Binding and wrapper generator for C/C++ libraries

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A C/C++/Qt binding and wrapper generator.

Platform support

| Arch | System | CI | Clang version | | ------- | ----------------- | ----------- | -------------- | | x86_64 | ArchLinux | Travis | Rolling | | x86_64 | Debian 7 | Travis | 4.0, 5.0 | | x86_64 | Ubuntu 17.04 | None | 5.0 | | x86_64 | Ubuntu 16.04 | Travis | 4.0, 5.0 | | | Other systems | Help wanted | ? |

You require the LLVM and Clang development libraries and headers. If you don't have them already installed, bindgen will tell you.


| Feature | Support | |--------------------------------------------------|---------| | Automatic Crystal binding generation | YES | | Automatic Crystal wrapper generation | YES | | Mapping C++ classes | | | +- Member methods | YES | | +- Static methods | YES | | +- Constructors | YES | | +- Overloaded operators | TBD | | +- Conversion functions | TBD | | Mapping C/C++ global functions | | | +- Mapping global functions | YES | | +- Wrapping as Crystal class | YES | | Overloaded methods (Also default arguments) | YES | | Copying default argument values | | | +- Integer, float, boolean types | YES | | +- String | YES | | Enumerations | YES | | Copying structures | YES | | Custom type conversions between C/++ and Crystal | YES | | Automatic type wrapping and conversion | YES | | Integration with Crystals GC | YES | | C++ Template instantiation for containers types | YES | | Virtual methods | YES | | Override virtual methods from Crystal | YES | | Abstract classes | YES | | Multiple inheritance wrapping | YES | | Qt integration | | | +- QObject signals | YES | | +- QFlags types | YES | | +- QMetaObject generation (mimic moc) | TBD | | #define macro support | | | +- Mapping as enumeration | YES | | +- Mapping as constant (Including strings) | YES | | Copying in-source docs | TBD | | Platform specific type binding rules | YES | | Portable path finding for headers, libs, etc. | YES |

Projects using bindgen

Created a published, stable-y binding with bindgen? Want to see it here? PR!

How To

  1. Add bindgen to your shard.yml and run crystal deps
  2. Copy assets/bindgen_helper.hpp into your ext/
  3. Copy and customize TEMPLATE.yml
  4. Run lib/bindgen/ your_template.yml
    github: Papierkorb/bindgen

Note: If you intend to ship the generated code with your shard, you can replace dependencies with development_dependencies.

See TEMPLATE.yml for configuration documentation.

Mapping behaviour

The following rules are automatically applied to all bindings:

  • Method names get underscored: addWidget() -> #add_widget
    • Setter methods are rewritten: setWindowTitle() -> #window_title=
    • Getter methods are rewritten: getWindowTitle() -> #window_title
    • Bool getters are rewritten: getAwesome() -> #awesome?
    • is getters are rewritten: isEmpty() -> #empty?
    • has getters are rewritten: hasSpace() -> #has_space?
  • On signal methods (For Qt signals):
    • Keep their name for the emit version: pressed() -> #pressed
    • Get an on_ prefix for the connect version: #on_pressed do .. end
  • Enum fields get title-cased if not already: color0 -> Color0


The processor pipeline can be configured through the processors: array. Its elements are run in the order they're defined, starting at the first element.

Note: Don't worry: The TEMPLATE.yml file already comes with the recommended pipeline pre-configured.

There are three kinds of processors:

  1. Refining ones modify the graph in some way, without a dependency to a later generator.
  2. Generation processors add data to the graph so the later ran generators have all data they need to work.
  3. Information processors don't modify the graph, but do checks or print data onto the screen for debugging purposes.

The order is having first Refining, and Generation processors second in the configured pipeline. Information processors can be run at any time.

The following list of processors is ordered alphabetically.


  • Kind: Refining
  • Run after: No specific dependency
  • Run before: InstantiateContainers

When encountering a known container class on an instantiation that is not registered yet, registers it.

Container classes still need to be declared in the configuration, but don't require an explicit instantiations attribute anymore:

containers: # At the top-level of the config
  - class: QList # Set the class name
    type: Sequential # And its type
    # instantiations: # Can be added, but doesn't need to be.


  • Kind: Refining
  • Run after: No specific dependency
  • Run before: No specific dependency

Copies structures of those types, that have copy_structure: true set in the configuration. A wrapper class of a copy_structure type will host the structure directly (instead of a pointer) to it.


  • Kind: Generation
  • Run after: Refining processors
  • Run before: CrystalBinding

Generates the C++ wrapper method Calls.


  • Kind: Generation
  • Run after: CppWrapper, VirtualOverride and CrystalWrapper
  • Run before: No specific dependency

Generates the lib Binding funs.


  • Kind: Generation
  • Run after: Refining processors
  • Run before: CrystalBinding and VirtualOverride

Generates the Crystal methods in the wrapper classes.


  • Kind: Refining
  • Run after: No specific dependency
  • Run before: No specific dependency

Clang doesn't expose default constructors methods for implicit default constructors. This processor finds these cases and adds an explicit constructor.


  • Kind: Information
  • Run after: Any time
  • Run before: Any time

Debugging processor dumping the current graph onto STDERR.


  • Kind: Refining
  • Run after: FunctionClass
  • Run before: No specific dependency

Adds the copied enums to the graph. Should be run after other processors adding classes, so that enums can be added into classes.


  • Kind: Refining
  • Run after: Functions and FunctionClass
  • Run before: No specific dependency

Checks if a method require a C/C++ wrapper. If not, marks the method to bind directly to the target method instead of writing a "trampoline" wrapper in C++.

Note: This processor is required for variadic functions to work. A variadic function looks like this: void func(int c, ...);

A method can be bound directly if all of these are true:

  1. It uses the C ABI (extern "C")
  2. No argument uses a to_cpp converter
  3. The return type doesn't use a from_cpp converter

Note: If all methods can be bound to directly, you can remove the cpp generator completely from your configuration.


  • Kind: Refining
  • Run after: No specific dependency
  • Run before: No specific dependency

Removes all methods using an argument, or returning something, which is configured as ignore: true. Also removes methods that show up in the ignore_methods: list.

This processor can be run at any time in theory, but should be run as first part of the pipeline.


  • Kind: Refining
  • Run after: FunctionClass and ExternC
  • Run before: No specific dependency

Maps C functions, configured through the functions: map in the configuration.


  • Kind: Refining
  • Run after: ExternC
  • Run before: Inheritance and Functions

Generates wrapper classes from OOP-like C APIs, using guidance from the user through configuration in the functions: map.


  • Kind: Refining
  • Run after: FunctionClass
  • Run before: FilterMethods and VirtualOverride

Implements Crystal wrapper inheritance and adds #as_X conversion methods. Also handles abstract classes in that it adds an Impl class, so code can return instances to the (otherwise) abstract class.


  • Kind: Refining
  • Run after: AutoContainerInstantiation if used
  • Run before: No specific dependency

Adds the container instantiation classes and wrappers.


  • Kind: Refining
  • Run after: No specific dependency
  • Run before: No specific dependency

Maps #define macros into the graph. The mapping is configured by the user in the macros: list. Only value-macros ("object-like macros") are supported, function-like macros are silently skipped.

// Okay:
#define SOME_INT 1
#define SOME_STRING "Hello"
#define SOME_BOOL true

// Not mapped:
#define SOME_FUNCTION(x) (x + 1)


  • Kind: Refining
  • Run after: No specific dependency
  • Run before: No specific dependency

Adds Qt specific behaviour:

  1. Removes the qt_check_for_QGADGET_macro fake method.
  2. Provides #on_SIGNAL signal connection method.
btn =
btn.on_clicked do |checked| # Generated by this processor
  puts "Checked: #{checked}"


  • Kind: Information
  • Run after: Any time, as very last pipeline element is ideal.
  • Run before: Any time

Does sanity checks on the graph, focusing on Crystal bindings and wrappers.

Checks are as follows:

  • Name of enums, libs, structures, classes, modules and aliases are valid
  • Name of constants are valid
  • Name of methods are valid
  • Enumerations have at least one constant
  • Flag-enumerations don't have All nor None constants
  • Method arguments and result types are reachable
  • Variadic methods are directly bound
  • Alias targets are reachable
  • Class base-classes are reachable


  • Kind: Refining, but ran after generation processors!
  • Run after: CrystalWrapper!
  • Run before: CrystalBinding and CppWrapper

Adds C++ and Crystal wrapper code to allow overriding C++ virtual methods from within Crystal. Requires the Inheritance processor.

Important Note: Make sure to run this processor after CrystalWrapper but before CrystalBinding.

It needs to modify the #initialize methods, and generate lib structures, bindings, and C++ code too.

This is the recommended processor order:

  # ...
  - crystal_wrapper
  - virtual_override
  - cpp_wrapper
  - crystal_binding

After this, usage is the same as with any method:

class MyAdder < VirtualCalculator
  # In C++: virtual int calculate(int a, int b);
  # In Crystal:
  def calculate(a, b)
    a + b

Advanced configuration features

Conditions and dependencies in YAML files

YAML configuration files support conditionals elements (So, ifs), and loading external dependency files.

Apart from this logic, the configuration file is still valid YAML.

Note: Conditionals and dependencies are only supported in mappings (Hash in Crystal). Any such syntax encountered in something other than a mapping will not trigger any special behaviour.

Condition syntax

YAML documents can define conditional parts in mappings by having a conditional key, with mapping value. If the condition matches, the mapping value will be transparently embedded. If it does not match, the value will be transparently skipped.

Condition keys look like if_X or elsif_X or else. X is the condition, and it looks like Y_is_Z or Y_match_Z. You can also use (one or more) spaces () instead of exactly one underscore (_) to separate the words.

  • Y_is_Z is true if the variable Y equals Z case-sensitively.
  • Y_isnt_Z is true if the variable Y doesn't equal Z case-sensitively.
  • Y_match_Z is true if the variable Y is matched by the regular expression in Z. The regular expression is created case-sensitively.

A condition block is opened by the first if. Later condition keys can use elsif or else (or if to open a new condition block).

Note: elsif or else without an if will raise an exception.

Their behaviour is like in Crystal: if starts a condition block, elsif starts an alternative condition block, and else is used if none of if or elsif matched. It's possible to mix condition key-values with normal key-values.

Note: Conditions can be used in every mapping, even in mappings of a conditional. Each mapping acts as its own scope.


Variables are set by the user of the class (Probably through ConfigReader.from_yaml). All variable values are strings.

Variable names are case-sensitive. A missing variable will be treated as having an empty value ("").


foo: # A normal mapping
  bar: 1

# A condition: Matches if `platform` equals "arm".
if_platform_is_arm: # In Crystal: `if platform == "arm"`
  company: ARM et al

# You can mix in values between conditionals.  It won't "break" following
# elsif or else blocks.
not_a_condition: Hello

# An elsif: Matches if 1) the previous conditions didn't match
# 2) its own condition matches.
elsif_platform_match_x86: # In Crystal: `elsif platform =~ /x86/`
  company: Many different

# An else: Matches if all previous conditions didn't match.
  company: No idea

# At any time, you can start a new if sequence.
"if today is friday": # You can use spaces instead of underscores too
  hooray: true


To modularize the configuration, you can require ("merge") external yaml files from within your configuration.

This is triggered by using a key named <<, and writing the file name as value: <<: my_dependency.yml. The file-extension can also be omitted: <<: my_dependency in which case an .yml extension is assumed.

The dependency path is relative to the currently processed YAML file.

You can also require multiple dependencies into the same mapping:

  Something: true # You can mix dependencies with normal fields.
  <<: simple_types.yml
  <<: complex_types.yml
  <<: ignores.yml

The dependency will be embedded into the open mapping: It's transparent to the client code.

It's perfectly possible to mix conditionals with dependencies:

  <<: windows-specific.yml


An exception will be raised if any of the following occur:

  • The maximum dependency depth of 10 (MAX_DEPTH) is exceeded.
  • The dependency name contains a dot: ../foo.yml won't work.
  • The dependency name is absolute: /foo/bar.yml won't work.

Architecture of bindgen

Bindgen employs a pipeline inspired code architecture, which is strikingly similar to what most compilers use.

The code-flow is basically Parser::Runner to Graph::Builder to Processor::Runner to Generator::Runner.

Architecture flow diagram

The Graph

An important data structure used throughout the program is the graph. Code-wise, it's represented by Graph::Node (And its sub-classes). The nodes can contain child nodes, making it a hierarchical structure.

This allows to represent (almost) arbitrary structures as defined by the user configuration.

Say, we're wrapping GreetLib. As any library, it comes with a bunch of classes (Greeter and Listener), enums (Greetings, Type) and other stuff like constants (PORT). The configuration file could look like this:

module: GreetLib
classes: # We copy the structure of classes
  Greeter: Greeter
  Listener: Listener
enums: # But map the enums differently
  Type: Greeter::Type
  Greeter::Greetings: Greetings

Which will generate a graph looking like this:

Graph example

Note: The concept is really similar to ASTs used by compilers.

Parser step

Begin of the actual execution pipeline. Calls out to the clang-based parser tool to read the C/C++ source code and write a JSON-formatted "database" onto standard output. This is directly caught by bindgen and subsequently parsed as Parser::Document.

Graph::Builder step

The second step takes the Parser::Document and transforms it into a Graph::Namespace. This step is where the user configuration mapping is used.

Processor step

The third step runs all configured processors in-order. These work with the Graph and mostly add methods and Calls so they can be bound later. But they're allowed to do whatever they want really, which makes it a good place to add more complex rewriting rules if desired.

Processors are responsible for many core features of bindgen. The TEMPLATE.yml has an already set-up pipeline.

Generator step

The final step now takes the finalized graph and writes the result into an output of one or more files. Generators do not change the graph in any way, and also don't build anything on their own. They only write to output.


  1. Talk to Papierkorb in #crystal-lang about what you're gonna do.
  2. You got the go-ahead? The project's in an early state: Things may change without notice under you.
  3. Read the for some tips.
  4. Then do the rest, PR and all. You know the drill :)


This project (bindgen) and all of its sources, except those otherwise noted, all fall under the GPLv3 license. You can find a copy of its complete license text in the LICENSE file.

The configuration used to generate code, and all code generated by this project, fall under the full copyright of the user of bindgen. bindgen does not claim any copyright, legal or otherwise, on your work. Established projects should define a license they want to use for the generated code and configuration.


  • Papierkorb Stefan Merettig - creator, maintainer
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GNU General Public License v3.0