support bazel complie this project and format code.

basic_content/extern/README.md

@@ -82,7 +82,7 @@ extern "C" {
     #include "add.h"
 }
 int main() {
-    add(2,3);
+    std::cout << add(2, 3) << std::endl;
     return 0;
 }
 ```
@@ -99,6 +99,8 @@ gcc -c add.c
 g++ add.cpp add.o -o main
 ```
 
+如果使用bazel编译，运行:`bazel run basic_content/extern/extern_c++:main`
+
 而通常为了C代码能够通用，即既能被C调用，又能被C++调用，头文件通常会有如下写法：
 
 ```c

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

basic_content/extern/extern_c/BUILD

@@ -0,0 +1,14 @@
+# 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.cpp"],
+    hdrs = ["add.h"],
+)
+
+cc_binary(
+    name = "main",
+    srcs = ["add.c"],
+    deps = [":add"],
+)

basic_content/extern/extern_c/add.cpp

@@ -1,5 +1,3 @@
 #include "add.h"
 
-int add(int x,int y) {
-    return x+y;
-}
+int add(int x, int y) { return x + y; }

basic_content/extern/extern_c/add.h

@@ -1,6 +1,6 @@
 #ifndef ADD_H
 #define ADD_H
 extern "C" {
-    int add(int x,int y);
+int add(int x, int y);
 }
 #endif