Fix issue #248: Align multiline help messages

include/argparse/argparse.hpp

@@ -668,7 +668,36 @@ class Argument {
         name_stream << " " << argument.m_metavar;
       }
     }
-    stream << name_stream.str() << "\t" << argument.m_help;
+
+    // align multiline help message
+    auto stream_width = stream.width();
+    auto name_padding = std::string(name_stream.str().size(), ' ');
+    auto pos = 0;
+    auto prev = 0;
+    auto first_line = true;
+    auto hspace = "  "; // minimal space between name and help message
+    stream << name_stream.str();
+    std::string_view help_view(argument.m_help);
+    while ((pos = argument.m_help.find('\n', prev)) != std::string::npos) {
+      auto line = help_view.substr(prev, pos - prev + 1);
+      if (first_line) {
+        stream << hspace << line;
+        first_line = false;
+      } else {
+        stream.width(stream_width);
+        stream << name_padding << hspace << line;
+      }
+      prev += pos - prev + 1;
+    }
+    if (first_line) {
+      stream << hspace << argument.m_help;
+    } else {
+      auto leftover = help_view.substr(prev, argument.m_help.size() - prev);
+      if (!leftover.empty()) {
+        stream.width(stream_width);
+        stream << name_padding << hspace << leftover;
+      }
+    }
 
     // print nargs spec
     if (!argument.m_help.empty()) {

test/test_help.cpp

@@ -73,3 +73,48 @@ TEST_CASE("Users can replace default -h/--help" * test_suite("help")) {
   program.parse_args({"test", "--help"});
   REQUIRE_FALSE(buffer.str().empty());
 }
+
+TEST_CASE("Multiline help message alignment") {
+  // '#' is used at the beginning of each help message line to simplify testing.
+  // It is important to ensure that this character doesn't appear elsewhere in the test case.
+  // Default arguments (e.g., -h/--help, -v/--version) are not included in this test.
+  argparse::ArgumentParser program("program");
+  program.add_argument("INPUT1")
+      .help(
+        "#This is the first line of help message.\n"
+        "#And this is the second line of help message."
+      );
+  program.add_argument("program_input2")
+      .help("#There is only one line.");
+  program.add_argument("-p", "--prog_input3")
+      .help(
+R"(#Lorem ipsum dolor sit amet, consectetur adipiscing elit.
+#Sed ut perspiciatis unde omnis iste natus error sit voluptatem
+#accusantium doloremque laudantium, totam rem aperiam...)"
+      );
+  program.add_argument("--verbose").default_value(false).implicit_value(true);
+
+  std::ostringstream stream;
+  stream << program;
+  std::istringstream iss(stream.str());
+
+  int help_message_start = -1;
+  std::string line;
+  while (std::getline(iss, line)) {
+    // Find the position of '#', which indicates the start of the help message line
+    auto pos = line.find('#');
+
+    if (pos == std::string::npos) {
+      continue;
+    }
+
+    if (help_message_start == -1) {
+      help_message_start = pos;
+    } else {
+      REQUIRE(pos == help_message_start);
+    }
+  }
+
+  // Make sure we have at least one help message
+  REQUIRE(help_message_start != -1);
+}