Add exit_on_default_arguments parameter to ArgumentParser

Allows users to opt-out of std::exit call in default arguments without
needing to replace with new --help and --version arguments.

Signed-off-by: Sean Robinson <sean.robinson@scottsdalecc.edu>

README.md

@@ -68,7 +68,7 @@ argparse::ArgumentParser program("program_name");
 
 **NOTE:** There is an optional second argument to the `ArgumentParser` which is the program version. Example: `argparse::ArgumentParser program("libfoo", "1.9.0");`
 
-**NOTE:** There is an optional third argument to the `ArgumentParser` which controls default arguments. Example: `argparse::ArgumentParser program("libfoo", "1.9.0", default_arguments::none);` See [Default Arguments](#default-arguments), below.
+**NOTE:** There are optional third and fourth arguments to the `ArgumentParser` which control default arguments. Example: `argparse::ArgumentParser program("libfoo", "1.9.0", default_arguments::help, false);` See [Default Arguments](#default-arguments), below.
 
 To add a new argument, simply call ```.add_argument(...)```. You can provide a variadic list of argument names that you want to group together, e.g., ```-v``` and ```--verbose```
 
@@ -559,7 +559,9 @@ The grammar follows `std::from_chars`, but does not exactly duplicate it. For ex
 
 ### Default Arguments
 
-`argparse` provides predefined arguments and actions for `-h`/`--help` and `-v`/`--version`. These default actions **exit** the program after displaying a help or version message, respectively. These defaults arguments can be disabled during `ArgumentParser` creation so that you can handle these arguments in your own way. (Note that a program name and version must be included when choosing default arguments.)
+`argparse` provides predefined arguments and actions for `-h`/`--help` and `-v`/`--version`. By default, these actions will **exit** the program after displaying a help or version message, respectively. This exit does not call destructors, skipping clean-up of taken resources.
+
+These default arguments can be disabled during `ArgumentParser` creation so that you can handle these arguments in your own way. (Note that a program name and version must be included when choosing default arguments.)
 
 ```cpp
 argparse::ArgumentParser program("test", "1.0", default_arguments::none);
@@ -578,6 +580,12 @@ The above code snippet outputs a help message and continues to run. It does not
 
 The default is `default_arguments::all` for included arguments. No default arguments will be added with `default_arguments::none`. `default_arguments::help` and `default_arguments::version` will individually add `--help` and `--version`.
 
+The default arguments can be used while disabling the default exit with these arguments. This forth argument to `ArgumentParser` (`exit_on_default_arguments`) is a bool flag with a default **true** value. The following call will retain `--help` and `--version`, but will not exit when those arguments are used.
+
+```cpp
+argparse::ArgumentParser program("test", "1.0", default_arguments::all, false)
+```
+
 ### Gathering Remaining Arguments
 
 `argparse` supports gathering "remaining" arguments at the end of the command, e.g., for use in a compiler:

include/argparse/argparse.hpp

@@ -1052,14 +1052,18 @@ class ArgumentParser {
 public:
   explicit ArgumentParser(std::string program_name = {},
                           std::string version = "1.0",
-                          default_arguments add_args = default_arguments::all)
+                          default_arguments add_args = default_arguments::all,
+                          bool exit_on_default_arguments = true)
       : m_program_name(std::move(program_name)), m_version(std::move(version)),
+        m_exit_on_default_arguments(exit_on_default_arguments),
         m_parser_path(m_program_name) {
     if ((add_args & default_arguments::help) == default_arguments::help) {
       add_argument("-h", "--help")
           .action([&](const auto & /*unused*/) {
             std::cout << help().str();
+            if (m_exit_on_default_arguments) {
               std::exit(0);
+            }
           })
           .default_value(false)
           .help("shows help message and exits")
@@ -1070,7 +1074,9 @@ public:
       add_argument("-v", "--version")
           .action([&](const auto & /*unused*/) {
             std::cout << m_version << std::endl;
+            if (m_exit_on_default_arguments) {
               std::exit(0);
+            }
           })
           .default_value(false)
           .help("prints version information and exits")
@@ -1676,6 +1682,7 @@ private:
   std::string m_version;
   std::string m_description;
   std::string m_epilog;
+  bool m_exit_on_default_arguments = true;
   std::string m_prefix_chars{"-"};
   std::string m_assign_chars{"="};
   bool m_is_parsed = false;

test/test_default_args.cpp

@@ -1,5 +1,7 @@
 #include <argparse/argparse.hpp>
 #include <doctest.hpp>
+#include <sstream>
+#include <streambuf>
 
 using doctest::test_suite;
 
@@ -17,3 +19,13 @@ TEST_CASE("Do not include default arguments" * test_suite("default_args")) {
   REQUIRE_THROWS_AS(parser.get("--help"), std::logic_error);
   REQUIRE_THROWS_AS(parser.get("--version"), std::logic_error);
 }
+
+TEST_CASE("Do not exit on default arguments" * test_suite("default_args")) {
+  argparse::ArgumentParser parser("test", "1.0",
+                                  argparse::default_arguments::all, false);
+  std::stringstream buf;
+  std::streambuf* saved_cout_buf = std::cout.rdbuf(buf.rdbuf());
+  parser.parse_args({"test", "--help"});
+  std::cout.rdbuf(saved_cout_buf);
+  REQUIRE(parser.is_used("--help"));
+}