see #1: concepts demo

code/10/10.2.concepts.cpp

@@ -0,0 +1,66 @@
+#include <iostream>
+#include <string>
+#include <vector>
+#include <list>
+
+using namespace std;
+
+ template<typename T>
+concept bool Stringable = requires(T a){
+    {a.to_string()} -> string;
+};
+
+ template<typename T>
+concept bool HasStringFunc = requires(T a){
+    { to_string(a) } -> string;
+};
+
+struct Person {
+    double height, weight;
+    Person(double a, double b) : height(a), weight(b) {}
+    string to_string(){
+        return "weight: "+ std::to_string(weight) + ", height: "+ std::to_string(height);
+	}
+};
+
+
+namespace std {
+
+string to_string(list<int> l){
+    string s = "";
+    for(int a : l ){
+        s+=(" " + to_string(a) + " ");
+    }
+    return s;
+}
+
+}
+
+string to_string(std::vector<int> v){
+    string s = "";
+    for(int a : v ){
+        s += (" " +  to_string(a) + " ");
+    }
+    return s;
+}
+
+
+ void print(Stringable a){
+    std::cout << a.to_string() << std::endl;
+}
+
+ void print(HasStringFunc a){
+    std::cout << to_string(a) << std::endl;
+}
+
+ int main() {
+    std::list<int> l {1, 2, 3};
+    Person p(57, 170.0);
+    std::vector<int> v { 34, 23, 34, 56, 78};
+
+    print(p); // uses concept Stringable
+    print(l);
+    print(3); // uses concept HasStringFunc
+    // print(v); // error
+    return 0;
+}