revision #1: 更新第二章中已维护的代码

code/2/2.2.constexpr.cpp

@@ -0,0 +1,50 @@
+//
+// 2.2.constexpr.cpp
+// chapter 2 language usability
+// modern cpp tutorial
+//
+// created by changkun at changkun.de
+//
+
+#include <iostream>
+#define LEN 10
+
+int len_foo() {
+    int i = 2;
+    return i;
+}
+constexpr int len_foo_constexpr() {
+    return 5;
+}
+
+// error in c++11
+// constexpr int fibonacci(const int n) {
+//     if(n == 1) return 1;
+//     if(n == 2) return 1;
+//     return fibonacci(n-1) + fibonacci(n-2);
+// }
+
+// ok
+constexpr int fibonacci(const int n) {
+    return n == 1 || n == 2 ? 1 : fibonacci(n-1) + fibonacci(n-2);
+}
+
+
+int main() {
+    char arr_1[10];                      // legal
+    char arr_2[LEN];                     // legal
+
+    int len = 10;
+    // char arr_3[len];                  // illegal
+
+    const int len_2 = len + 1;
+    char arr_4[len_2];                   // legal
+
+    // char arr_5[len_foo()+5];          // illegal
+    char arr_6[len_foo_constexpr() + 1]; // legal
+    
+    // 1, 1, 2, 3, 5, 8, 13, 21, 34, 55
+    std::cout << fibonacci(10) << std::endl;
+
+    return 0;
+}