Update 003_stl_std_hash_std_string.cpp

Author: ls-Brynn

cpp_11/003_stl_std_hash_std_string.cpp

@@ -1 +1,59 @@
-1
+#include <iostream>
+#include <iomanip>
+#include <functional>
+#include <string>
+#include <unordered_set>
+ 
+struct S {
+    std::string first_name;
+    std::string last_name;
+};
+bool operator==(const S& lhs, const S& rhs) {
+    return lhs.first_name == rhs.first_name && lhs.last_name == rhs.last_name;
+}
+ 
+// custom hash can be a standalone function object:
+struct MyHash
+{
+    std::size_t operator()(S const& s) const noexcept
+    {
+        std::size_t h1 = std::hash<std::string>{}(s.first_name);
+        std::size_t h2 = std::hash<std::string>{}(s.last_name);
+        return h1 ^ (h2 << 1); // or use boost::hash_combine (see Discussion)
+    }
+};
+ 
+// custom specialization of std::hash can be injected in namespace std
+namespace std
+{
+    template<> struct hash<S>
+    {
+        std::size_t operator()(S const& s) const noexcept
+        {
+            std::size_t h1 = std::hash<std::string>{}(s.first_name);
+            std::size_t h2 = std::hash<std::string>{}(s.last_name);
+            return h1 ^ (h2 << 1); // or use boost::hash_combine
+        }
+    };
+}
+ 
+int main()
+{
+    std::string str = "Meet the new boss...";
+    std::size_t str_hash = std::hash<std::string>{}(str);
+    std::cout << "hash(" << std::quoted(str) << ") = " << str_hash << '\n';
+ 
+    S obj = { "Hubert", "Farnsworth" };
+    // using the standalone function object
+    std::cout << "hash(" << std::quoted(obj.first_name) << ", "
+              << std::quoted(obj.last_name) << ") = "
+              << MyHash{}(obj) << " (using MyHash)\n" << std::setw(31) << "or "
+              << std::hash<S>{}(obj) << " (using injected std::hash<S> specialization)\n";
+ 
+    // custom hash makes it possible to use custom types in unordered containers
+    // The example will use the injected std::hash<S> specialization above,
+    // to use MyHash instead, pass it as a second template argument
+    std::unordered_set<S> names = {obj, {"Bender", "Rodriguez"}, {"Turanga", "Leela"} };
+    for(auto& s: names)
+        std::cout << std::quoted(s.first_name) << ' ' << std::quoted(s.last_name) << '\n';
+}