book: translation of ch04

Update #12

code/4/4.3.tuples.cpp

@@ -0,0 +1,83 @@
+//
+// 4.3.tuples.cpp
+// chapter 04 containers
+// modern c++ tutorial
+//
+// created by changkun at changkun.de
+// https://github.com/changkun/modern-cpp-tutorial
+//
+
+
+#include <tuple>
+#include <iostream>
+#include <variant>
+
+auto get_student(int id)
+{
+    if (id == 0)
+        return std::make_tuple(3.8, 'A', "John");
+    if (id == 1)
+        return std::make_tuple(2.9, 'C', "Jack");
+    if (id == 2)
+        return std::make_tuple(1.7, 'D', "Ive");
+    // return type is std::tuple<double, char, std::string>
+    return std::make_tuple(0.0, 'D', "null");
+}
+
+template <size_t n, typename... T>
+constexpr std::variant<T...> _tuple_index(const std::tuple<T...>& tpl, size_t i) {
+    if constexpr (n >= sizeof...(T))
+        throw std::out_of_range("out of range.");
+    if (i == n)
+        return std::variant<T...>{ std::in_place_index<n>, std::get<n>(tpl) };
+    return _tuple_index<(n < sizeof...(T)-1 ? n+1 : 0)>(tpl, i);
+}
+template <typename... T>
+constexpr std::variant<T...> tuple_index(const std::tuple<T...>& tpl, size_t i) {
+    return _tuple_index<0>(tpl, i);
+}
+
+template <typename T>
+auto tuple_len(T &tpl) {
+    return std::tuple_size<T>::value;
+}
+
+template <typename T0, typename ... Ts>
+std::ostream & operator<< (std::ostream & s, std::variant<T0, Ts...> const & v) { 
+    std::visit([&](auto && x){ s << x;}, v); 
+    return s;
+}
+
+int main()
+{
+    auto student = get_student(0);
+    std::cout << "ID: 0, "
+              << "GPA: "   << std::get<0>(student) << ", "
+              << "Grade: " << std::get<1>(student) << ", "
+              << "Name: "  << std::get<2>(student) << '\n';
+    
+    double gpa;
+    char grade;
+    std::string name;
+    
+    // tuple unpack
+    std::tie(gpa, grade, name) = get_student(1);
+    std::cout << "ID: 1, "
+              << "GPA: "   << gpa   << ", "
+              << "Grade: " << grade << ", "
+              << "Name: "  << name  << '\n';
+    
+    
+    std::tuple<std::string, double, double, int> t("123", 4.5, 6.7, 8);
+    std::cout << std::get<std::string>(t) << std::endl;
+    // std::cout << std::get<double>(t) << std::endl;   // illegal, runtime error
+    std::cout << std::get<3>(t) << std::endl;
+    
+    // concat
+    auto new_tuple = std::tuple_cat(get_student(1), std::move(t));
+    
+    // iteration
+    for(int i = 0; i != tuple_len(new_tuple); ++i) {
+        std::cout << tuple_index(new_tuple, i) << std::endl; // runtime indexing
+    }
+}