Adding the ability to set custom failure messages

include/CLI/App.hpp

@@ -40,6 +40,11 @@ enum class Classifer { NONE, POSITIONAL_MARK, SHORT, LONG, SUBCOMMAND };
 struct AppFriend;
 } // namespace detail
 
+namespace FailureMessage {
+std::string simple(const App *app, const Error &e);
+std::string help(const App *app, const Error &e);
+} // namespace FailureMessage
+
 class App;
 
 using App_p = std::unique_ptr<App>;
@@ -89,6 +94,9 @@ class App {
     /// A pointer to the help flag if there is one INHERITABLE
     Option *help_ptr_{nullptr};
 
+    /// The error message printing function INHERITABLE
+    std::function<std::string(const App *, const Error &e)> failure_message_ = FailureMessage::simple;
+
     ///@}
     /// @name Parsing
     ///@{
@@ -157,6 +165,7 @@ class App {
             option_defaults_ = parent_->option_defaults_;
 
             // INHERITABLE
+            failure_message_ = parent_->failure_message_;
             allow_extras_ = parent_->allow_extras_;
             prefix_command_ = parent_->prefix_command_;
             ignore_case_ = parent_->ignore_case_;
@@ -712,6 +721,11 @@ class App {
         run_callback();
     }
 
+    /// Provide a function to print a help message. The function gets access to the App pointer and error.
+    void set_failure_message(std::function<std::string(const App *, const Error &e)> function) {
+        failure_message_ = function;
+    }
+
     /// Print a nice error message and return the exit code
     int exit(const Error &e) const {
 
@@ -719,15 +733,16 @@ class App {
         if(dynamic_cast<const CLI::RuntimeError *>(&e) != nullptr)
             return e.get_exit_code();
 
-        if(e.exit_code != static_cast<int>(ExitCodes::Success)) {
+        if(dynamic_cast<const CLI::CallForHelp *>(&e) != nullptr) {
-            std::cerr << "ERROR: ";
+            std::cout << help();
-            std::cerr << e.what() << std::endl;
+            return e.get_exit_code();
-            if(e.print_help)
-                std::cerr << help();
-        } else {
-            if(e.print_help)
-                std::cout << help();
         }
+
+        if(e.exit_code != static_cast<int>(ExitCodes::Success)) {
+            if(failure_message_)
+                std::cerr << failure_message_(this, e) << std::flush;
+        }
+
         return e.get_exit_code();
     }
 
@@ -895,7 +910,7 @@ class App {
                 out << std::endl << group << ":" << std::endl;
                 for(const Option_p &opt : options_) {
                     if(opt->nonpositional() && opt->get_group() == group)
-                        detail::format_help(out, opt->help_name(), opt->get_description(), wid);
+                        detail::format_help(out, opt->help_name(true), opt->get_description(), wid);
                 }
             }
         }
@@ -1107,23 +1122,20 @@ class App {
             // Required
             if(opt->get_required()) {
                 if(opt->count() == 0) {
-                    throw RequiredError(opt->get_name() + " is required");
+                    throw RequiredError(opt->help_name() + " is required");
                 } else if(static_cast<int>(opt->count()) < opt->get_expected()) {
-                    if(opt->get_expected() == 1)
+                    throw RequiredError(opt->help_name() + " required at least " + std::to_string(opt->get_expected()) +
-                        throw RequiredError(opt->get_name() + " requires an argument");
+                                        " arguments");
-                    else
-                        throw RequiredError(opt->get_name() + " requires at least " +
-                                            std::to_string(opt->get_expected()) + " arguments");
                 }
             }
             // Requires
             for(const Option *opt_req : opt->requires_)
                 if(opt->count() > 0 && opt_req->count() == 0)
-                    throw RequiresError(opt->get_name(), opt_req->get_name());
+                    throw RequiresError(opt->single_name(), opt_req->single_name());
             // Excludes
             for(const Option *opt_ex : opt->excludes_)
                 if(opt->count() > 0 && opt_ex->count() != 0)
-                    throw ExcludesError(opt->get_name(), opt_ex->get_name());
+                    throw ExcludesError(opt->single_name(), opt_ex->single_name());
         }
 
         auto selected_subcommands = get_subcommands();
@@ -1415,6 +1427,20 @@ class App {
     }
 };
 
+namespace FailureMessage {
+inline std::string simple(const App *app, const Error &e) {
+    std::string header = std::string("ERROR: ") + e.what() + "\n";
+    if(app->get_help_ptr() != nullptr)
+        header += "Run with " + app->get_help_ptr()->single_name() + " for more help\n";
+    return header;
+};
+inline std::string help(const App *app, const Error &e) {
+    std::string header = std::string("ERROR: ") + e.what() + "\n";
+    header += app->help();
+    return header;
+};
+} // namespace FailureMessage
+
 namespace detail {
 /// This class is simply to allow tests access to App's protected functions
 struct AppFriend {

include/CLI/Error.hpp

@@ -41,25 +41,19 @@ enum class ExitCodes {
 /// All errors derive from this one
 struct Error : public std::runtime_error {
     int exit_code;
-    bool print_help;
     int get_exit_code() const { return exit_code; }
-    Error(std::string parent, std::string name, ExitCodes exit_code = ExitCodes::BaseClass, bool print_help = true)
+
-        : runtime_error(parent + ": " + name), exit_code(static_cast<int>(exit_code)), print_help(print_help) {}
+    Error(std::string parent, std::string name, ExitCodes exit_code = ExitCodes::BaseClass)
-    Error(std::string parent,
+        : runtime_error(parent + ": " + name), exit_code(static_cast<int>(exit_code)) {}
-          std::string name,
+    Error(std::string parent, std::string name, int exit_code = static_cast<int>(ExitCodes::BaseClass))
-          int exit_code = static_cast<int>(ExitCodes::BaseClass),
+        : runtime_error(parent + ": " + name), exit_code(exit_code) {}
-          bool print_help = true)
-        : runtime_error(parent + ": " + name), exit_code(exit_code), print_help(print_help) {}
 };
 
 /// Construction errors (not in parsing)
 struct ConstructionError : public Error {
     // Using Error::Error constructors seem to not work on GCC 4.7
-    ConstructionError(std::string parent,
+    ConstructionError(std::string parent, std::string name, ExitCodes exit_code = ExitCodes::BaseClass)
-                      std::string name,
+        : Error(parent, name, exit_code) {}
-                      ExitCodes exit_code = ExitCodes::BaseClass,
-                      bool print_help = true)
-        : Error(parent, name, exit_code, print_help) {}
 };
 
 /// Thrown when an option is set to conflicting values (non-vector and multi args, for example)
@@ -83,20 +77,17 @@ struct OptionAlreadyAdded : public ConstructionError {
 
 /// Anything that can error in Parse
 struct ParseError : public Error {
-    ParseError(std::string parent, std::string name, ExitCodes exit_code = ExitCodes::BaseClass, bool print_help = true)
+    ParseError(std::string parent, std::string name, ExitCodes exit_code = ExitCodes::BaseClass)
-        : Error(parent, name, exit_code, print_help) {}
+        : Error(parent, name, exit_code) {}
-    ParseError(std::string parent,
+    ParseError(std::string parent, std::string name, int exit_code = static_cast<int>(ExitCodes::BaseClass))
-               std::string name,
+        : Error(parent, name, exit_code) {}
-               int exit_code = static_cast<int>(ExitCodes::BaseClass),
-               bool print_help = true)
-        : Error(parent, name, exit_code, print_help) {}
 };
 
 // Not really "errors"
 
 /// This is a successful completion on parsing, supposed to exit
 struct Success : public ParseError {
-    Success() : ParseError("Success", "Successfully completed, should be caught and quit", ExitCodes::Success, false) {}
+    Success() : ParseError("Success", "Successfully completed, should be caught and quit", ExitCodes::Success) {}
 };
 
 /// -h or --help on command line
@@ -107,7 +98,7 @@ struct CallForHelp : public ParseError {
 
 /// Does not output a diagnostic in CLI11_PARSE, but allows to return from main() with a specific error code.
 struct RuntimeError : public ParseError {
-    RuntimeError(int exit_code = 1) : ParseError("RuntimeError", "runtime error", exit_code, false) {}
+    RuntimeError(int exit_code = 1) : ParseError("RuntimeError", "runtime error", exit_code) {}
 };
 
 /// Thrown when parsing an INI file and it is missing

include/CLI/Option.hpp

@@ -359,10 +359,20 @@ class Option : public OptionBase<Option> {
         return out;
     }
 
-    /// The first half of the help print, name plus default, etc
+    /// The most discriptive name available
-    std::string help_name() const {
+    std::string single_name() const {
+        if(!lnames_.empty())
+            return lnames_[0];
+        else if(!snames_.empty())
+            return snames_[0];
+        else
+            return pname_;
+    }
+
+    /// The first half of the help print, name plus default, etc. Setting opt_only to true avoids the positional name.
+    std::string help_name(bool opt_only = false) const {
         std::stringstream out;
-        out << get_name(true) << help_aftername();
+        out << get_name(opt_only) << help_aftername();
         return out.str();
     }