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tes 2 value 3 uses toggle uses subsection 3 1 helper 3 2 control flow 3 3 higher order functions 4 examples toggle examples subsection 4 1 simple example 4 2 quicksort 5 languages toggle languages subsection 5 1 functional languages 6 alternatives toggle alternatives subsection 6 1 modularity 6 2 parameters 6 3 lambda 6 4 alternatives by language 7 implementation toggle implementation subsection 7 1 access of non local objects 7 2 functions as values 7 3 no execute stacks 8 see also 9 references 10 external links toggle the table of contents nested function 5 languages ελληνικά français português русский українська edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia named function defined within a function in computer programming a nested function or nested procedure or subroutine is a named function that is defined within another enclosing block and is lexically scoped within the enclosing block meaning it is only callable by name within the body of the enclosing block and can use identifiers declared in outer blocks including outer functions the enclosing block is typically but not always another function programming language support for nested functions varies with respect to structured programming languages it is supported in some outdated languages such as algol simula 67 and pascal and in the commonly used javascript it is commonly supported in dynamic and functional languages however it is not supported in some commonly used languages including standard c and c other programming technologies provide similar benefit for example a lambda function also allows for a function to be defined inside of a function as well as elsewhere and allows for similar data hiding and encapsulation notably a lambda function has no name is anonymous and therefore cannot be called by name and has no visibility aspect attributes edit the scope of a nested function is the block that contains it be it a function block or block within a function body it is not visible cannot be called by name outside its containing block a nested function can use identifiers i e the name of functions variables types classes declared in any enclosing block except when they are masked by inner declarations with the same names a nested function can be declared within a nested function recursively to form a deeply nested structure a deeply nested function can access identifiers declared in all of its enclosing blocks including enclosing functions nested functions may in certain situations lead to the creation of a closure if it is possible for the nested function to escape the enclosing function for example if functions are first class objects and a nested function is passed to another function or returned from the enclosing function then a closure is created and calls to this function can access the environment of the original function the frame of the immediately enclosing function must continue to be alive until the last referencing closure dies and non local automatic variables referenced in closures can therefore not be stack allocated in languages that allow the closure to persist beyond the lifetime of the enclosing block this is known as the funarg problem and is a key reason why nested functions was not implemented in some simpler languages as it significantly complicates code generation and analysis especially when functions are nested to various levels sharing different parts of their environment value edit the nested function technology allows a programmer to write source code that includes beneficial attributes such as information hiding encapsulation and decomposition the programmer can divide a task into subtasks which are only meaningful within the context of the task such that the subtask functions are hidden from callers that are not designed to use them block scoping allows functions to share the state of enclosing blocks including enclosing functions without passing parameters or using global variables 1 uses edit helper edit a nested function typically acts as a helper function or a recursive function control flow edit nested functions can be used for unstructured control flow by using the return statement for general unstructured control flow this can be used for finer grained control than is possible with other built in features of the language for example it can allow early termination of a for loop if break is not available or early termination of a nested for loop if a multi level break or exceptions are not available higher order functions edit main article higher order function in some languages it is possible to create a nested function that accesses a set of parameters from the outer function that is a closure and have that function be the outer function s return value thus it is possible to return a function that is set to fulfill a certain task with little or no further parameters given to it which can increase performance quite significantly 2 examples edit simple example edit a simple example in pascal function e x real real function f y real real begin f x y end begin e f 3 f 4 end the function f is nested within e note that e s parameter x is also visible in f as f is a part of e while both x and y are invisible outside e and f respectively similarly in standard ml fun e x real let fun f y x y in f 3 f 4 end in haskell e float float e x f 3 f 4 where f y x y in pl i e procedure x returns float declare x float f procedure y returns float declare y float return x y end return f 3 0 f 4 0 end in python def e x float float def f y float float return x y return f 3 0 f 4 0 in gnu c 3 which extends standard c with nested functions float e float x float f float y return x y return f 3 0f f 4 0f quicksort edit the following is an implementation of quicksort 4 void sort int items int size void quicksort int first int last void swap int p int q int tmp items p items p items q items q tmp int partition int pivot items first int index first swap index last for int i first i last i if items i pivot swap index i swap index last return index if first last int pivotindex partition quicksort first pivotindex 1 quicksort pivotindex 1 last quicksort 0 size 1 the following is an implementation of the hoare partition based quicksort using c 11 lambda expression syntax which is an alternative technology that also allows hiding a function inside a function iter represents a random access iterator template typename iter void sort iter begin iter end auto partition iter hoare partition scheme iter pivot begin iter forwardcursor begin iter backwardcursor end 1 iter partitionpositionfound false auto locatepartitionposition void while forwardcursor pivot forwardcursor while pivot backwardcursor backwardcursor if forwardcursor backwardcursor partitionpositionfound true else swap forwardcursor backwardcursor trivial helper function auto moveonandtryagain void forwardcursor backwardcursor brief outline of the actual partition process while true locatepartitionposition if partitionpositionfound return backwardcursor 1 else moveonandtryagain brief outline of the quicksort algorithm if begin end 1 iter partitionposition partition sort begin partitionposition sort partitionposition end languages edit notable languages supporting nested functions include algol based languages such as algol 68 simula pascal modula 2 modula 3 oberon pl i and ada modern versions of lisp with lexical scope such as scheme and common lisp ecmascript javascript and actionscript dart 5 kotlin local functions 6 rust scala nested functions 7 various degrees of support in scripting languages such as ruby python lua php and perl gcc supports nested functions in c as a language extension 8 c starting with c 7 0 the d language a c related language with nested functions fortran starting with fortran 90 supports a single level of nested contained subroutines and functions matlab full support wolfram language golang function closures 9 functional languages edit in most functional programming languages such as scheme nested functions are a common way of implementing algorithms with loops in them a simple tail recursive inner function is created which behaves as the algorithm s main loop while the outer function performs startup actions that only need to be done once in more complex cases a number of mutually recursive functions may be created as inner functions alternatives edit various alternative techniques can be used to achieve similar programming results as via nested functions modularity edit a common alternative is to leverage a language s modularity technology some functions are exposed for use outside of the module and some are only visible within the module in c this can be implemented by declaring functions and variables as static to hide them from code outside the file 10 this allows for data hiding encapsulation and decomposition but at a different level of granularity than with nested functions this modularity does not support more than one level of nesting in object oriented languages a class typically provides a scope in which functions and state can be hidden from consumers of the class but accessible within the class some languages allow classes to be nested parameters edit to implement data hiding functions can pass around shared data as parameters but this increases the complexity of function calls 1 in c this is generally implemented by passing a pointer to a structure containing the shared data 10 lambda edit in php and other languages the lambda is an alternative a function is defined in a code statement rather than declared with the usual function syntax it has no name but is callable via a function reference such functions can be defined inside of a function as well as in other scopes to use local variables in the anonymous function use closure alternatives by language edit the following languages provide features that are similar to nested functions c classes allow for similar data hiding and encapsulation defining a class within a class provides similar structure see function object in c c 11 and later via lambda expressions see quicksort example above 11 eiffel explicitly disallows nesting of routines to keep the language simple does allow the convention of using a special variable result to denote the result of a value returning function c and visual basic via lambda expressions java since java 8 via lambda expressions 12 and in older versions via an anonymous class containing a single method implementation edit see also man or boy test implementation of nested functions can be more involved than it may appear as a reference to a nested function that references non local variables creates a closure for this reason nested functions are not supported in some languages such as c c or java as this makes compilers more difficult to implement 10 13 however some compilers do support them as a compiler specific extension a well known example of this is the gnu c implementation of c which shares code with compilers for languages such as pascal ada and modula access of non local objects edit there are several ways to implement nested procedures in a lexically scoped language but the classic way is as follows any non local object x is reached via access links in the activation frames on the machine stack the caller c assists the called procedure p by pushing a direct link to the latest activation of p s immediate lexical encapsulation p prior to the call itself p may then quickly find the right activation for a certain x by following a fixed number p depth x depth of links normally a small number the caller creates this direct link by itself following c depth p depth 1 older links leading up to the latest activation of p and then temporarily bridging over these with a direct link to that activation the link later disappears together with p whereby the older links beneath it may come into use again note that p is visible for and may therefore be called by c if p c c c etc this original method is faster than it may seem but it is nevertheless often optimized in practical modern compilers using displays or similar techniques another way to implement nested functions that is used by some compilers is to convert lift nested functions into non nested functions where extra hidden parameters replace the access links using a process known as lambda lifting during an intermediate stage in the compilation functions as values edit in order for local functions with lexically scoped nonlocals to be passed as results the language runtime code must also implicitly pass the environment data that the function sees inside its encapsulating function so that it is reachable also when the current activation of the enclosing function no longer exists 14 this means that the environment must be stored in another memory area than the subsequently reclaimed parts of a chronologically based execution stack which in turn implies some sort of freely dynamic memory allocation many older algol based languages or dialects thereof does therefore not allow local functions that access nonlocals to be passed as return values or do they not allow functions as return values at all although passing of such functions as arguments may still be possible no execute stacks edit gcc s implementation of nested functions in c causes a loss of no execute stacks nx stacks therefore software engineered using secure development lifecycle often do not allow the use of gcc s nested functions 15 by default gcc will issue a brief notification when a generated object file requires an executable stack for a clear warning when compiling c code use the wtrampolines option the problem occurs because gcc uses trampolines executable code on the stack to jump to nested functions 16 the gcc project has an alternative method that could provide nested functions via non executable descriptors however it is currently only used in gcc s ada compiler see also edit call stack closure computer science function composition computer science inner class nesting computing references edit 1 2 bright 2004 higher order functions and lambdas kotlin programming language rothwell trevis j 2011 the gnu c reference manual free software foundation inc p 63 re nesting functions why link removed baavgai 14 january 2012 a tour of the dart language functions kotlin nested methods nested functions using the gnu compiler collection gcc gnu project retrieved 2007 01 06 a tour of go 1 2 3 question 20 24 why doesn t c have nested functions comp lang c faq nested function rosetta code nested function rosetta code answer by dave vandervies aug 28 09 at 17 45 to why are nested functions not supported by the c stand...