cpp: rewrite example to have library, binary, and test (#243)

A single file is too simple.

cpp/BUILD

@@ -1,4 +1,19 @@
 cc_binary(
     name = "cpp",
     srcs = ["main.cc"],
+    deps = [":cpp_lib"],
+)
+
+cc_library(
+    name = "cpp_lib",
+    srcs = [
+        "hello.cc",
+        "hello.h",
+    ],
+)
+
+cc_test(
+    name = "cpp_test",
+    srcs = ["hello_test.cc"],
+    deps = [":cpp_lib"],
 )

cpp/hello.cc

@@ -0,0 +1,7 @@
+#include "cpp/hello.h"
+
+#include <string>
+
+std::string hello() {
+  return "hello";
+}

cpp/hello.h

@@ -0,0 +1,8 @@
+#ifndef cpp_hello_h
+#define cpp_hello_h
+
+#include <string>
+
+std::string hello();
+
+#endif

cpp/hello_test.cc

@@ -0,0 +1,11 @@
+#include <iostream>
+#include "cpp/hello.h"
+
+int main() {
+  auto got = hello();
+  if (got != "hello") {
+    std::cerr << "got '" << got << "', want 'hello'" << std::endl;
+    return 1;
+  }
+  return 0;
+}

cpp/main.cc

@@ -1,5 +1,7 @@
-#include <stdio.h>
-
-int main(int argc, char** argv) {
-  printf("Hello World\n");
-}
+#include <iostream>
+#include "cpp/hello.h"
+
+int main() {
+  std::cout << hello() << std::endl;
+  return 0;
+}