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  1. #include <iostream>
  2. using namespace std;
  3.  
  4. template<typename Fn>
  5. void call_value(Fn f) { f(); }
  6. template<typename Fn>
  7. void call_ref(Fn & f) { f(); }
  8. template<typename Fn>
  9. void call_cref(Fn const & f) { f(); }
  10.  
  11. struct Data {
  12.   Data() {}
  13.   Data(Data const &) {
  14.     cout << "copy" << endl;
  15.   }
  16.   Data(Data &&) {
  17.     cout << "move" << endl;
  18.   }
  19. };
  20.  
  21. int main(int, char **) {
  22.   Data data;
  23.  
  24.   auto capref = [&data] () {};
  25.   cout << "capture by value, so we get a ";
  26.   auto capcp = [data] () {};
  27.  
  28.   cout << " the lambda with a reference ... ";
  29.   call_value(capref);
  30.   cout << " could now be called and mutate .. ";
  31.   call_ref(capref);
  32.   call_cref(capref);
  33.   cout << " but won't, as it had to be declared mutable " << endl;
  34.  
  35.   cout << "the lambda with an instance: ";
  36.   call_value(capcp);
  37.   cout << "but not ";
  38.   call_ref(capcp);
  39.   call_cref(capcp);
  40.   cout << " the reference versions " << endl;
  41.  
  42.   bool en = false;
  43.   auto trigger = [en](bool enable = true) mutable {
  44.     if (en) {
  45.       cout << "fire!" << endl;
  46.     }
  47.     if (en or enable) {
  48.       en = true;
  49.     }
  50.   };
  51.   cout << "won't shoot" << endl;
  52.   trigger(false);
  53.   call_value(trigger);
  54.   trigger(false);
  55.   call_ref(trigger);
  56.   cout << "and now ... ";
  57.   trigger(false);
  58.   // const ref won't work
  59.   return 0;
  60. }
Success #stdin #stdout 0s 3096KB
comments (?)
stdin
copy 
Standard input is empty
stdout
copy 
capture by value, so we get a copy
 the lambda with a reference ...  could now be called and mutate ..  but won't, as it had to be declared mutable 
the lambda with an instance: copy
but not  the reference versions 
won't shoot
and now ... fire!
https://ideone.com/9egDKI
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
public

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