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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.
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;.
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.
C++17 is a version of the ISO/IEC 14882 standard for the C++ ... or an additional helper template function std:: ... Lambda expressions can capture "*this" by value ...
Unlike ordinary C function definitions, their value can capture state from their surrounding context. A block definition produces an opaque value which contains both a reference to the code within the block and a snapshot of the current state of local stack variables at the time of its definition.
The second, treats lambda abstractions which are applied to a parameter as defining a function. Lambda abstractions applied to a parameter have a dual interpretation as either a let expression defining a function, or as defining an anonymous function. Both interpretations are valid. These two predicates are needed for both definitions. lambda ...
Calling f with a regular function argument first applies this function to the value 2, then returns 3. However, when f is passed to call/cc (as in the last line of the example), applying the parameter (the continuation) to 2 forces execution of the program to jump to the point where call/cc was called, and causes call/cc to return the value 2.
The function that accepts a callback may be designed to store the callback so that it can be called back after returning which is known as asynchronous, non-blocking or deferred. Programming languages support callbacks in different ways such as function pointers, lambda expressions and blocks.