update

modern_C++_30/constexpr/sqrt.cpp

@@ -0,0 +1,47 @@
+//
+// Created by light on 20-1-7.
+//
+
+#include <iostream>
+#include <array>
+
+int sqr(int n) {
+    return n * n;
+}
+
+int rand() {
+    return std::rand();
+}
+
+// constant expression
+constexpr int sqr1(int n) {
+    return n * n;
+}
+
+constexpr int factorial(int n) {
+    // n是普通的int 不能使用static_assert
+//    static_assert(n>=0); //error
+    // 正确方式
+    if (n < 0) {
+        throw std::invalid_argument(
+                "Arg must be non-negative");
+    }
+
+    if (n == 0) {
+        return 1;
+    } else {
+        return n * factorial(n - 1);
+    }
+}
+
+int main() {
+    int a[sqr(3)];  // ok
+    const int n1 = sqr(3);  // ok
+
+    constexpr int n = sqr1(3); // constexpr=constant expression 常量表达式
+    std::array<int, n> aa;  // ok
+
+    std::array<int, factorial(10)> b;   // ok
+    int cc[rand()];
+    return 0;
+}