fork(2) download
copy 
  1. #include <type_traits>
  2. #include <iostream>
  3.  
  4. // interface class
  5. class A
  6. {
  7. public:
  8.     virtual void foo() = 0;
  9.     virtual ~A() {}
  10. };
  11.  
  12. namespace detail
  13. {
  14.     // an helper for traits
  15.     template <typename T>
  16.     decltype(T{}, std::cout << T{}, std::true_type{})
  17.     helper_has_default_constructor_and_foo(int);
  18.  
  19.     template <typename T>
  20.     std::false_type helper_has_default_constructor_and_foo(...);
  21.  
  22.     // the traits
  23.     template <typename T>
  24.     using has_default_constructor_and_foo = decltype(helper_has_default_constructor_and_foo<T>(0));
  25.  
  26.     // genaral case (so when traits is false)
  27.     template <typename T, typename = has_default_constructor_and_foo<T>>
  28.     struct C : public A {};
  29.  
  30.     // specialization when traits is true
  31.     template <typename T>
  32.     struct C<T, std::true_type> : public A
  33.     {
  34.         void foo() override { std::cout << T{}; }
  35.     };
  36.  
  37. }
  38.  
  39. template <typename T>
  40. class B : public detail::C<T>
  41. {
  42. };
  43.  
  44. // concrete classes
  45. // will work on it's own
  46. typedef B<std::string> C;
  47.  
  48. class D : public B<void>
  49. {
  50. public:
  51.     // B<void>::foo won't instantiate on it's own
  52.     // so we provide help here
  53.     void foo() override {}
  54. };
  55.  
  56. int main()
  57. {
  58.     //B<void> b; // error as expected
  59.     B<int> b;
  60.     C c;
  61.     D d;
  62.  
  63.     b.foo(); // 0
  64.     c.foo(); // empty string
  65.     d.foo(); // nothing
  66. }
  67.  
Success #stdin #stdout 0s 3140KB
comments (?)
stdin
copy 
Standard input is empty
stdout
copy 
Standard output is empty
https://ideone.com/KkU0Tw
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
public

Share or Embed source code

Discover > Sphere Engine API

The brand new service which powers Ideone!

Discover > IDE Widget

Widget for compiling and running the source code in a web browser!