[CPP20]Add format header file examples

CMakeLists.txt

@@ -59,6 +59,7 @@ add_executable(cpp_weekly
         cpp20_features/barrier_test.cc
         cpp20_features/semaphores_test.cc
         cpp20_features/placeholder_types.cc
+        cpp20_features/format_test.cc
 
         cpp_challenges/chap1_math_problems.cc
 

cpp20_features/format_test.cc

@@ -0,0 +1,34 @@
+#include "internal_check_conds.h"
+
+#include <gtest/gtest.h>
+#include <format>
+#include <string>
+
+namespace {
+
+void basic_example() {
+  std::string str{"hello"};
+
+  std::cout << std::format("String '{}' has {} chars\n", str, str.size()) ;
+  std::cout << std::format("String '{1}' has {0} chars\n", str, str.size()) ;
+}
+
+}
+
+TEST(format_test, basic_test) {
+  std::stringstream oss;
+
+  testing::internal::CaptureStdout();
+  basic_example();
+
+  auto act_output = testing::internal::GetCapturedStdout();
+
+  oss << "String 'hello' has 5 chars\n"
+         "String '5' has hello chars\n";
+
+#ifndef NDEBUG
+  debug_msg(oss, act_output);
+#endif
+
+  EXPECT_EQ(oss.str(), act_output);
+}