Can we have functions inside functions in C

Question

I mean something like   int main        void a                 code       a       return 0

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For all intents and purposes  C   supports this via lambdas 1  int main         auto f          return 42         std  cout  lt  lt   f        lt  lt  f    lt  lt  std  endl      Here  f is a lambda object that acts as a local function in main  Captures can be specified to allow the function to access local objects   Behind the scenes  f is a function object  i e  an object of a type that provides an operator     The function object type is created by the compiler based on the lambda     1 since C  11

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You can t have local functions in C    However  C  11 has lambdas  Lambdas are basically variables that work like functions  A lambda has the type std  function  actually that s not quite true  but in most cases you can suppose it is   To use this type  you need to  include  lt functional gt   std  function is a template  taking as template argument the return type and the argument types  with the syntax std  function lt ReturnType ArgumentTypes  gt   For example  std  function lt int std  string  float  gt  is a lambda returning an int and taking two arguments  one std  string and one float  The most common one is std  function lt void   gt   which returns nothing and takes no arguments  Once a lambda is declared  it is called just like a normal function  using the syntax lambda arguments   To define a lambda  use the syntax  captures  arguments  code   there are other ways of doing it  but I won t mention them here   arguments is what arguments the lambda takes  and code is the code that should be run when the lambda is called  Usually you put     or   amp   as captures      means that you capture all variables in the scope in which the value is defined by value  which means that they will keep the value that they had when the lambda was declared    amp   means that you capture all variables in the scope by reference  which means that they will always have their current value  but if they are erased from memory the program will crash  Here are some examples   include  lt functional gt   include  lt iostream gt   int main        int x   1       std  function lt void   gt  lambda1                  std  cout  lt  lt  x  lt  lt  std  endl             std  function lt void   gt  lambda2     amp              std  cout  lt  lt  x  lt  lt  std  endl              x   2      lambda1         Prints 1 since that was the value of x when it was captured and x was captured by value with         lambda2         Prints 2 since that s the current value of x and x was captured by reference with   amp        std  function lt void   gt  lambda3           lambda4                I prefer to initialize these since calling an uninitialized lambda is undefined behavior                                                                            is the empty lambda                 int y   3       y will be deleted from the memory at the end of this scope         lambda3                      std  cout  lt  lt  y  lt  lt  endl                     lambda4     amp                  std  cout  lt  lt  y  lt  lt  endl                        lambda3         Prints 3  since that s the value y had when it was captured      lambda4         Causes the program to crash  since y was captured by reference and y doesn t exist anymore                      This is a bit like if you had a pointer to y which now points nowhere because y has been deleted from the memory                      This is why you should be careful when capturing by reference       return 0     You can also capture specific variables by specifying their names  Just specifying their name will capture them by value  specifying their name with a  amp  before will capture them by reference  For example       amp foo  will capture all variables by value except foo which will be captured by reference  and   amp   foo  will capture all variables by reference except foo which will be captured by value  You can also capture only specific variables  for example   amp foo  will capture foo by reference and will capture no other variables  You can also capture no variables at all by using     If you try to use a variable in a lambda that you didn t capture  it won t compile  Here is an example   include  lt functional gt   int main        int x   4  y   5       std  function lt void int  gt  myLambda    y  int z           int xSquare   x   x       Compiler error because x wasn t captured         int ySquare   y   y       OK because y was captured         int zSquare   z   z       OK because z is an argument of the lambda             return 0     You can t change the value of a variable that was captured by value inside a lambda  variables captured by value have a const type inside the lambda   To do so  you need to capture the variable by reference  Here is an exampmle   include  lt functional gt   int main        int x   3  y   5      std  function lt void   gt  myLambda    x   amp y             x   2       Compiler error because x is captured by value and so it s of type const int inside the lambda         y   2       OK because y is captured by reference            x   2       This is of course OK because we re not inside the lambda     return 0     Also  calling uninitialized lambdas is undefined behavior and will usually cause the program to crash  For example  never do this  std  function lt void   gt  lambda  lambda         Undefined behavior because lambda is uninitialized   Examples Here is the code for what you wanted to do in your question using lambdas   include  lt functional gt       Don t forget this  otherwise you won t be able to use the std  function type  int main        std  function lt void   gt  a                    code           a        return 0     Here is a more advanced example of a lambda   include  lt functional gt       For std  function  include  lt iostream gt         For std  cout  int main        int x   4      std  function lt float int  gt  divideByX    x  int y           return  float y    float x       x is a captured variable  y is an argument           std  cout  lt  lt  divideByX 3   lt  lt  std  endl       Prints 0 75     return 0

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You cannot define a free function inside another in C

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No   What are you trying to do    workaround   int main void      struct foo         void operator       int a   1            foo b    b       call the operator

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Local classes have already been mentioned  but here is a way to let them appear even more as local functions  using an operator   overload and an anonymous class  int main         struct           unsigned int operator    unsigned int val  const               return val lt  1   1   val   this  val-1                   fac       std  cout  lt  lt  fac 5   lt  lt    n      I don t advise on using this  it s just a funny trick  can do  but imho shouldn t    2014 Update  With the rise of C  11 a while back  you can now have local functions whose syntax is a little reminiscient of JavaScript  auto fac       unsigned int val        return val 42

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As others have mentioned  you can use nested functions by using the gnu language extensions in gcc  If you  or your project  sticks to the gcc toolchain  your code will be mostly portable across the different architectures targeted by the gcc compiler   However  if there is a possible requirement that you might need to compile code with a different toolchain  then I d stay away from such extensions     I d also tread with care when using nested functions  They are a beautiful solution for managing the structure of complex  yet cohesive blocks of code  the pieces of which are not meant for external general use   They are also very helpful in controlling namespace pollution  a very real concern with naturally complex long classes in verbose languages    But like anything  they can be open to abuse    It is sad that C C   does not support such features as an standard  Most pascal variants and Ada do  almost all Algol-based languages do   Same with JavaScript  Same with modern languages like Scala  Same with venerable languages like Erlang  Lisp or Python    And just as with C C    unfortunately  Java  with which I earn most of my living  does not   I mention Java here because I see several posters suggesting usage of classes and class  methods as alternatives to nested functions  And that s also the typical workaround in Java   Short answer  No   Doing so tend to introduce artificial  needless complexity on a class hierarchy  With all things being equal  the ideal is to have a class hierarchy  and its encompassing namespaces and scopes  representing an actual domain as simple as possible   Nested functions help deal with  private   within-function complexity  Lacking those facilities  one should try to avoid propagating that  private  complexity out and into one s class model   In software  and in any engineering discipline   modeling is a matter of trade-offs  Thus  in real life  there will be justified exceptions to those rules  or rather guidelines   Proceed with care  though

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Let me post a solution here for C  03 that I consider the cleanest possible     define DECLARE LAMBDA NAME  RETURN TYPE  FUNCTION        struct   RETURN TYPE operator    FUNCTION   NAME        int main      DECLARE LAMBDA demoLambda  void      cout lt  lt  I m a lambda   lt  lt endl        demoLambda       DECLARE LAMBDA plus  int   int i  int j       return i j          cout  lt  lt   plus 1 2     lt  lt  plus 1 2   lt  lt  endl    return 0          in the C   world using macros is never considered clean

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No  it s not allowed  Neither C nor C   support this feature by default  however TonyK points out  in the comments  that there are extensions to the GNU C compiler that enable this behavior in C

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Starting with C   11 you can use proper lambdas  See the other answers for more details     Old answer  You can  sort-of  but you have to cheat and use a dummy class   void moo         class dummy           public           static void a     printf  I m in a  n                  dummy  a        dummy  a

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All this tricks just look  more or less  as local functions  but they don t work like that  In a local function you can use local variables of it s super functions  It s kind of semi-globals  Non of these tricks can do that  The closest is the lambda trick from c  0x  but it s closure is bound in definition time  not the use time

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But we can declare a function inside main     int main         void a        Although the syntax is correct  sometimes it can lead to the  Most vexing parse     include  lt iostream gt    struct U       U     val 0         U int val    val val          int val      struct V       V U a  U b                std  cout  lt  lt   V    lt  lt  a val  lt  lt        lt  lt  b val  lt  lt      n              V                 std  cout  lt  lt    V    n             int main         int five   5      V v U five   U            no program output    Only Clang warning after compilation    C    39 s most vexing parse again

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Modern C   - Yes with lambdas   In current versions of c    C  11  C  14  and C  17   you can have functions inside functions in the form of a lambda   int main            This declares a lambda  which can be called just like a function     auto print message      std  string message                  std  cout  lt  lt  message  lt  lt    n                   Prints  Hello   10 times     for int i   0  i  lt  10  i              print message  Hello                Lambdas can also modify local variables through   capture-by-reference   With capture-by-reference  the lambda has access to all local variables declared in the lambda s scope  It can modify and change them normally    int main         int i   0         Captures i by reference  increments it by one     auto addOne     amp                i                 while i  lt  10            addOne      Add 1 to i         std  cout  lt  lt  i  lt  lt    n             C  98 and C  03 - Not directly  but yes with static functions inside local classes  C   doesn t support that directly    That said  you can have local classes  and they can have functions  non-static or static   so you can get this to some extend  albeit it s a bit of a kludge    int main      it s int  dammit      struct X      struct s as good as class     static void a                       X  a       return 0      However  I d question the praxis  Everyone knows  well  now that you do  anyway     C   doesn t support local functions  so they are used to not having them  They are not used  however  to that kludge  I would spend quite a while on this code to make sure it s really only there to allow local functions  Not good

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