try to give a solution of pezy#135

.gitignore

@@ -1,3 +1,6 @@
+#vscode
+.vs
+
 # Mac OS
 .DS_Store
 

ch16/ex16_41_sum.cpp

@@ -0,0 +1,92 @@
+#include <climits>
+#include <iostream>
+
+template <typename T> struct promote;
+
+template <> struct promote<short int> {
+    using type = int;
+};
+
+template <> struct promote<unsigned short int> {
+    using type = unsigned int;
+};
+
+template <> struct promote<int> {
+    using type = long long int;
+};
+
+template <> struct promote<unsigned int> {
+    using type = unsigned long int;
+};
+
+template <> struct promote<long int> {
+    using type = long long int;
+};
+
+template <> struct promote<unsigned long int> {
+    using type = unsigned long int;
+};
+
+template <> struct promote<long long int> {
+    using type = unsigned long long int;
+};
+
+template <> struct promote<unsigned long long int> {
+    using type = unsigned long long int;
+};
+
+template <> struct promote<float> {
+    using type = double;
+};
+
+template <> struct promote<double> {
+    using type = long double;
+};
+
+template <> struct promote<long double> {
+    using type = long double;
+};
+
+template <typename T> using promote_t = typename promote<T>::type;
+
+template <typename T> auto sum(T lhs, T rhs) -> promote_t<T>
+{
+    return static_cast<promote_t<T>>(lhs) + rhs;
+}
+
+int main()
+{
+    std::cout << sum(std::numeric_limits<short int>::max(),
+                     std::numeric_limits<short int>::max())
+              << std::endl;
+    std::cout << sum(std::numeric_limits<unsigned short int>::max(),
+                     std::numeric_limits<unsigned short int>::max())
+              << std::endl;
+    std::cout << sum(std::numeric_limits<int>::max(),
+                     std::numeric_limits<int>::max())
+              << std::endl;
+    std::cout << sum(std::numeric_limits<unsigned int>::max(),
+                     std::numeric_limits<unsigned int>::max())
+              << std::endl;
+    std::cout << sum(std::numeric_limits<long int>::max(),
+                     std::numeric_limits<long int>::max())
+              << std::endl;
+    std::cout << sum(std::numeric_limits<unsigned long int>::max(),
+                     std::numeric_limits<unsigned long int>::max())
+              << std::endl;
+    std::cout << sum(std::numeric_limits<long long int>::max(),
+                     std::numeric_limits<long long int>::max())
+              << std::endl;
+    std::cout << sum(std::numeric_limits<unsigned long long int>::max(),
+                     std::numeric_limits<unsigned long long int>::max())
+              << std::endl;
+    std::cout << sum(std::numeric_limits<float>::max(),
+                     std::numeric_limits<float>::max())
+              << std::endl;
+    std::cout << sum(std::numeric_limits<double>::max(),
+                     std::numeric_limits<double>::max())
+              << std::endl; // too large
+    std::cout << sum(std::numeric_limits<long double>::max(),
+                     std::numeric_limits<long double>::max())
+              << std::endl; // too large
+}