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  1. #include <functional>
  2. #include <iostream>
  3. #include <string>
  4. #include <cassert>
  5.  
  6. // Call f with one argument
  7. template <class Fn, class Arg>
  8. auto call(const Fn &f, const Arg & arg) -> decltype(f(arg)) {
  9.     return f(arg);
  10. }
  11.  
  12. // Helper functor for the function below
  13. template<class Fn1, class Fn2>
  14. class CompFn {
  15.     Fn1 a;
  16.     Fn2 b;
  17.  
  18. public:
  19.     CompFn(const Fn1 &f1, const Fn2 &f2) : a(f1), b(f2) {}
  20.  
  21.     template<class Arg> inline
  22.     auto operator()(const Arg & arg) const -> decltype(call(b, call(a, arg))) {
  23.         return call(b, call(a, arg));
  24.     }
  25. };
  26.  
  27. /** Composition of f1 and f2 (f2 after f1). */
  28. template<class Fn1, class Fn2>
  29. CompFn<Fn1,Fn2> comp(const Fn1 &f1, const Fn2 &f2) {
  30.     return CompFn<Fn1,Fn2>(f1, f2);
  31. }
  32.  
  33.  
  34.  
  35. int main() {
  36.     // Example: Take the length of the string and compare it against zero.
  37.     std::function<int(std::string)> stringLength = [](std::string s) { return s.size(); };
  38.     std::function<bool(int)> greaterZero = [](int x) { return x > 0; };
  39.     auto stringNotEmpty = comp(stringLength, greaterZero);
  40.  
  41.     std::string testInput1 = "foo";
  42.     std::string testInput2 = "";
  43.  
  44.     assert(stringNotEmpty(testInput1) == true);
  45.     assert(stringNotEmpty(testInput2) == false);
  46. }
Success #stdin #stdout 0s 3028KB
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https://ideone.com/0URRy3
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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