[api] Default response only string fields to a null StringRef (#19025)

tests/components/core/test_string_ref.cpp

@@ -2,6 +2,9 @@
 
 #include "esphome/core/string_ref.h"
 
+#include <iterator>
+#include <string>
+
 namespace esphome::core::testing {
 
 TEST(StringRefStartsWith, ProperPrefixMatches) {
@@ -59,4 +62,103 @@ TEST(StringRefStartsWith, RefOverloadComparesOnlyTheViewedLength) {
   EXPECT_TRUE(ref.starts_with(prefix));
 }
 
+// The generated api messages start their encode only string fields as a null pointer with zero
+// length; every member must treat that exactly like the default constructed empty string.
+TEST(StringRefNullEmpty, BehavesAsEmptyString) {
+  const StringRef null_empty{nullptr, 0};
+  const StringRef empty;
+  EXPECT_TRUE(null_empty.empty());
+  EXPECT_EQ(null_empty.size(), 0u);
+  EXPECT_EQ(null_empty.c_str(), nullptr);
+  EXPECT_EQ(null_empty.byte(), nullptr);
+  EXPECT_TRUE(null_empty == empty);
+  EXPECT_FALSE(null_empty < empty);
+  EXPECT_FALSE(empty < null_empty);
+  EXPECT_TRUE(null_empty == "");             // NOLINT(readability-container-size-empty) - operator under test
+  EXPECT_TRUE(null_empty == std::string());  // NOLINT(readability-container-size-empty) - operator under test
+  EXPECT_EQ(null_empty.compare(empty), 0);
+  EXPECT_EQ(null_empty.compare(""), 0);
+  EXPECT_LT(null_empty.compare("a"), 0);
+  EXPECT_TRUE(null_empty.starts_with(""));
+  EXPECT_FALSE(null_empty.starts_with("a"));
+  EXPECT_EQ(null_empty.str(), std::string());
+  EXPECT_EQ(null_empty.substr(0), std::string());
+  EXPECT_EQ(null_empty.find('a'), std::string::npos);
+  EXPECT_EQ(null_empty.find("a"), std::string::npos);
+  char buf[4] = "xyz";
+  EXPECT_EQ(null_empty.copy(buf, sizeof(buf)), 0u);
+  EXPECT_EQ(null_empty.begin(), null_empty.end());
+}
+
+TEST(StringRefNullEmpty, ComparesAgainstText) {
+  const StringRef null_empty{nullptr, 0};
+  const StringRef text("abc", 3);
+  EXPECT_FALSE(null_empty == text);
+  EXPECT_FALSE(text == null_empty);
+  EXPECT_LT(null_empty.compare(text), 0);
+  EXPECT_GT(text.compare(null_empty), 0);
+  EXPECT_TRUE(null_empty < text);
+  EXPECT_FALSE(text < null_empty);
+  EXPECT_TRUE(text.starts_with(null_empty));
+}
+
+TEST(StringRefNullEmpty, TwoNullViewsAreEqual) {
+  const StringRef a{nullptr, 0};
+  const StringRef b{nullptr, 0};
+  EXPECT_TRUE(a == b);
+  EXPECT_FALSE(a < b);
+  EXPECT_EQ(a.compare(b), 0);
+  EXPECT_TRUE(a.starts_with(b));
+}
+
+// Every iterator endpoint of a null view is the same null position: nothing is dereferenced and
+// no offset is applied to the null pointer, so the range is simply empty.
+TEST(StringRefNullEmpty, IteratorEndpointsFormAnEmptyRange) {
+  const StringRef null_empty{nullptr, 0};
+  EXPECT_EQ(null_empty.cbegin(), null_empty.cend());
+  EXPECT_EQ(null_empty.rbegin(), null_empty.rend());
+  EXPECT_EQ(null_empty.crbegin(), null_empty.crend());
+  EXPECT_EQ(std::distance(null_empty.begin(), null_empty.end()), 0);
+  size_t visited = 0;
+  for (char c : null_empty) {
+    (void) c;
+    visited++;
+  }
+  EXPECT_EQ(visited, 0u);
+  // NOLINTNEXTLINE(bugprone-string-constructor) - empty range under test
+  EXPECT_EQ(std::string(null_empty.begin(), null_empty.end()), std::string());
+}
+
+// The pointer and length constructor accepts an empty range at a null pointer; the copy into a
+// std::string reads nothing.
+TEST(StringRefNullEmpty, ConvertsToEmptyStdString) {
+  const StringRef null_empty{nullptr, 0};
+  const std::string copy = null_empty.str();
+  EXPECT_TRUE(copy.empty());
+  EXPECT_EQ(static_cast<std::string>(null_empty), std::string());
+  EXPECT_EQ(null_empty.substr(0, 5), std::string());
+  std::string target("keep");
+  target += null_empty;
+  EXPECT_EQ(target, "keep");
+}
+
+// The number conversions hand the pointer to the C library; a null view must stop before that.
+TEST(StringRefNullEmpty, NumericConversionsReturnZero) {
+  const StringRef null_empty{nullptr, 0};
+  size_t pos = 99;
+  EXPECT_EQ(stoi(null_empty, &pos), 0);
+  EXPECT_EQ(pos, 0u);
+  pos = 99;
+  EXPECT_EQ(stol(null_empty, &pos, 16), 0L);
+  EXPECT_EQ(pos, 0u);
+  pos = 99;
+  EXPECT_EQ(stof(null_empty, &pos), 0.0f);
+  EXPECT_EQ(pos, 0u);
+  pos = 99;
+  EXPECT_EQ(stod(null_empty, &pos), 0.0);
+  EXPECT_EQ(pos, 0u);
+  EXPECT_EQ(stoi(null_empty), 0);
+  EXPECT_EQ(stof(null_empty), 0.0f);
+}
+
 }  // namespace esphome::core::testing