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libs/cli11/book/chapters/toolkits.md

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+# Using CLI11 in a Toolkit
+
+CLI11 was designed to be integrate into a toolkit, providing a native experience
+for users. This was used in GooFit to provide `GooFit::Application`, an class
+designed to make ROOT users feel at home.
+
+## Custom namespace
+
+If you want to provide CLI11 in a custom namespace, you'll want to at least put
+`using CLI::App` in your namespace. You can also include Option, some errors,
+and validators. You can also put `using namespace CLI` inside your namespace to
+import everything.
+
+You may also want to make your own copy of the `CLI11_PARSE` macro. Something
+like:
+
+```cpp
+ #define MYPACKAGE_PARSE(app, argv, argc)    \
+     try {                                   \
+         app.parse(argv, argc);              \
+     } catch(const CLI::ParseError &e) {     \
+         return app.exit(e);                 \
+     }
+```
+
+## Subclassing App
+
+If you subclass `App`, you'll just need to do a few things. You'll need a
+constructor; calling the base `App` constructor is a good idea, but not
+necessary (it just sets a description and adds a help flag.
+
+You can call anything you would like to configure in the constructor, like
+`option_defaults()->take_last()` or `fallthrough()`, and it will be set on all
+user instances. You can add flags and options, as well.
+
+## Virtual functions provided
+
+You are given a few virtual functions that you can change (only on the main
+App). `pre_callback` runs right before the callbacks run, letting you print out
+custom messages at the top of your app.