support bazel complie this project and format code.

basic_content/extern/extern_c++/BUILD

@@ -0,0 +1,15 @@
+# please run `bazel run basic_content/extern/extern_c++:main`
+load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_library")
+
+cc_library(
+    name = "add",
+    srcs = ["add.c"],
+    hdrs = ["add.h"],
+)
+
+cc_binary(
+    name = "main",
+    srcs = ["add.cpp"],
+    deps = [":add"],
+    copts = ["-std=c++11"]
+)

basic_content/extern/extern_c++/add.cpp

@@ -1,9 +1,9 @@
 #include <iostream>
 using namespace std;
 extern "C" {
-    #include "add.h"
+#include "add.h"
 }
 int main() {
-    add(2,3);
-    return 0;
+  std::cout << add(2, 3) << std::endl;
+  return 0;
 }

basic_content/extern/extern_c++/add.h

@@ -1,4 +1,4 @@
 #ifndef ADD_H
 #define ADD_H
-extern int add(int x,int y);
+extern int add(int x, int y);
 #endif