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In computer programming, an anonymous function (function literal, expression or block) is a function definition that is not bound to an identifier. Anonymous functions are often arguments being passed to higher-order functions or used for constructing the result of a higher-order function that needs to return a function. [ 1 ]
The term closure is often used as a synonym for anonymous function, though strictly, an anonymous function is a function literal without a name, while a closure is an instance of a function, a value, whose non-local variables have been bound either to values or to storage locations (depending on the language; see the lexical environment section below).
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;.
Notably, the delivery need not be made by the clerk who took the order. A callback need not be called by the function that accepted the callback as a parameter. Also, the delivery need not be made directly to the customer. A callback need not be to the calling function. In fact, a function would generally not pass itself as a callback.
A function definition starts with the name of the type of value that it returns or void to indicate that it does not return a value. This is followed by the function name, formal arguments in parentheses, and body lines in braces. In C++, a function declared in a class (as non-static) is called a member function or method.
The lambda lift is the substitution of the lambda abstraction S for a function application, along with the addition of a definition for the function. l a m b d a - l i f t [ S , L ] ≡ let V : d e - l a m b d a [ G = S ] in L [ S := G ] {\displaystyle \operatorname {lambda-lift} [S,L]\equiv \operatorname {let} V:\operatorname ...
Anonymous recursion can also be used for named functions, rather that calling them by name, say to specify that one is recursing on the current function, or to allow one to rename the function without needing to change the name where it calls itself. However, as a matter of programming style this is generally not done.
The C++ Standard Library provides base classes unary_function and binary_function to simplify the definition of adaptable unary functions and adaptable binary functions. Adaptable function objects are important, because they can be used by function object adaptors: function objects that transform or manipulate other function objects.