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CLI11/book/chapters/internals.md
2019-09-06 14:25:27 -04:00

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# 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:
```cpp
Option* add_option(string name, T item) {
this->function = [&item](string value){
item = detail::lexical_cast<T>(value);
}
}
```
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 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 (new in 1.7). This ensures that interactions between options and subcommand options is consistent.
1. `_process_ini`: This reads an INI file and fills/changes options as needed.
2. `_process_env`: Look for environment variables.
3. `_process_callbacks`: Run the callback functions - this fills out the variables.
4. `_process_help_flags`: Run help flags if present (and quit).
5. `_process_requirements`: Make sure needs/excludes, required number of options present.
## Exceptions
The library immediately returns a C++ exception when it detects a problem, such as an incorrect construction or a malformed command line.