What is a lambda expression in C 11

Question

What is a lambda expression in C  11  When would I use one  What class of problem do they solve that wasn t possible prior to their introduction   A few examples  and use cases would be useful

User · Answer

The lambda s in c   are treated as  on the go available function   yes its literally on the go  you define it  use it  and as the parent function scope finishes the lambda function is gone   c   introduced it in c   11 and everyone started using it like at every possible place  the example and what is lambda can be find here https   en cppreference com w cpp language lambda  i will describe which is not there but essential to know for every c   programmer   Lambda is not meant to use everywhere and every function cannot be replaced with lambda  It s also not the fastest one compare to normal function  because it has some overhead which need to be handled by lambda   it will surely help in reducing number of lines in some cases  it can be basically used for the section of code  which is getting called in same function one or more time and that piece of code is not needed anywhere else so that you can create standalone function for it   Below is the basic example of lambda and what happens in background   User code   int main          Lambda  amp  auto   int member 10    auto endGame       int a  int b   return a b member       endGame 4 5      return 0       How compile expands it   int main       int member   10     class   lambda 6 18         int member      public       inline   constexpr    int operator   int a  int b  const             return a   b   member             public    lambda 6 18 int  member        member  member                    lambda 6 18 endGame     lambda 6 18 member     endGame operator   4  5      return 0      so as you can see  what kind of overhead it adds when you use it  so its not good idea to use them everywhere  it can be used at places where they are applicable

User · Answer

Answers  Q  What is a lambda expression in C  11   A  Under the hood  it is the object of an autogenerated class with overloading operator   const  Such object is called closure and created by compiler  This  closure  concept is near with the bind concept from C  11   But lambdas typically generate better code  And calls through closures allow full inlining   Q  When would I use one   A  To define  simple and small logic  and ask compiler perform generation from previous question  You give a compiler some expressions which you want to be inside operator    All other stuff compiler will generate to you   Q  What class of problem do they solve that wasn t possible prior to their introduction   A  It is some kind of syntax sugar like operators overloading instead of functions for custom add  subrtact operations   But it save more lines of unneeded code to wrap 1-3 lines of real logic to some classes  and etc   Some engineers think that if the number of lines is smaller then there is a less chance to make errors in it  I m also think so   Example of usage  auto x       int arg1  printf   i   arg1      void  f  int    x  f 1   x 1       Extras about lambdas  not covered by question  Ignore this section if you re not interest  1  Captured values  What you can to capture  1 1  You can reference to a variable with static storage duration in lambdas  They all are captured   1 2  You can use lambda for capture values  by value   In such case captured vars will be copied to the function object  closure     captureVar1 captureVar2  int arg1      1 3  You can capture be reference   amp  -- in this context mean reference  not pointers        amp captureVar1  amp captureVar2  int arg1      1 4  It exists notation to capture all non-static vars by value  or by reference        int arg1       capture all not-static vars by value      amp   int arg1       capture all not-static vars by reference   1 5   It exists notation to capture all non-static vars by value  or by reference and specify smth  more  Examples  Capture all not-static vars by value  but by reference capture Param2      amp Param2  int arg1       Capture all not-static vars by reference  but by value capture Param2    amp  Param2  int arg1       2  Return type deduction  2 1  Lambda return type can be deduced if lambda is one expression  Or you can explicitly specify it       int arg1 - gt trailing return type return trailing return type       If lambda has more then one expression  then return type must be specified via trailing return type     Also  similar syntax can be applied to auto functions and member-functions  3  Captured values  What you can not capture  3 1  You can capture only local vars  not member variable of the object   4   onversions  4 1    Lambda is not a function pointer and it is not an anonymous function  but capture-less lambdas can be implicitly converted to a function pointer   p s     More about lambda grammar information can be found in Working draft for Programming Language C    337  2012-01-16  5 1 2  Lambda Expressions  p 88 In C  14 the extra feature which has named as  init capture  have been added  It allow to perform arbitarily declaration of closure data members   auto toFloat      int value    return float value     auto interpolate    min   toFloat 0   max   toFloat 255   int value - gt float   return  value - min     max - min

User · Answer

One problem it solves  Code simpler than lambda for a call in constructor that uses an output parameter function for initializing a const member  You can initialize a const member of your class  with a call to a function that sets its value by giving back its output as an output parameter

User · Answer

What is a lambda function   The C   concept of a lambda function originates in the lambda calculus and functional programming  A lambda is an unnamed function that is useful  in actual programming  not theory  for short snippets of code that are impossible to reuse and are not worth naming   In C   a lambda function is defined like this              barebone lambda   or in all its glory       mutable - gt  T        T is the return type  still lacking throw        is the capture list     the argument list and    the function body   The capture list  The capture list defines what from the outside of the lambda should be available inside the function body and how  It can be either    a value   x  a reference   amp x  any variable currently in scope by reference   amp   same as 3  but by value       You can mix any of the above in a comma separated list  x   amp y    The argument list  The argument list is the same as in any other C   function    The function body  The code that will be executed when the lambda is actually called   Return type deduction  If a lambda has only one return statement  the return type can be omitted and has the implicit type of decltype return statement    Mutable  If a lambda is marked mutable  e g       mutable      it is allowed to mutate the values that have been captured by value   Use cases  The library defined by the ISO standard benefits heavily from lambdas and raises the usability several bars as now users don t have to clutter their code with small functors in some accessible scope   C  14  In C  14 lambdas have been extended by various proposals   Initialized Lambda Captures  An element of the capture list can now be initialized with    This allows renaming of variables and to capture by moving  An example taken from the standard   int x   4  auto y     amp r   x  x   x 1   - gt int               r    2              return x 2                    Updates   x to 6  and initializes y to 7    and one taken from Wikipedia showing how to capture with std  move   auto ptr   std  make unique lt int gt  10      See below for std  make unique auto lambda    ptr   std  move ptr    return  ptr      Generic Lambdas  Lambdas can now be generic  auto would be equivalent to T here if T were a type template argument somewhere in the surrounding scope    auto lambda      auto x  auto y   return x   y      Improved Return Type Deduction  C  14 allows deduced return types for every function and does not restrict it to functions of the form return expression   This is also extended to lambdas

User · Answer

Lambda expressions are typically used to encapsulate algorithms so that they can be passed to another function   However  it is possible to execute a lambda immediately upon definition     amp         your code            immediately executed lambda expression   is functionally equivalent to       your code         simple code block   This makes lambda expressions a powerful tool for refactoring complex functions   You start by wrapping a code section in a lambda function as shown above   The process of explicit parameterization can then be performed gradually with intermediate testing after each step   Once you have the code-block fully parameterized  as demonstrated by the removal of the  amp    you can move the code to an external location and make it a normal function   Similarly  you can use lambda expressions to initialize variables based on the result of an algorithm     int a       int b    int r 1  while  b gt 0  r  b--  return r    5      5    As a way of partitioning your program logic  you might even find it useful to pass a lambda expression as an argument to another lambda expression       amp    std  function lt void   gt  algorithm      wrapper section            your wrapper code       algorithm          your wrapper code            amp        algorithm section            your algorithm code             Lambda expressions also let you create named nested functions  which can be a convenient way of avoiding duplicate logic   Using named lambdas also tends to be a little easier on the eyes  compared to anonymous inline lambdas  when passing a non-trivial function as a parameter to another function   Note  don t forget the semicolon after the closing curly brace   auto algorithm     amp    double x  double m  double b   - gt  double         return m x b         int a algorithm 1 2 3   b algorithm 4 5 6     If subsequent profiling reveals significant initialization overhead for the function object  you might choose to rewrite this as a normal function

User · Answer

Well  one practical use I ve found out is reducing boiler plate code  For example   void process z vec vector lt int gt  amp  vec      auto print 2d      const vector lt int gt  amp  board  int bsize          for int i   0  i lt bsize  i                for int j 0  j lt bsize  j                    cout  lt  lt  board bsize i j   lt  lt                     cout  lt  lt    n                   Do sth with the vec    print 2d vec x size        Do sth else with the vec    print 2d vec y size                Without lambda  you may need to do something for different bsize cases  Of course you could create a function but what if you want to limit the usage within the scope of the soul user function  the nature of lambda fulfills this requirement and I use it for that case

User · Answer

A lambda function is an anonymous function that you create in-line  It can capture variables as some have explained   e g  http   www stroustrup com C  11FAQ html lambda  but there are some limitations  For example  if there s a callback interface like this    void apply void   f  int         f 10       f 20       f 30       you can write a function on the spot to use it like the one passed to apply below   int col 0  void output         apply    int data            cout  lt  lt  data  lt  lt      col   10            n                But you can t do this   void output int n        int col 0      apply   amp col n  int data            cout  lt  lt  data  lt  lt      col   10            n                because of limitations in the C  11 standard  If you want to use captures  you have to rely on the library and    include  lt functional gt      or some other STL library like algorithm to get it indirectly  and then work with std  function instead of passing normal functions as parameters like this    include  lt functional gt  void apply std  function lt void int  gt  f        f 10       f 20       f 30     void output int width        int col      apply  width  amp col  int data            cout  lt  lt  data  lt  lt      col   width            n

User · Answer

One of the best explanation of lambda expression is given from author of C   Bjarne Stroustrup in his book    The C   Programming Language    chapter 11  ISBN-13  978-0321563842    What is a lambda expression       A lambda expression  sometimes also referred to as a lambda   function or  strictly speaking incorrectly  but colloquially  as a   lambda  is a simplified notation for defining and using an anonymous function object  Instead of defining a named class with an operator    later making an object of that class  and finally   invoking it  we can use a shorthand    When would I use one      This is particularly useful when we want to pass an operation as an   argument to an algorithm  In the context of graphical user interfaces    and elsewhere   such operations are often referred to as callbacks    What class of problem do they solve that wasn t possible prior to their introduction   Here i guess every action done with lambda expression can be solved without them  but with much more code and much bigger complexity  Lambda expression this is the way of optimization for your code and a way of making it more attractive  As sad by Stroustup       effective ways of optimizing   Some examples  via lambda expression  void print modulo const vector lt int gt  amp  v  ostream amp  os  int m     output v i  to os if v i  m  0       for each begin v  end v             amp os m  int x                if  x m  0  os  lt  lt  x  lt  lt    n                    or via function  class Modulo print            ostream amp  os     members to hold the capture list int m       public           Modulo print ostream amp  s  int mm   os s   m mm               void operator   int x  const                            if  x m  0  os  lt  lt  x  lt  lt    n                      or even  void print modulo const vector lt int gt  amp  v  ostream amp  os  int m           output v i  to os if v i  m  0       class Modulo print           ostream amp  os     members to hold the capture list         int m           public             Modulo print  ostream amp  s  int mm   os s   m mm                void operator   int x  const                              if  x m  0  os  lt  lt  x  lt  lt    n                             for each begin v  end v  Modulo print os m         if u need u can name lambda expression like below   void print modulo const vector lt int gt  amp  v  ostream amp  os  int m         output v i  to os if v i  m  0         auto Modulo print     amp os m   int x    if  x m  0  os  lt  lt  x  lt  lt    n            for each begin v  end v  Modulo print        Or assume another simple sample  void TestFunctions  simpleLambda         bool sensitive   true      std  vector lt int gt  v   std  vector lt int gt   1 33 3 4 5 6 7         sort v begin   v end              sensitive  int x  int y                 printf   n i n    x  lt  y                return sensitive   x  lt  y   abs x   lt  abs y                      printf  sorted        for each v begin    v end                    int x                     printf  x -  i    x                                      will generate next     0      1      0      1      0      1      0      1      0      1      0 sortedx - 1 x - 3 x - 4 x - 5 x - 6 x - 7 x - 33       - this is capture list or lambda introducer  if lambdas require no access to their local environment we can use it   Quote from book      The first character of a lambda expression is always    A lambda   introducer can take various forms               an empty capture list  This   implies that no local names from the surrounding context can be used   in the lambda body  For such lambda expressions  data is obtained from   arguments or from nonlocal variables              amp    implicitly capture by   reference  All local names can be used  All local variables are   accessed by reference                implicitly capture by value  All local   names can be used  All names refer to copies of the local variables   taken at the point of call of the lambda expression            capture-list    explicit capture  the capture-list is the list of names of local  variables to be captured  i e   stored in the object  by reference or by value  Variables with names preceded by  amp  are captured by   reference  Other variables are captured by value  A capture list can   also contain this and names followed by     as elements              amp    capture-list   implicitly capture by reference all local variables  with names not men- tioned in the list  The capture list can contain  this  Listed names cannot be preceded by  amp   Variables named in the   capture list are captured by value                capture-list   implicitly capture by value all local variables with names not mentioned in the  list  The capture list cannot contain this  The listed names must be preceded by  amp   Vari- ables named in the capture list are captured by  reference        Note that a local name preceded by  amp  is always captured by   reference and a local name not pre- ceded by  amp  is always captured by   value  Only capture by reference allows modification of variables in   the calling environment    Additional  Lambda expression format    Additional references    Wiki open-std org  chapter 5 1 2

User · Answer

The problem  C   includes useful generic functions like std  for each and std  transform  which can be very handy  Unfortunately they can also be quite cumbersome to use  particularly if the functor you would like to apply is unique to the particular function    include  lt algorithm gt   include  lt vector gt   namespace     struct f       void operator   int             do something               void func std  vector lt int gt  amp  v      f f    std  for each v begin    v end    f       If you only use f once and in that specific place it seems overkill to be writing a whole class just to do something trivial and one off   In C  03 you might be tempted to write something like the following  to keep the functor local   void func2 std  vector lt int gt  amp  v      struct       void operator   int              do something           f    std  for each v begin    v end    f       however this is not allowed  f cannot be passed to a template function in C  03   The new solution  C  11 introduces lambdas allow you to write an inline  anonymous functor to replace the struct f  For small simple examples this can be cleaner to read  it keeps everything in one place  and potentially simpler to maintain  for example in the simplest form   void func3 std  vector lt int gt  amp  v      std  for each v begin    v end       int       do something here           Lambda functions are just syntactic sugar for anonymous functors   Return types  In simple cases the return type of the lambda is deduced for you  e g    void func4 std  vector lt double gt  amp  v      std  transform v begin    v end    v begin                        double d    return d  lt  0 00001   0   d                            however when you start to write more complex lambdas you will quickly encounter cases where the return type cannot be deduced by the compiler  e g    void func4 std  vector lt double gt  amp  v        std  transform v begin    v end    v begin               double d                if  d  lt  0 0001                    return 0                else                   return d                                To resolve this you are allowed to explicitly specify a return type for a lambda function  using - gt  T   void func4 std  vector lt double gt  amp  v        std  transform v begin    v end    v begin               double d  - gt  double               if  d  lt  0 0001                    return 0                else                   return d                                 Capturing  variables  So far we ve not used anything other than what was passed to the lambda within it  but we can also use other variables  within the lambda  If you want to access other variables you can use the capture clause  the    of the expression   which has so far been unused in these examples  e g    void func5 std  vector lt double gt  amp  v  const double amp  epsilon        std  transform v begin    v end    v begin             epsilon  double d  - gt  double               if  d  lt  epsilon                    return 0                else                   return d                                You can capture by both reference and value  which you can specify using  amp  and   respectively      amp epsilon  capture by reference   amp   captures all variables used in the lambda by reference     captures all variables used in the lambda by value   amp   epsilon  captures variables like with   amp    but epsilon by value      amp epsilon  captures variables like with      but epsilon by reference   The generated operator   is const by default  with the implication that captures will be const when you access them by default  This has the effect that each call with the same input would produce the same result  however you can mark the lambda as mutable to request that the operator   that is produced is not const

[c++] What is a lambda expression in C++11?

Examples related to c++

Examples related to lambda

Examples related to c++11

Examples related to c++-faq