support bazel complie this project and format code.

learn_class/modern_cpp_30/constexpr/sqrt.cpp

@@ -0,0 +1,40 @@
+//
+// Created by light on 20-1-7.
+//
+
+#include <array>
+#include <iostream>
+
+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; // cordump
+  int cc[rand()];
+  return 0;
+}