Recursive lambda functions in C++11

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I am new to C++11. I am writing the following recursive lambda function, but it doesn’t compile.

sum.cpp

#include <iostream>
#include <functional>

auto term = [](int a)->int {
  return a*a;
};

auto next = [](int a)->int {
  return ++a;
};

auto sum = [term,next,&sum](int a, int b)mutable ->int {
  if(a>b)
    return 0;
  else
    return term(a) + sum(next(a),b);
};

int main(){
  std::cout<<sum(1,10)<<std::endl;
  return 0;
}

compilation error:

vimal@linux-718q:~/Study/09C++/c++0x/lambda> g++ -std=c++0x sum.cpp

sum.cpp: In lambda function:
sum.cpp:18:36: error: ‘((<lambda(int, int)>*)this)-><lambda(int, int)>::sum’ cannot be used as a function

gcc version

gcc version 4.5.0 20091231 (experimental) (GCC)

But if I change the declaration of sum() as below, it works:

std::function<int(int,int)> sum = [term,next,&sum](int a, int b)->int {
   if(a>b)
     return 0;
   else
     return term(a) + sum(next(a),b);
};

Could someone please throw light on this?

14 Answers
14

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