00---
title: "Generators"
---

Some templates within the ETL are designed to support a variable number of template parameters. To accommodate this, these template classes have a number of specialisations to handle each variant. The ETL sets this number to a reasonable default, but there will almost certainly be circumstances where these are too small for the project at hand.  

To this end, the ETL supplies 'generators' to enable the creation of customised variants of these template classes.  

The code generation is handled by a Python library called `COG`. This allows Python code to be embedded within the C++. When `COG` is run on the generator header, it will produce a customised version of that header.  

The naming convention used is  
The generator file: `xxxx_generator.h`  
The output file: `xxxx.h`  
The script: `generate_xxxx.bat`  

The script `generate.bat` will run all of the generate scripts.  

e.g.
Running the `COG` script on the generator `type_traits_generator.h` will create `type_traits.h`  

The syntax for processing a generator file is  
`python -m cogapp -d -e -o<output> -D<parameter>=<N> <generator>`  

e.g.
`python -m cogapp -d -e -otype_traits.h -DIsOneOf=16 type_traits_generator.h`  
The line above will create type_traits.h with the ability to handle up to 16 types in the `etl::is_one_of` structure.  

All of the generator files may be processed by running the script `generate.bat`  
There are generator scripts for each generator header.  
This can be run as a command line batch file under Windows or as a script under Linux (if set to 'executable')  

**Defined generators**  


| Functionality	       | Generator	                  | Output             | Parameters | Notes |
| -------------------- | ---------------------------- | ------------------ | ---------- | ----- |
| Finite State Machine | `fsm_generator.h`	          | `fsm.h`            | Handlers   | Defines the maximum number of events that can be handled |
| Message Router	   | `message_router_generator.h` | `message_router.h` | Handlers	| Defines the maximum number of messages that can be handled |
| Smallest	           | `smallest_generator.h`	      | `smallest.h`	   | NTypes     | Defines the maximum number of types that can be declared |
| Largest              | `largest_generator.h`        | `largest.h`        | NTypes     | Defines the maximum number of types that can be declared |
| Type Traits          | `type_traits_generator.h`    | `type_traits.h`    | NTypes     | Defines the maximum number of types that can be declared in the class `etl::is_one_of` |
| Type Lookup          | `type_lookup_generator.h`    | `type_lookup.h`	   | NTypes     | Defines the maximum number of types that can be declared in the classes `etl::type_id_lookup` and `etl::type_type_lookup` |
| Type Select          | `type_select_generator.h`	  | `type_select.h`    | NTypes     | Defines the maximum number of types that can be declared in the class `etl::type_select` |
| Variant Pool         | `variant_pool_generator.h`   | `variant_pool.h`   | NTypes     | Defines the maximum number of types that can be declared |