes we don’t. Christian S. Perone () C++0x 16 March 2011 58 / 65

New proposals Rvalue References Move Semantics Consider the follow example:1 template <class T>2 swap(T& a, T& b)3 {4 T tmp(a); // two copies of a5 a = b; // two copies of b6 b = tmp; // two copies of tmp7 } In this case, we just want to swap variables and not make expansive copies here and there. Christian S. Perone () C++0x 16 March 2011 59 / 65

New proposals Rvalue References Move Semantics Now consider the follow example using move semantics:1 template <class T>2 swap(T& a, T& b)3 {4 T tmp(std::move(a));5 a = std::move(b);6 b = std::move(tmp);7 } What std::move does is the conversion of the argument from a lvalue/rvalue to a rvalue, but it doesn’t guarantee the preservation of the argument data; you can imagine the std::move method as a potentially destructive read. Christian S. Perone () C++0x 16 March 2011 60 / 65

New proposals Rvalue References Move Semantics Some examples to illustrate the std::move semantic:1 std::vector<int> v = {1,2,3,4,5};2 std::vector<int> v2;3 v2 = std::move(v); // Move, doesn’t copy !4 std::cout << v.size(); // Size: 0 Another example:1 std::vector<int> return_vector()2 {3 std::vector<int> tmp {1,2,3,4,5};4 return tmp;5 }6 std::vector<int> vect = return_vector(); What std::move does is the conversion of the argument from a lvalue/rvalue to a rvalue, but it doesn’t guarantee the preservation of the argument data; you can imagine the std::move method as a potentially destructive read. Christian S. Perone () C++0x 16 March 2011 61 / 65

Conclusion C++0x Feature AvailabilityOutline1 Introduction What is C++0x ?2 New proposals Type Inference Trailing return types Static Assertion Constant Expressions Variadic Templates Strongly-typed enums Initializer Lists Range-based for Naming the NULL Lambda Rvalue References3 Conclusion C++0x Feature Availability Links Christian S. Perone () C++0x 16 March 2011 62 / 65

Conclusion C++0x Feature AvailabilityC++0x Feature AvailabilitySome compilers already have support for some of the new C++0x features.GCCGCC started the implementation of the C++0x proposals since the version4.3, the current development branch (4.6) is supporting more than 39C++0x proposals, including but not limited to: Lambdas, range-based for, initializer lists, auto-typed variables, null pointer constant, constant expressions, static assertions,. . .In order to use GCC compiler with C++0x support, you must use theparameter -std=c++0x. Christian S. Perone () C++0x 16 March 2011 63 / 65

Conclusion LinksOutline1 Introduction What is C++0x ?2 New proposals Type Inference Trailing return types Static Assertion Constant Expressions Variadic Templates Strongly-typed enums Initializer Lists Range-based for Naming the NULL Lambda Rvalue References3 Conclusion C++0x Feature Availability Links Christian S. Perone () C++0x 16 March 2011 64 / 65

Conclusion LinksLinks C++0x FAQ, from the author of C++. http://www2.research.att.com/ bs/C++0xFAQ.html Apache list of C++0x Compiler Support. http://wiki.apache.org/stdcxx/C++0xCompilerSupport Eq Solution: Weekly Snapshots of GCC dev. branch. http://www.equation.com/servlet/equation.cmd?call=fortran GCC C++0x Status page (with proposals used in this presentation). http://gcc.gnu.org/gcc-4.6/cxx0x status.html Explicating the new C++ standard. http://www.codeproject.com/KB/cpp/cpp10.aspx A Tutorial Introduction to the Lambda Calculus. http://www.inf.fu-berlin.de/lehre/WS03/alpi/lambda.pdf Christian S. Perone () C++0x 16 March 2011 65 / 65

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C++0x :: Introduction to some amazing features

by Christian S. Perone on Jul 28, 2011

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C++0x :: Introduction to some amazing features — Presentation Transcript