Fix parsing empty argument vectors

include/argparse/argparse.hpp

@@ -2356,7 +2356,9 @@ class ArgumentParser {
     }
     auto end = std::end(arguments);
     auto positional_argument_it = std::begin(m_positional_arguments);
-    for (auto it = std::next(std::begin(arguments)); it != end;) {
+    for (auto it = arguments.empty() ? std::begin(arguments)
+                                     : std::next(std::begin(arguments));
+         it != end;) {
       const auto &current_argument = *it;
       if (Argument::is_positional(current_argument, m_prefix_chars)) {
         if (positional_argument_it == std::end(m_positional_arguments)) {
@@ -2473,7 +2475,9 @@ class ArgumentParser {
     }
     auto end = std::end(arguments);
     auto positional_argument_it = std::begin(m_positional_arguments);
-    for (auto it = std::next(std::begin(arguments)); it != end;) {
+    for (auto it = arguments.empty() ? std::begin(arguments)
+                                     : std::next(std::begin(arguments));
+         it != end;) {
       const auto &current_argument = *it;
       if (Argument::is_positional(current_argument, m_prefix_chars)) {
         if (positional_argument_it == std::end(m_positional_arguments)) {

test/test_parse_args.cpp

@@ -6,9 +6,25 @@ import argparse;
 #include <doctest.hpp>
 
 #include <optional>
+#include <vector>
 
 using doctest::test_suite;
 
+TEST_CASE("Parse empty argument vector" * test_suite("parse_args")) {
+  argparse::ArgumentParser program("test");
+
+  REQUIRE_NOTHROW(program.parse_args(std::vector<std::string>{}));
+}
+
+TEST_CASE("Parse empty argument vector with required argument" *
+          test_suite("parse_args")) {
+  argparse::ArgumentParser program("test");
+  program.add_argument("config");
+
+  REQUIRE_THROWS_AS(program.parse_args(std::vector<std::string>{}),
+                    std::runtime_error);
+}
+
 TEST_CASE("Missing argument" * test_suite("parse_args")) {
   argparse::ArgumentParser program("test");
   program.add_argument("--config").nargs(1);

test/test_parse_known_args.cpp

@@ -10,6 +10,14 @@ import argparse;
 
 using doctest::test_suite;
 
+TEST_CASE("Parse empty argument vector without exceptions" *
+          test_suite("parse_known_args")) {
+  argparse::ArgumentParser program("test");
+
+  auto unknown_args = program.parse_known_args(std::vector<std::string>{});
+  REQUIRE(unknown_args.empty());
+}
+
 TEST_CASE("Parse unknown optional and positional arguments without exceptions" *
           test_suite("parse_known_args")) {
   argparse::ArgumentParser program("test");