Can lambda functions be templated

Question

In C  11  is there a way to template a lambda function  Or is it inherently too specific to be templated   I understand that I can define a classic templated class functor instead  but the question is more like  does the language allow templating lambda functions

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In C  20 this is possible using the following syntax  auto lambda      lt typename T gt  T t          do something

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I am aware that this question is about C  11  However  for those who googled and landed on this page  templated lambdas are now supported in C  14 and go by the name Generic Lambdas    info  Most of the popular compilers support this feature now  Microsoft Visual Studio 2015 supports  Clang supports  GCC supports

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Here is one solution that involves wrapping the lamba in a structure   template  lt typename T gt                                                     struct LamT                                                                                                                                          static void Go                                                                                                                                       auto lam                                                                                                                                             T var                                                                    std  cout  lt  lt   lam  type      lt  lt  typeid var  name    lt  lt  std  endl                                                                                  lam                                                                                                                                                 To use do   LamT lt int gt   Go      LamT lt char gt   Go      This prints  lam  type   i lam  type   c   The main issue with this  besides the extra typing  you cannot embed this structure definition inside another method or you get  gcc 4 9   error  a template declaration cannot appear at block scope   I also tried doing this   template  lt typename T gt  using LamdaT   decltype                                  void                                                                                                                                         std  cout  lt  lt   LambT type      lt  lt  typeid T  name    lt  lt  std  endl             With the hope that I could use it like this   LamdaT lt int gt           LamdaT lt char gt       But I get the compiler error   error  lambda-expression in unevaluated context   So this doesn t work     but even if it did compile it would be of limited use because we would still have to put the  using LamdaT  at file scope  because it is a template  which sort of defeats the purpose of lambdas

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I ve been playing with the latest clang version 5 0 1 compiling with the -std c  17 flag and there is now some nice support for auto type parameters for lambdas    include  lt iostream gt   include  lt vector gt   include  lt stdexcept gt   int main         auto slice      auto input  int beg  int end            using T   decltype input           const auto size   input size            if  beg  gt  size    end  gt  size    beg  lt  0    end  lt  0                throw std  out of range  beg end must be between  0  input size                         if  beg  gt  end                throw std  invalid argument  beg must be less than end                      return T input begin     beg  input begin     end              auto v   std  vector lt int gt    1 2 3 4 5        for  auto e   slice v  1  4             std  cout  lt  lt  e  lt  lt                 std  cout  lt  lt  std  endl

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C  11 lambdas can t be templated as stated in other answers but decltype   seems to help when using a lambda within a templated class or function      include  lt iostream gt   include  lt string gt   using namespace std   template lt typename T gt  void boring template fn T t       auto identity      decltype t  t   return t        std  cout  lt  lt  identity t   lt  lt  std  endl     int main int argc  char  argv          std  string s  My string        boring template fn s       boring template fn 1024       boring template fn true       Prints   My string 1024 1   I ve found this technique is helps when working with templated code but realize it still means lambdas themselves can t be templated

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There is a gcc extension which allows lambda templates      create the widgets and set the label base  for each  widgets      lt typename Key T  typename Widget T gt                            boost  fusion  pair lt Key T  Widget T  gt  amp  pair  - gt  void                                pair second   new Widget T                                 pair second- gt set label str Key T  label                                                  where  widgets is a std  tuple lt  fusion  pair lt Key T  Widget T gt      gt

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I m not sure why nobody else has suggested this  but you can write a templated function that returns lambda functions  The following solved my problem  the reason I came to this page   template  lt typename DATUM gt  std  function lt double DATUM  gt  makeUnweighted       return    DATUM datum  return 1 0        Now whenever I want a function that takes a given type of argument  e g  std  string   I just say  auto f   makeUnweighted lt std  string gt      and now f  any string   returns 1 0   That s an example of what I mean by  templated lambda function    This particular case is used to automatically provide an inert weighting function when somebody doesn t want to weight their data  whatever their data might be

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I wonder what about this   template  lt class something gt  inline std  function lt void   gt  templateLamda       return       std  cout  lt  lt  something memberfunc          I used similar code like this  to generate a template and wonder if the compiler will optimize the  wrapping  function out

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In C  11  lambda functions can not be templated  but in the next version of the ISO C   Standard  often called C  14   this feature will be introduced   Source   Usage example   auto get container size       auto container    return container size         Note that though the syntax uses the keyword auto  the type deduction will not use the rules of auto type deduction  but instead use the rules of template argument deduction  Also see the proposal for generic lambda expressions and the update to this

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Have a look at Boost Phoenix for polymorphic lambdas  http   www boost org doc libs 1 44 0 libs spirit phoenix doc html index html Does not require C  0x  by the way

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UPDATE 2018  C  20 will come with templated and conceptualized lambdas  The feature has already been integrated into the standard draft     UPDATE 2014  C  14 has been released this year and now provides Polymorphic lambdas with the same syntax as in this example  Some major compilers already implement it     At it stands  in C  11   sadly no  Polymorphic lambdas would be excellent in terms of flexibility and power   The original reason they ended up being monomorphic was because of concepts  Concepts made this code situation difficult   template  lt Constraint T gt  void foo T x        auto bar      auto x        imaginary syntax     In a constrained template you can only call other constrained templates   Otherwise the constraints couldn t be checked   Can foo invoke bar x   What constraints does the lambda have  the parameter for it is just a template  after all    Concepts weren t ready to tackle this sort of thing  it d require more stuff like late check  where the concept wasn t checked until invoked  and stuff  Simpler was just to drop it all and stick to monomorphic lambdas   However  with the removal of concepts from C  0x  polymorphic lambdas become a simple proposition again  However  I can t find any proposals for it

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