doc/doc.md: updated doc

doc/doc.md

@@ -580,7 +580,7 @@ string argument:
 
 class cli_parser_t : public clapp::basic_main_parser_t {
    public:
-    cli_parser_t() = default;
+    cli_parser_t();
 
     ~cli_parser_t() override;
 
@@ -601,6 +601,7 @@ class cli_parser_t : public clapp::basic_main_parser_t {
     cli_parser_t &operator=(cli_parser_t &&) noexcept = delete;
 };
 
+cli_parser_t::cli_parser_t() = default;
 cli_parser_t::~cli_parser_t() = default;
 
 int main(int argc, char *argv[]) {
@@ -660,7 +661,7 @@ Caught Exception: Mandatory argument 'string-arg' not given.
 
 # Give no mandatory option throws:
 $ ./libclapp_doc_simple_main_parser -b my-string-arg
-Caught Exception: Only the following mandatory options '-b|--bool' were given, but at least the following options are required: '-b|--bool', '-s|--str'
+Caught Exception: Only the following mandatory options -b|--bool were given, but at least the following options are required: -b|--bool -s|--str=<param>
 
 ```
 [//]:#end_calls_simple_main_parser
@@ -959,6 +960,7 @@ But if you want to be conformant to [CppCoreGuideline C.21](https://github.com/i
 If the previous example listing is executed, you get the following output:
 [//]:#begin_calls_parser_container
 ```bash
+# Print the help message:
 $ ./libclapp_doc_parser_container -h  # this is the same as with option `--help`
 Usage:
 ./libclapp_doc_parser_container -h|--help | ( -b|--bool -s|--str=<param> ) <string-arg>
@@ -1096,12 +1098,14 @@ this sub-parser.
 
 class cli_parser_t : public clapp::basic_main_parser_t {
    public:
-    cli_parser_t() = default;
+    cli_parser_t();
     ~cli_parser_t() override;
 
-    clapp::help_option_t help{*this, "help", 'h', "Show help options."};
+    clapp::help_option_t help{*this, "help", 'h', "Show help options.",
+                              clapp::parser::types::purpose_t::optional};
 
-    clapp::int32_param_option_t int_opt{*this, 'i', "Int option"};
+    clapp::int32_param_option_t int_opt{
+        *this, 'i', "Int option", clapp::parser::types::purpose_t::optional};
 
     class mode1_parser_t : public clapp::basic_sub_parser_t {
        public:
@@ -1112,7 +1116,9 @@ class cli_parser_t : public clapp::basic_main_parser_t {
         clapp::help_option_t help{*this, "help", 'h', "Show help options.",
                                   clapp::parser::types::purpose_t::optional};
 
-        clapp::string_param_option_t string{*this, 's', "String param option."};
+        clapp::string_param_option_t string{
+            *this, 's', "String param option.",
+            clapp::parser::types::purpose_t::optional};
 
         // delete copy/mv-ctors and assignmet operators (CppCoreGuideline C.21):
         explicit mode1_parser_t(const mode1_parser_t &) = delete;
@@ -1150,6 +1156,7 @@ class cli_parser_t : public clapp::basic_main_parser_t {
     cli_parser_t &operator=(cli_parser_t &&) noexcept = delete;
 };
 
+cli_parser_t::cli_parser_t() = default;
 cli_parser_t::~cli_parser_t() = default;
 cli_parser_t::mode1_parser_t::~mode1_parser_t() = default;
 cli_parser_t::mode2_parser_t::~mode2_parser_t() = default;