revision #1: 更新第二章中已维护的代码

exercises/2/fold.expresion.cpp

@@ -0,0 +1,8 @@
+#include <iostream>
+template<typename ... T>
+auto average(T ... t) {
+    return (t + ... ) / sizeof...(t);
+}
+int main() {
+    std::cout << average(1, 2, 3, 4, 5, 6, 7, 8, 9, 10) << std::endl;
+}

exercises/2/structured.binding.cpp

@@ -0,0 +1,22 @@
+#include <iostream>
+#include <map>
+#include <string>
+#include <functional>
+
+template <typename Key, typename Value, typename F>
+void update(std::map<Key, Value>& m, F foo) {
+    for (auto&& [key, value] : m ) value = foo(key);
+}
+
+int main() {
+    std::map<std::string, long long int> m {
+        {"a", 1},
+        {"b", 2},
+        {"c", 3}
+    };
+    update(m, [](std::string key) -> long long int {
+        return std::hash<std::string>{}(key);
+    });
+    for (auto&& [key, value] : m)
+        std::cout << key << ":" << value << std::endl;
+}