Dimensional Data

A defaulted function is a function that contains =default; in its prototype. This construction indicates that the function’s default definition should be used. Defaulted functions are a C++11 specific feature. Defaulted functions example class A { A() = default; // OK A& operator = (A & a) = default; // OK void f() = default; // ill-formed, only special member function may be defaulted }; class A {         A() = default;                    // OK         A& operator = (A & a) = default;  // OK         void f() = default;               // ill-formed, only special member function may be defaulted }; By default, C++ provides four default special member functions. Users may override these defaults. destructor default constructor copy constructor copy assignment operator = Also by default, C++ applies several global operators to classes. Users may provide class-specific operators. sequence operator , address-of operator & indirection operator * member access operator -> member indirection operator ->* free-store allocation operator new free-store deallocation operator delete The management of defaults has several problems: Constructor definitions are coupled; declaring any constructor suppresses the default constructor. The destructor default is inappropriate to polymorphic classes, requiring an explicit definition. Once a default is suppressed, there is no means to resurrect it. Default implementations are often more efficient than manually specified implementations. Non-default implementations are non-trivial, which affects type semantics, e.g. makes a type non-POD. There is no means to prohibit a special member function or global operator without declaring a (non-trivial) substitute. The most common encounter with these problems is when disabling copying of a class. The accepted technique is to declare a private copy constructor and a private copy assignment operator, and then fail to define either. Head over and check out more information about defaulted functions on Windows in C++.

C++17 has a new feature that consolidates syntactic sugar and automatic type deduction: structured bindings. This helps to assign values from tuples, pairs, and structs into individual variables. In another programming language, you can find this as unpacking. Applying a structured binding to specify various variables from one bundled structure is one step. Structured bindings always applied with the same pattern: auto [var1, var2, …] = ; auto [var1, var2, …] = pair, tuple, struct, or array expression>; The list of variables var1, var2 – must exactly match the number of variables contained by the expression being assigned from Then there should be one of the following std::pair std::tuple struct or array The type can be auto, const auto, or even auto&& If you write too many or not enough variables between the square brackets, the compiler will give an error, telling us about our mistake std::tuple tup {3.55, 1, 99}; auto [a, b] = tup; // gives an error std::tuplefloat, int, long> tup {3.55, 1, 99}; auto [a, b] = tup; // gives an error Here is a complete real example of a use case for structured bindings: #ifdef _WIN32 #include #else typedef char _TCHAR; #define _tmain main #endif #include #include #include // Demonstrate structured bindings – here, a method might in the past // have had a bool success result, and an error param written to. // Now, can return both (all error info) as one tuple, bound automatically // to local vars through the auto keyword (much nicer than std::tie // in C++11/14) auto connect_to_network() { // The connection could succeed, or fail with an error message // Here, mimic it failing bool connected{ false }; // failed! std::string str_error{ “404: resource not found” }; return std::make_tuple(connected, str_error); } int _tmain(int argc, _TCHAR* argv[]) { auto [connected, str_error] = connect_to_network(); if (!connected) { std::cout 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 #ifdef _WIN32 #include #else typedef char _TCHAR; #define _tmain main #endif   #include #include #include   // Demonstrate structured bindings – here, a method might in the past // have had a bool success result, and an error param written to. // Now, can return both (all error info) as one tuple, bound automatically // to local vars through the auto keyword (much nicer than std::tie // in C++11/14)   auto connect_to_network() {   // The connection could succeed, or fail with an error message   // Here, mimic it failing   bool connected{ false }; // failed!     std::string str_error{ “404: resource not found” };     return std::make_tuple(connected, str_error); }   int _tmain(int argc, _TCHAR* argv[]) {   auto [connected, str_error] = connect_to_network();     if (!connected) {     std::cout str_error;   }     system(“pause”);     return 0; } As you can see, by using structured bindings, you can write more efficient and clean code. Be sure to check out the official documentation on structured bindings here! Check out the structured bindings demo for C++Builder on GitHub!

rocedure TForm1.Button1Click(Sender: TObject); var   P : Variant; begin   PythonEngine1.ExecStrings( Memo1.Lines );   P := MainModule.Person(‘John’, ‘Doe’);   Assert(P.first_name = ‘John’);   Assert(P.last_name = ‘Doe’);   Assert(VarIsNone(P.weight));   Assert(VarIsNone(P.height));   Assert(VarIsNone(P.age));   P := MainModule.Person(‘John’, ‘Doe’, weight := 70);   Assert(P.first_name = ‘John’);   Assert(P.last_name = ‘Doe’);   Assert(P.weight = 70);   Assert(VarIsNone(P.height));   Assert(VarIsNone(P.age));   P := MainModule.Person(‘John’, ‘Doe’, weight := 70, height := 172);   Assert(P.first_name = ‘John’);   Assert(P.last_name = ‘Doe’);   Assert(P.weight = 70);   Assert(P.height = 172);   Assert(VarIsNone(P.age));   P := MainModule.Person(‘John’, ‘Doe’, weight := 70, height := 172, age := 35);   Assert(P.first_name = ‘John’);   Assert(P.last_name = ‘Doe’);   Assert(P.weight = 70);   Assert(P.height = 172);   Assert(P.age = 35);   P := MainModule.Person(last_name := ‘Doe’, first_name := ‘John’, weight := 70, height := 172, age := 35);   Assert(P.first_name = ‘John’);   Assert(P.last_name = ‘Doe’);   Assert(P.weight = 70);   Assert(P.height = 172);   Assert(P.age = 35);   P := MainModule.Person(‘John’, ‘Doe’, 35, 172, 70);   Assert(P.first_name = ‘John’);   Assert(P.last_name = ‘Doe’);   Assert(P.weight = 70);   Assert(P.height = 172);   Assert(P.age = 35);   Memo2.Lines.Add(‘Success’) end;

Substitution failure is not an error (SFINAE) refers to a situation in C++ where an invalid substitution of template parameters is not in itself an error. We’re talking here about something related to templates, template substitution rules and metaprogramming… A quick example: template struct A {}; char xxx(int); char xxx(float); template A f(T){} int main() { f(1); }   template <int I> struct A {};     char xxx(int);   char xxx(float);     template <class T> A<sizeof(xxx((T)0))> f(T){}     int main()   {     f(1);   } This example is rejected by all major compilers because template deduction/substitution has historically used a simplified model of semantic checking, i.e., the SFINAE rules (which are mostly about types), instead of full semantic checking. But in C++ 11, fully general expressions allowed, and that most errors in such expressions be treated as SFINAE failures rather than errors. There’s a continuum of errors, some errors being clearly SFINAE failures, and some clearly “real” errors, with lots of unclear cases in between. We decided it’s easier to write the definition by listing the errors that are not treated as SFINAE failures, and the list we came up with is as follows: errors that occur while processing some entity external to the expression, e.g., an instantiation of a template or the generation of the definition of an implicitly-declared copy constructor. errors due to implementation limits (it’s probably a category error to list these here, since they’re not errors in the normal sense, but we wanted to make it very clear that compilers don’t have to take steps to capture and recover from violations of implementation limits; such violations cause hard errors, compiler crashes, etc., the same as anywhere else in a program). errors due to access violations (this is a judgment call, but the philosophy of access has always been that it doesn’t affect visibility)Everything else produces a SFINAE failure rather than a hard error. At certain points in the template argument deduction process it is is necessary to take a function type that makes use of template parameters and replace those template parameters with the corresponding template arguments. This is done at the beginning of template argument deduction when any explicitly specified template arguments are substituted into the function type, and again at the end of template argument deduction when any template arguments that were deduced or obtained from default arguments are substituted. The substitution occurs in all types and expressions that are used in the function type and in template parameter declarations. The expressions include not only constant expressions such as those that appear in array bounds or as nontype template arguments but also general expressions (i.e., non-constant expressions) inside sizeof, decltype, and other contexts that allow non-constant expressions. [Note: The equivalent substitution in exception specifications is done only when the function is instantiated, at which point a program is ill-formed if the substitution results in an invalid type or expression.] For example, template auto f(T t1, T t2) -> decltype(t1 + t2); template <class T> auto f(T t1, T t2) -> decltype(t1 + t2); Head over and check out more information about SFINAE problems for expressions in Windows Development.