specify get_line behavior on out of range versus empty line

When the specified line is empty, the returned `string_view` will still have a valid (non-null) data pointer. When the line number is out of range, the data pointer will be null.

Author: N-Dekker

README.md

@@ -290,7 +290,7 @@ UTF-8 decoding is performed using a state machine based on Bjoern Hoehrmann's '[
 -   **[@levicki](https://github.com/levicki)** - Helped design some new features
 -   **[@moorereason](https://github.com/moorereason)** - Reported a whole bunch of bugs
 -   **[@mosra](https://github.com/mosra)** - Created the awesome [m.css] used to generate the API docs
--   **[@N-Dekker](https://github.com/N-Dekker)** - Added a workaround for the legacy lambda processor of MSVC 2019/2022
+-   **[@N-Dekker](https://github.com/N-Dekker)** - Added a workaround for the legacy lambda processor of MSVC 2019/2022, added `get_line`
 -   **[@ned14](https://github.com/ned14)** - Reported a bunch of bugs and helped design some new features
 -   **[@okureta](https://github.com/okureta)** - Reported a bug
 -   **[@prince-chrismc](https://github.com/prince-chrismc)** - Added toml++ to ConanCenter, and fixed some typos

include/toml++/impl/source_region.hpp

@@ -235,7 +235,11 @@ TOML_NAMESPACE_START
 	/// \param 	line_num	The line number (1-based).
 	///
 	/// \returns	The specified line, excluding any possible trailing carriage return or line feed character.
-	/// \remarks Returns an empty string_view when there is no line at the specified line number, in the specified document.
+	/// \remarks	The data pointer of the returned `string_view` will be null when the specified line
+	///				number is out of range (i.e., when the line number is zero, or greater than the
+	///				total number of lines of the specified document). Otherwise, when the line
+	///				number is within the document and the line is empty, the returned `string_view` will
+	///				also be empty, but then its	data pointer will not be null.
 	TOML_NODISCARD
 	constexpr std::string_view get_line(std::string_view doc, source_index line_num) noexcept
 	{

tests/user_feedback.cpp

@@ -455,12 +455,16 @@ b = []
 	SECTION("tomlplusplus/issues/254") // https://github.com/marzer/tomlplusplus/issues/254
 	{
 		// Check constexpr support.
-		static_assert(toml::get_line(""sv, 1) == std::string_view{});
+		static_assert(toml::get_line(""sv, 1).data() == nullptr);
 		static_assert(toml::get_line("alpha = 1\nbeta = 2\n # last line # "sv, 1) == "alpha = 1"sv);
 
+		constexpr auto expected_empty_line = toml::get_line("# line one\n\n# line three"sv, 2);
+		static_assert(expected_empty_line.empty());
+		static_assert(expected_empty_line.data() != nullptr);
+
 		for (const toml::source_index line_num : { 0u, 1u, 2u })
 		{
-			CHECK(toml::get_line(""sv, line_num) == std::string_view{});
+			CHECK(toml::get_line(""sv, line_num).data() == nullptr);
 		}
 
 		for (const auto input : {
@@ -469,9 +473,9 @@ b = []
 				 "# \r (embedded carriage return)\r\n"sv,
 			 })
 		{
-			CHECK(toml::get_line(input, 0) == std::string_view{});
+			CHECK(toml::get_line(input, 0).data() == nullptr);
 			CHECK(toml::get_line(input, 1) == "# \r (embedded carriage return)"sv);
-			CHECK(toml::get_line(input, 2) == std::string_view{});
+			CHECK(toml::get_line(input, 2).data() == nullptr);
 		}
 
 		for (const auto input : {
@@ -480,11 +484,11 @@ b = []
 				 "alpha = 1\r\nbeta = 2\r\n # last line # \r\n"sv,
 			 })
 		{
-			CHECK(toml::get_line(input, 0) == std::string_view{});
+			CHECK(toml::get_line(input, 0).data() == nullptr);
 			CHECK(toml::get_line(input, 1) == "alpha = 1"sv);
 			CHECK(toml::get_line(input, 2) == "beta = 2"sv);
 			CHECK(toml::get_line(input, 3) == " # last line # "sv);
-			CHECK(toml::get_line(input, 4) == std::string_view{});
+			CHECK(toml::get_line(input, 4).data() == nullptr);
 		}
 	}
 }