CLI11 Internals

Callbacks

The library was designed to bind to existing variables without requiring typed classes or inheritance. This is accomplished through lambda functions.

This looks like:

Option* add_option(string name, T &item) {
    this->function = [&item](string value){
        return lexical_cast(value, item);
    }
}

Obviously, you can't access T after the add_ method is over, so it stores the string representation of the default value if it receives the special true value as the final argument (not shown above).

Parsing

Parsing follows the following procedure:

1. _validate: Make sure the defined options and subcommands are self consistent.
2. _parse: Main parsing routine. See below.
3. _run_callback: Run an App callback if present.

The parsing phase is the most interesting:

1. _parse_single: Run on each entry on the command line and fill the options/subcommands.
2. _process: Run the procedure listed below.
3. _process_extra: This throws an error if needed on extra arguments that didn't fit in the parse.

The _process procedure runs the following steps; each step is recursive and completes all subcommands before moving to the next step. This ensures that interactions between options and subcommand options is consistent.

CLI11_INLINE void App::_process() {
    // help takes precedence over other potential errors and config and environment shouldn't be processed if help
    // throws
    _process_callbacks(CallbackPriority::FirstPreHelp);
    _process_help_flags(CallbackPriority::First);
    _process_callbacks(CallbackPriority::First);

    std::exception_ptr config_exception;
    try {
        // the config file might generate a FileError but that should not be processed until later in the process
        // to allow for help, version and other errors to generate first.
        _process_config_file();

        // process env shouldn't throw but no reason to process it if config generated an error
        _process_env();
    } catch(const CLI::FileError &) {
        config_exception = std::current_exception();
    }
    // callbacks and requirements processing can generate exceptions which should take priority
    // over the config file error if one exists.
    _process_callbacks(CallbackPriority::PreRequirementsCheckPreHelp);
    _process_help_flags(CallbackPriority::PreRequirementsCheck);
    _process_callbacks(CallbackPriority::PreRequirementsCheck);

    _process_requirements();

    _process_callbacks(CallbackPriority::NormalPreHelp);
    _process_help_flags(CallbackPriority::Normal);
    _process_callbacks(CallbackPriority::Normal);

    if(config_exception) {
        std::rethrow_exception(config_exception);
    }

    _process_callbacks(CallbackPriority::LastPreHelp);
    _process_help_flags(CallbackPriority::Last);
    _process_callbacks(CallbackPriority::Last);
}

Option callbacks can be executed at many different stages depending on the priority specified. The default is Normal so they will execute after processing requirements. The default for help and version flags is to execute First. Both can be changed to execute in different steps of the process.

Exceptions

The library immediately returns a C++ exception when it detects a problem, such as an incorrect construction or a malformed command line. Errors from config processing are delayed until after other processing, to give priority to any help or version flags, or other types of callback errors.