Homework-18

hw18/Exploreconcepts.cpp

+#include<iostream>
+#include<vector>
+
+using namespace std;
+
+template <class T>
+concept bool EqualityComparable() { 
+  return requires(T a, T b) {
+    {a == b} -> bool;
+    {a != b} -> bool;
+  };
+}
+
+void f(EqualityComparable x) {
+  cout << x << endl;
+}
+
+void g() {}
+
+template<typename T>
+  concept bool Integer = is_integral<T>::value;
+template<typename T>
+  concept bool Unsigned_integer = Integer<T> && is_unsigned<T>::value;
+
+template<typename T> class S { };
+
+template<Integer T> class S<T> { }; // #1
+template<Unsigned_integer T> class S<T> { }; // #2
+
+template<Integer T> void f(S<T>); // A
+template<Unsigned_integer T> void f(S<T>); // B
+
+int main() {
+  int i = 10;
+  f(i);
+  
+  char s[] = "apple";
+  f(s);
+  
+  f(g);
+  
+  return 0;
+}
+
+/*
+I tried some examples from the references on the slides and Wikipedia and ran them on wandbox.org.
+*/
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hw18/conceptSort.cpp

+#include<iostream>
+#include<array>
+#include<list>
+#include<iterator>
+#include<type_traits>
+
+using namespace std;
+/*
+template<typename T, enable_if_t<is_same_v<typename iterator_traits<T>::iterator_category, random_access_iterator_tag>> * = nullptr>
+void
+mySort(T beg, T end) {
+  cout << "Sort->random_access_iterator" << endl;
+}
+
+template<typename T, enable_if_t<is_same_v<typename iterator_traits<T>::iterator_category, forward_iterator_tag>> * = nullptr>
+void
+mySort(T beg, T end) {
+  cout << "Sort->forward_iterator" << endl;
+}
+
+template<typename T, enable_if_t<is_same_v<typename iterator_traits<T>::iterator_category, bidirectional_iterator_tag>> * = nullptr>
+void
+mySort(T beg, T end) {
+  cout << "Sort->bidirectional_iterator" << endl;
+}
+*/
+//method #1:
+
+template<typename T>
+concept bool ran = is_same_v<typename iterator_traits<T>::iterator_category, random_access_iterator_tag>;
+
+template<typename T>
+concept bool bi = is_same_v<typename iterator_traits<T>::iterator_category, bidirectional_iterator_tag>;
+
+void
+mySort(ran beg, ran end)  {
+  cout << "Sort->random_access_iterator" << endl;
+}
+
+void
+mySort(bi beg, bi end)  {
+  cout << "Sort->bidirectional_iterator" << endl;
+}
+
+/*
+//method #2:
+
+template<typename T>
+requires(is_same_v<typename iterator_traits<T>::iterator_category, random_access_iterator_tag>)
+void
+mySort(T beg, T end)  {
+  cout << "Sort->random_access_iterator" << endl;
+}
+
+template<typename T>
+requires(is_same_v<typename iterator_traits<T>::iterator_category, bidirectional_iterator_tag>)
+void
+mySort(T beg, T end)  {
+  cout << "Sort->bidirectional_iterator" << endl;
+}
+*/
+int main() {
+  array<int, 3> a = {3, 2, 1};
+  mySort(a.begin(), a.end());
+
+  list<int> b = {3, 2, 1};
+  mySort(b.begin(), b.end());
+
+  return 0;
+}
+/*
+I use gcc Head 9.0.0 201805 with "-fconcepts" on wandbox.org to run the above code since I cannot run it on my own MacBook.
+
+I like the concepts-based approach more than enable_if-based approach because it is more generic and also it is more clear.
+
+If I do not define the second concept bool bi, the error messages are following.
+
+Start
+prog.cc: In function 'int main()':
+prog.cc:67:28: error: cannot call function 'void mySort(auto:1, auto:1) [with auto:1 = std::_List_iterator<int>]'
+   mySort(b.begin(), b.end());
+                            ^
+prog.cc:36:1: note:   constraints not satisfied
+ mySort(ran beg, ran end)  {
+ ^~~~~~
+prog.cc:30:14: note: within 'template<class T> concept const bool ran<T> [with T = std::_List_iterator<int>]'
+ concept bool ran = is_same_v<typename iterator_traits<T>::iterator_category, random_access_iterator_tag>;
+              ^~~
+prog.cc:30:14: note: 'std::is_same_v' evaluated to false
+1
+Finish
+
+I think it is useful and clear for me.
+*/
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