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C++20 replaced the prior version of the C++ standard, called C++17, and was later replaced by C++23. [1] The standard was technically finalized [ 2 ] by WG21 at the meeting in Prague in February 2020, [ 3 ] had its final draft version announced in March 2020, [ 4 ] was approved on 4 September 2020, [ 5 ] [ 6 ] and published in December 2020.
The above example is an expression that evaluates to a first-class function. The symbol lambda creates an anonymous function, given a list of parameter names, x – just a single argument in this case, and an expression that is evaluated as the body of the function, x**2. Anonymous functions are sometimes called lambda expressions.
As of the 2011 revision, the C++ language also supports closures, which are a type of function object constructed automatically from a special language construct called lambda-expression. A C++ closure may capture its context either by storing copies of the accessed variables as members of the closure object or by reference.
The names "lambda abstraction", "lambda function", and "lambda expression" refer to the notation of function abstraction in lambda calculus, where the usual function f (x) = M would be written (λx. M), and where M is an expression that uses x. Compare to the Python syntax of lambda x: M.
In such a case, a common solution is to create a clone (or similar) virtual function that creates and returns a copy of the derived class when called. A member function can also be made "pure virtual" by appending it with = 0 after the closing parenthesis and before the semicolon. A class containing a pure virtual function is called an abstract ...
C++11 allowed lambda functions to deduce the return type based on the type of the expression given to the return statement. C++14 provides this ability to all functions. It also extends these facilities to lambda functions, allowing return type deduction for functions that are not of the form return expression;.
In the untyped lambda calculus, where the basic types are functions, lifting may change the result of beta reduction of a lambda expression. The resulting functions will have the same meaning, in a mathematical sense, but are not regarded as the same function in the untyped lambda calculus. See also intensional versus extensional equality.
Any member function of a class, such as copy constructors, operator overloads, etc., can be declared as constexpr, so long as they meet the requirements for constexpr functions. This allows the compiler to copy objects at compile time, perform operations on them, etc. If a constexpr function or constructor is called with arguments which aren't ...