CxxWrap

This package aims to provide a Boost.Python-like wrapping for C++ types and functions to Julia. The idea is to write the code for the Julia wrapper in C++, and then use a one-liner on the Julia side to make the wrapped C++ library available there.

The mechanism behind this package is that functions and types are registered in C++ code that is compiled into a dynamic library. This dynamic library is then loaded into Julia, where the Julia part of this package uses the data provided through a C interface to generate functions accessible from Julia. The functions are passed to Julia either as raw function pointers (for regular C++ functions that don't need argument or return type conversion) or std::functions (for lambda expressions and automatic conversion of arguments and return types). The Julia side of this package wraps all this into Julia methods automatically.

For this to work, the user must have a C++ compiler installed which supports C++17 (e.g. GCC 7, clang 5; for macOS users that means Xcode 9.3).

What's the difference with Cxx.jl?

With Cxx.jl it is possible to directly access C++ using the @cxx macro from Julia. So when facing the task of wrapping a C++ library in a Julia package, authors now have two options:

• Use Cxx.jl to write the wrapper package in Julia code (much like one uses ccall for wrapping a C library)
• Use CxxWrap to write the wrapper completely in C++ (and one line of Julia code to load the .so)

Boost.Python also uses the latter (C++-only) approach, so translating existing Python bindings based on Boost.Python may be easier using CxxWrap.

Features

• Support for C++ functions, member functions and lambdas
• Classes with single inheritance, using abstract base classes on the Julia side
• Trivial C++ classes can be converted to a Julia isbits immutable
• Template classes map to parametric types, for the instantiations listed in the wrapper
• Automatic wrapping of default and copy constructor (mapped to copy) if defined on the wrapped C++ class
• Facilitate calling Julia functions from C++

Installation

Just like any registered package, in pkg mode (] at the REPL)

add CxxWrap

CxxWrap v0.10 and later depends on the libcxxwrap_julia_jll JLL package to manage the libcxxwrap-julia binaries. See the libcxxwrap-julia Readme for information on how to build this library yourself and force CxxWrap to use your own version.

Boost Python Hello World example

Let's try to reproduce the example from the Boost.Python tutorial. Suppose we want to expose the following C++ function to Julia in a module called CppHello:

std::string greet()
{
   return "hello, world";
}

Using the C++ side of CxxWrap, this can be exposed as follows:

#include "jlcxx/jlcxx.hpp"

JLCXX_MODULE define_julia_module(jlcxx::Module& mod)
{
  mod.method("greet", &greet);
}

Once this code is compiled into a shared library (say libhello.so) it can be used in Julia as follows:

# Load the module and generate the functions
module CppHello
  using CxxWrap
  @wrapmodule(() -> joinpath("path/to/built/lib","libhello"))

  function __init__()
    @initcxx
  end
end

# Call greet and show the result
@show CppHello.greet()

The code for this example can be found in [hello.cpp] in the examples directory of the libcxxwrap-julia project and test/hello.jl. Note that the __init__ function is necessary to support precompilation, which is on by default since Julia 1.0.

Compiling the C++ code

The recommended way to compile the C++ code is to use CMake to discover libcxxwrap-julia and the Julia libraries. A full example is in the testlib directory of libcxxwrap-julia. The following sequence of commands can be used to build:

mkdir build && cd build
cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH=/path/to/libcxxwrap-julia-prefix /path/to/sourcedirectory
cmake --build . --config Release

The path for CMAKE_PREFIX_PATH can be obtained from Julia using:

julia> using CxxWrap
julia> CxxWrap.prefix_path()

Windows and MSVC

The default binaries installed with CxxWrap are cross-compiled using GCC, and thus incompatible with Visual Studio C++ (MSVC). In MSVC 2019, it is easy to check out libcxxwrap-julia from git, and then build it and the wrapper module from source. Details are provided in the README.

Module entry point

Above, we defined the module entry point as a function JLCXX_MODULE define_julia_module(jlcxx::Module& mod). In the general case, there may be multiple modules defined in a single library, and each should have its own entry point, called within the appropriate module:

JLCXX_MODULE define_module_a(jlcxx::Module& mod)
{
  // add stuff for A
}

JLCXX_MODULE define_module_b(jlcxx::Module& mod)
{
  // add stuff for B
}

In Julia, the name of the entry point must now be specified explicitly:

module A
  using CxxWrap
  @wrapmodule(() -> "mylib.so",:define_module_a)
end

module B
  using CxxWrap
  @wrapmodule(() -> "mylib.so",:define_module_b)
end

In specific cases, it may also be necessary to specify dlopen flags such as RTLD_GLOBAL. These can be supplied in a third, optional argument to @wrapmodule, e.g:

@wrapmodule(() -> CxxWrapCore.libcxxwrap_julia_stl, :define_cxxwrap_stl_module, Libdl.RTLD_GLOBAL)

More extensive example and function call performance

A more extensive example, including wrapping a C++11 lambda and conversion for arrays can be found in examples/functions.cpp and test/functions.jl. This test also includes some performance measurements, showing that the function call overhead is the same as using ccall on a C function if the C++ function is a regular function and does not require argument conversion. When std::function is used (e.g. for C++ lambdas) extra overhead appears, as expected.

Exposing classes

Consider the following C++ class to be wrapped:

struct World
{
  World(const std::string& message = "default hello") : msg(message){}
  void set(const std::string& msg) { this->msg = msg; }
  std::string greet() { return msg; }
  std::string msg;
  ~World() { std::cout << "Destroying World with message " << msg << std::endl; }
};

Wrapped in the entry point function as before and defining a module CppTypes, the code for exposing the type and some methods to Julia is:

types.add_type<World>("World")
  .constructor<const std::string&>()
  .method("set", &World::set)
  .method("greet", &World::greet);

Here, the first line just adds the type. The second line adds the non-default constructor taking a string. Finally, the two method calls add member functions, using a pointer-to-member. The member functions become free functions in Julia, taking their object as the first argument. This can now be used in Julia as

w = CppTypes.World()
@test CppTypes.greet(w) == "default hello"
CppTypes.set(w, "hello")
@test CppTypes.greet(w) == "hello"

The manually added constructor using the constructor function also creates a finalizer. This can be disabled by adding the argument jlcxx::finalize_policy::no:

types.add_type<World>("World")
  .constructor<const std::string&>(jlcxx::finalize_policy::no);

The add_type function actually builds two Julia types related to World. The first is an abstract type:

abstract type World end

The second is a mutable type (the "allocated" or "boxed" type) with the following structure:

mutable struct WorldAllocated <: World
  cpp_object::Ptr{Cvoid}
end

This type needs to be mutable, because it must have a finalizer attached to it that deletes the held C++ object.

This means that the variable w in the above example is of concrete type WorldAllocated and letting it go out of scope may trigger the finalizer and delete the object. When calling a C++ constructor, it is the responsibility of the caller to manage the lifetime of the resulting variable.

The above types are used in method generation as follows, considering for example the greet method taking a World argument:

greet(w::World) = ccall($fpointer, Any, (Ptr{Cvoid}, WorldRef), $thunk, cconvert(WorldRef, w))

Here, the cconvert from WorldAllocated to WorldRef is defined automatically when creating the type.

Warning: The ordering of the C++ code matters: types used as function arguments or return types must be added before they are used in a function.

The full code for this example and more info on immutables and bits types can be found in examples/types.cpp and test/types.jl.

Checking for null

Values returned from C++ can be checked for being null using the isnull function.

Setting the module to which methods are added

It is possible to add methods directly to e.g. the Julia Base module, using set_override_module. After calling this, all methods will be added to the specified module. To revert to the default behavior of adding methods to the current module, call unset_override_module.

mod.add_type<A>("A", jlcxx::julia_type("AbstractFloat", "Base"))
    .constructor<double>();
mod.set_override_module(mod.julia_module());
// == will be in the wrapped module:
mod.method("==", [](A& a, A& b) { return a == b; });
mod.set_override_module(jl_base_module);
// The following methods will be in Base
mod.method("+", [](A& a, A& b) { return a + b; });
mod.method("float", [](A& a) { return a.get_val(); });
// Revert to default behavior
mod.unset_override_module();
mod.method("val", [](A& a) { return a.get_val(); });

Inheritance

To encapsulate inheritance, types must first inherit from each other in C++, so a static_cast to the base type can work:

struct A
{
  virtual std::string message() const = 0;
  std::string data = "mydata";
};

struct B : A
{
  virtual std::string message() const
  {
    return "B";
  }
};

When adding the type, add the supertype as a second argument:

types.add_type<A>("A").method("message", &A::message);
types.add_type<B>("B", jlcxx::julia_base_type<A>());

The supertype is of type jl_datatype_t* and using the template function jlcxx::julia_base_type looks up the abstract type associated with A here. Since the concrete arguments given to ccall are the reference types, we need a way to convert BRef into ARef. To allow CxxWrap to figure out the correct static_cast to use, the hierarchy must be defined at compile time as follows:

namespace jlcxx
{
  template<> struct SuperType<B> { typedef A type; };
}

There is also a variant taking a string for the type name and an optional Julia module name as second argument, which is useful for inheriting from a type defined in Julia, e.g.:

mod.add_type<Teuchos::ParameterList>("ParameterList", jlcxx::julia_type("AbstractDict", "Base"))

The value returned by add_type also had a dt() method, useful in the case of template types:

auto multi_vector_base = mod.add_type<Parametric<TypeVar<1>>>("MultiVectorBase");
auto vector_base = mod.add_type<Parametric<TypeVar<1>>>("VectorBase", multi_vector_base.dt());

Conversion

Conversion to the base type happens automatically, or can be forced by calling convert, e.g.

convert(A,b)

Where we have b::B and B <: A

For the equivalent of a C++ dynamic_cast, we need to use pointers because the conversion may fail, i.e:

convert(CxxPtr{B},CxxPtr(a))

This is equivalent to the C++ code:

dynamic_cast<B*>(&a);

Use isnull on the result to check if the conversion was successful or not.

See the test at examples/inheritance.cpp and test/inheritance.jl.

Enum types

Enum types are converted to strongly-typed bits types on the Julia side. Consider the C++ enum:

enum MyEnum
{
  EnumValA,
  EnumValB
};

This is registered as follows:

JLCXX_MODULE define_types_module(jlcxx::Module& types)
{
  types.add_bits<MyEnum>("MyEnum", jlcxx::julia_type("CppEnum"));
  types.set_const("EnumValA", EnumValA);
  types.set_const("EnumValB", EnumValB);
}

The enum constants will be available on the Julia side as CppTypes.EnumValA and CppTypes.EnumValB, both of type CppTypes.MyEnum. Wrapped C++ functions taking a MyEnum will only accept a value of type CppTypes.MyEnum in Julia.

Template (parametric) types

The natural Julia equivalent of a C++ template class is the parametric type. The mapping is complicated by the fact that all possible parameter values must be compiled in advance, requiring a deviation from the syntax for adding a regular class. Consider the following template class:

template<typename A, typename B>
struct TemplateType
{
  typedef typename A::val_type first_val_type;
  typedef typename B::val_type second_val_type;

  first_val_type get_first()
  {
    return A::value();
  }

  second_val_type get_second()
  {
    return B::value();
  }
};

The code for wrapping this is:

types.add_type<Parametric<TypeVar<1>, TypeVar<2>>>("TemplateType")
  .apply<TemplateType<P1,P2>, TemplateType<P2,P1>>([](auto wrapped)
{
  typedef typename decltype(wrapped)::type WrappedT;
  wrapped.method("get_first", &WrappedT::get_first);
  wrapped.method("get_second", &WrappedT::get_second);
});

The first line adds the parametric type, using the generic placeholder Parametric and a TypeVar for each parameter. On the second line, the possible instantiations are created by calling apply on the result of add_type. Here, we allow for TemplateType<P1,P2> and TemplateType<P2,P1> to exist, where P1 and P2 are C++ classes that also must be wrapped and that fulfill the requirements for being a parameter to TemplateType. The argument to apply is a functor (generic C++14 lambda here) that takes the wrapped instantiated type (called wrapped here) as argument. This object can then be used as before to define methods. In the case of a generic lambda, the actual type being wrapped can be obtained using decltype as shown on the 4th line.

Use on the Julia side:

import ParametricTypes.TemplateType, ParametricTypes.P1, ParametricTypes.P2

p1 = TemplateType{P1, P2}()
p2 = TemplateType{P2, P1}()

@test ParametricTypes.get_first(p1) == 1
@test ParametricTypes.get_second(p2) == 1

There is also an apply_combination method to make applying all combinations of parameters shorter to write.

Full example and test including non-type parameters at: examples/parametric.cpp and test/parametric.jl.

Memory management

Constructors and destructors

The default constructor and any manually added constructor using the constructor function will automatically create a Julia object that has a finalizer attached that calls delete to free the memory. To write a C++ function that returns a new object that can be garbage-collected in Julia, use the jlcxx::create function:

jlcxx::create<Class>(constructor_arg1, ...);

This will return the new C++ object wrapped in a jl_value_t* that has a finalizer. The default constructor can be explicitly disabled by specializing the DefaultConstructible type trait, for example:

namespace jlcxx {
  template<> struct DefaultConstructible<Class> : std::false_type { };
}

Copy constructor

The copy constructor is mapped to Julia's standard copy function. Using the .-notation it can be used to easily create a Julia arrays from the elements of e.g. an std::vector:

wvec = cpp_function_returning_vector()
julia_array = copy.(wvec)

It can be explicitly disabled for a type by specializing the CopyConstructible type trait, for example:

namespace jlcxx {
  template<> struct CopyConstructible<Class> : std::false_type { };
}

Return values

If a wrapped C++ function returns an object by value, the wrapped object gets a finalizer and is owned by Julia. The same holds if a smart pointer such as shared_ptr (automatically wrapped in a SharedPtr) is returned by value. In contrast to that, if a reference or raw pointer is returned from C++, then the default assumption is that the pointed-to object lifetime is managed by C++.

Call operator overload

Since Julia supports overloading the function call operator (), this can be used to wrap operator() by just omitting the method name:

struct CallOperator
{
  int operator()() const
  {
    return 43;
  }
};

// ...

types.add_type<CallOperator>("CallOperator").method(&CallOperator::operator());

Use in Julia:

call_op = CallOperator()
@test call_op() == 43

The C++ function does not even have to be operator(), but of course it is most logical use case.

Automatic argument conversion

By default, overloaded signatures for wrapper methods are generated, so a method taking a double in C++ can be called with e.g. an Int in Julia. Wrapping a function like this:

mod.method("half_lambda", [](const double a) {return a*0.5;});

then yields the methods:

half_lambda(arg1::Int64)
half_lambda(arg1::Float64)

In some cases (e.g. when a template parameter depends on the number type) this is not desired, so the behavior can be disabled on a per-argument basis using the StrictlyTypedNumber type. Wrapping a function like this:

mod.method("strict_half", [](const jlcxx::StrictlyTypedNumber<double> a) {return a.value*0.5;});

will only yield the Julia method:

strict_half(arg1::Float64)

Note that in C++ the number value is accessed using the value member of StrictlyTypedNumber.

Customization

The automatic overloading can be customized. For example, to allow passing an Int64 where a UInt64 is normally expected, the following method can be added:

CxxWrap.argument_overloads(t::Type{UInt64}) = [Int64]

Integer types

Due to the fact that built-in integer types don't have an imposed size, they can't be mapped to Julia integer types in the same way on every platform. For CxxWrap, we take the following approach:

• Fixed-size types such as int32_t are mapped directly to their Julia equivalents
• Built-in types are mapped to a named type, e.g. the C++ type long becomes CxxLong in Julia. If in the given C++ implementation we have long == int64_t, then in Julia CxxLong will be an alias for Int64, otherwise it is its own bits type.

The following table gives an overview of the mapping, where some of the Cxx* types may actually be aliases for a Julia type:

C++	Julia
int8_t	Int8
uint8_t	UInt8
int16_t	Int16
uint16_t	UInt16
int32_t	Int32
uint32_t	UInt32
int64_t	Int64
uint64_t	UInt64
bool	CxxBool
char	CxxChar
wchar_t	CxxWchar
signed char	CxxSignedChar
unsigned char	CxxUChar
short	CxxShort
unsigned short	CxxUShort
int	CxxInt
unsigned int	CxxUInt
long	CxxLong
unsigned long	CxxULong
long long	CxxLongLong
unsigned long long	CxxULongLong

Pointers and references

Simple pointers and references are treated the same way, and wrapped in a struct with as a single member the pointer to the C++ object.

References to pointers

A reference to a pointer allows changing the referred object, e.g.:

void writepointerref(MyData*& ptrref)
{
  delete ptrref;
  ptrref = new MyData(30);
}

is called from Julia as:

d = PtrModif.MyData()
writepointerref(Ref(d))

Note that this modifies d itself, so d must be a MyDataAllocated. More details are in the pointer_modification example.

Reference to bool

In the Julia C calling convention, a boolean is a Cuchar, so to pass a reference to a boolean to C++ you need:

bref = Ref{Cuchar}(0)
boolref(bref)

Where boolref on the C++ side is:

mod.method("boolref", [] (bool& b)
{
  b = !b;
});

Strictly speaking, the representation of bool in C++ is implementation-defined, so this conversion relies on undefined behavior. Passing references to boolean is therefore not recommended, it is better to sidestep this by writing e.g. a wrapper function in C++ that returns a boolean by value.

Smart pointers

Currently, std::shared_ptr, std::unique_ptr and std::weak_ptr are supported transparently. Returning one of these pointer types will return an object inheriting from SmartPointer{T}:

types.method("shared_world_factory", []()
{
  return std::shared_ptr<World>(new World("shared factory hello"));
});

The shared pointer can then be used in a function taking an object of type World like this (the module is named CppTypes here):

swf = CppTypes.shared_world_factory()
CppTypes.greet(swf[])

Adding a custom smart pointer

Suppose we have a "smart" pointer type defined as follows:

template<typename T>
struct MySmartPointer
{
  MySmartPointer(T* ptr) : m_ptr(ptr)
  {
  }

  MySmartPointer(std::shared_ptr<T> ptr) : m_ptr(ptr.get())
  {
  }

  T& operator*() const
  {
    return *m_ptr;
  }

  T* m_ptr;
};

Specializing in the jlcxx namespace:

namespace jlcxx
{
  template<typename T> struct IsSmartPointerType<cpp_types::MySmartPointer<T>> : std::true_type { };
  template<typename T> struct ConstructorPointerType<cpp_types::MySmartPointer<T>> { typedef std::shared_ptr<T> type; };
}

Here, the first line marks our type as a smart pointer, enabling automatic conversion from the pointer to its referenced type and adding the dereferencing pointer. If the type uses inheritance and the hierarchy is defined using SuperType, automatic conversion to the pointer or reference of the base type is also supported. The second line indicates that our smart pointer can be constructed from a std::shared_ptr, also adding auto-conversion for that case. This is useful for a relation as in std::weak_ptr and std::shared_ptr, for example.

Function arguments

Because C++ functions often return references or pointers, writing Julia functions that operate on C++ types can be tricky. For example, writing a function like:

julia_greet(w::World) = greet_cpp(w)

If World is a type from C++, this will only work with objects that have been constructed directly or that were returned by value from C++. To make it work with references and pointers, we would need an additional method:

julia_greet(w::CxxWrap.CxxBaseRef{World}) = greet_cpp(w[])

Note that in the general case, both the signature and the implementation need to change, making this cumbersome when there are many functions like this. Enter the @cxxdereference macro. Declaring the function like this makes sure it can accept both values and references:

@cxxdereference julia_greet(w::World) = greet_cpp(w)

The @cxxdereference macro changes the function into:

function julia_greet(w::CxxWrap.reference_type_union(World))
    w = CxxWrap.dereference_argument(w)
    greet_cpp(w)
end

The type of w is now calculated by the CxxWrap.reference_type_union function, which resolves to Union{World, CxxWrap.CxxBaseRef{World}, CxxWrap.SmartPointer{World}}. The behavior of the macro can be customized by adding methods to CxxWrap.reference_type_union and CxxWrap.dereference_argument.

Exceptions

When directly adding a regular free C++ function as a method, it will be called directly using ccall and any exception will abort the Julia program. To avoid this, you can force wrapping it in an std::function to intercept the exception automatically by setting the jlcxx::calling_policy argument to std_function:

mod.method("test_exception", test_exception, jlcxx::calling_policy::std_function);

Member functions and lambdas are automatically wrapped in an std::function and so any exceptions thrown there are always intercepted and converted to a Julia exception.

Tuples

C++11 tuples can be converted to Julia tuples by including the containers/tuple.hpp header:

#include "jlcxx/jlcxx.hpp"
#include "jlcxx/tuple.hpp"

JLCXX_MODULE define_types_module(jlcxx::Module& containers)
{
  containers.method("test_tuple", []() { return std::make_tuple(1, 2., 3.f); });
}

Use in Julia:

using CxxWrap
using Base.Test

module Containers
  @wrapmodule(() -> libcontainers)
  export test_tuple
end
using Containers

@test test_tuple() == (1,2.0,3.0f0)

Working with arrays

Reference native Julia arrays

The ArrayRef type is provided to work conveniently with array data from Julia. Defining a function like this in C++:

void test_array_set(jlcxx::ArrayRef<double> a, const int64_t i, const double v)
{
  a[i] = v;
}

This can be called from Julia as:

ta = [1.,2.]
test_array_set(ta, 0, 3.)

The ArrayRef type provides basic functionality:

• iterators
• size
• [] read-write accessor
• push_back for appending elements

Note that ArrayRef only works with primitive types, if you need a "boxed" type it has to be made an array of Any with type ArrayRef<jl_value_t*> in C++.

Const arrays

Sometimes, a function returns a const pointer that is an array, either of fixed size or with a size that can be determined from elsewhere in the API. Example:

const double* const_vector()
{
  static double d[] = {1., 2., 3};
  return d;
}

In this simple case, the most logical way to translate this would be as a tuple:

mymodule.method("const_ptr_arg", []() { return std::make_tuple(const_vector().ptr[0], const_vector().ptr[1], const_vector().ptr[2]); });

In the case of a larger blob of heap-allocated data it makes more sense to convert this to a ConstArray, which implements the read-only part of the Julia array interface, so it exposes the data safely to Julia in a way that can be used natively:

mymodule.method("const_vector", []() { return jlcxx::make_const_array(const_vector(), 3); });

For multi-dimensional arrays, the make_const_array function takes multiple sizes, e.g.:

const double* const_matrix()
{
  static double d[2][3] = {{1., 2., 3}, {4., 5., 6.}};
  return &d[0][0];
}

// ...module definition skipped...

mymodule.method("const_matrix", []() { return jlcxx::make_const_array(const_matrix(), 3, 2); });

Note that because of the column-major convention in Julia, the sizes are in reversed order from C++, so the Julia code:

display(const_matrix())

shows:

3x2 ConstArray{Float64,2}:
 1.0  4.0
 2.0  5.0
 3.0  6.0

An extra file has to be included to have constant array functionality: #include "jlcxx/const_array.hpp".

Mutable arrays

Replacing make_const_array in the examples above by make_julia_array creates a mutable, regular Julia array with memory owned by C++.

Returning a Julia array

A Julia-owned Array can be created and returned from C++ using the jlcxx::Array class:

mymodule.method("array", [] () {
    jlcxx::Array<int> data{ };
    data.push_back(1);
    data.push_back(2);
    data.push_back(3);

    return data;
});

Calling Julia functions from C++

Direct call to Julia

Directly calling Julia functions uses jl_call from julia.h but with a more convenient syntax and automatic argument conversion and boxing. Use a JuliaFunction to get a functor that can be invoked directly. Example for calling the max function from Base:

mymodule.method("julia_max", [](double a, double b)
{
  jlcxx::JuliaFunction max("max");
  return max(a, b);
});

Internally, the arguments and return value are boxed, making this method convenient but slower than calling a regular C function.

Safe cfunction

The macro CxxWrap.@safe_cfunction provides a wrapper around Base.@cfunction that checks the type of the function pointer. Example C++ function:

mymodule.method("call_safe_function", [](double(*f)(double,double))
{
  if(f(1.,2.) != 3.)
  {
    throw std::runtime_error("Incorrect callback result, expected 3");
  }
});

Use from Julia:

testf(x,y) = x+y
c_func = @safe_cfunction(testf, Float64, (Float64,Float64))
MyModule.call_safe_function(c_func)

Using types different from the expected function pointer call will result in an error. This check incurs a runtime overhead, so the idea here is that the function is converted only once and then applied many times on the C++ side.

If the result of @safe_cfunction needs to be stored before the calling signature is known, direct conversion of the created structure (type SafeCFunction) is also possible. It can then be converted later using jlcxx::make_function_pointer:

mymodule.method("call_safe_function", [](jlcxx::SafeCFunction f_data)
{
  auto f = jlcxx::make_function_pointer<double(double,double)>(f_data);
  if(f(1.,2.) != 3.)
  {
    throw std::runtime_error("Incorrect callback result, expected 3");
  }
});

This method of calling a Julia function is less convenient, but the call overhead should be no larger than calling a regular C function through its pointer.

Adding Julia code to the module

Sometimes, you may want to write additional Julia code in the module that is built from C++. To do this, call the wrapmodule method inside an appropriately named Julia module:

module ExtendedTypes

using CxxWrap
@wrapmodule(() -> "libextended")
export ExtendedWorld, greet

end

Here, ExtendedTypes is a name that matches the module name passed to create_module on the C++ side. The @wrapmodule call works as before, but now the functions and types are defined in the existing ExtendedTypes module, and additional Julia code such as exports and macros can be defined.

It is also possible to replace the @wrapmodule call with a call to @readmodule and then separately call @wraptypes and @wrapfunctions. This allows using the types before the functions get called, which is useful for overloading the argument_overloads with types defined on the C++ side.

Overriding finalization behavior

By default, objects that are allocated from Julia are also destroyed through a finalizer that calls delete. If you want to override this behavior, you can specialize the jlcxx::Finalizer class as follows, for example only doing something special in the case a tye has a getImpl function:

namespace jlcxx
{
  template<typename T>
  struct Finalizer<T, SpecializedFinalizer>
  {
    static void finalize(T* to_delete)
    {
      constexpr bool has_getImpl = requires(const T& t) {
        t.getImpl();
      };

      if constexpr(has_getImpl) {
        std::cout << "calling specialized delete" << std::endl;
        delete to_delete;
      } else {
        delete to_delete;
      }
    }
  };
}

You can also further specialize on T to get specific behavior depending on the concrete type.

STL support

Julia Type Name	STL container	CxxWrap Version
StdString	std::string	v0.9.0+
StdVector	std::vector	v0.9.0+
StdValArray	std::valarray	v0.9.0+
StdDeque	std::deque	v0.13.4+
StdQueue	std::queue	v0.15.0+
StdPriorityQueue	std::priority_queue	To be released
StdStack	std::stack	To be released
StdSet	std::set	v0.16.0+
StdMultiset	std::multiset	v0.16.0+
StdUnorderedSet	std::unordered_set	To be released
StdUnorderedMultiset	std::unordered_multiset	To be released
StdList	std::list	To be released
StdForwardList	std::forward_list	To be released

View StdLib to check available methods. The containers have iterators defined, and hence are iterable.

To add support for e.g. vectors of your own type World, either just add methods that use an std::vector<World> as an argument, or manually wrap them using jlcxx::stl::apply_stl<World>(mod);. For this to work, add #include "jlcxx/stl.hpp" to your C++ file.

If the type World contains methods that take or return std:: collections of type World or World*, however, you must first complete the type, so that CxxWrap can generate the type and the template specializations for the std:: collections. In this case, you can add those methods to your type like this:

jlcxx::stl::apply_stl<World*>(mod);
mod.method("getSecondaryWorldVector", [](const World* p)->const std::vector<World*>& {
    return p->getSecondaries();
});

Linking wrappers using STL support requires adding JlCxx::cxxwrap_julia_stl to the target_link_libraries command in CMakeLists.txt.

Working with StdString

The StdString implements the Julia string interface and interprets std::string data as UTF-8 data. Since C++ strings do not require the use of the null-character to denote the end of a string the StdString constructors usually rely on the ncodeunits to determin the size of the string. When constructing a StdString from a Cstring, Base.CodeUnits, or Vector{UInt8} the first null-character present will denote the end of the string.

Release procedure

Often, new releases of CxxWrap also require a new release of the C++ component libcxxwrap-julia, and a rebuild of its JLL package. To make sure everything is tested properly, the following procedure should be followed for each release that requires changing both the Julia and the C++ component:

1. Merge the changes to CxxWrap into the testjll branch
2. Create a PR in libcxxwrap-julia with the required changes there and make sure it passes all tests. These tests will run using the CxxWrap#testjll branch.
3. Merge the libcxxwrap-julia PR. This will build and publish a JLL, available through the CxxWrapTestRegistry
4. Make a PR in CxxWrap to merge testjll into prerelease. Verify that the tests pass (rerun them if needed, since the first push to testjll will have used the old JLL version). Don't merge this PR yet.
5. Tag the next libcxxwrap-julia release and update to this new release in Yggdrasil
6. Wait for the new JLL to appear in the registry and then merge the PR from point 4. Verify that the tests running on the prerelease branch pass, by merging the PR from point 4. The difference with the tests in the testjll branch is that the prerelease branch tests using the JLL in the Julia General repository.
7. Merge the CxxWrap prerelease branch into main and create a new release using Registrator.

Breaking changes for CxxWrap 0.7

• JULIA_CPP_MODULE_BEGIN and JULIA_CPP_MODULE_END no longer exists, define a function with return type JLCXX_MODULE in the global namespace instead. By default, the Julia side expects this function to be named define_julia_module, but another name can be chosen and passed as a second argument to @wrapmodule.

• wrap_modules is removed, replace wrap_modules(lib_file_path) with:

  module Foo
    using CxxWrap
    @wrapmodule(lib_file_path)
  end

• export_symbols is removed, since all C++ modules are now wrapped in a corresponding module declared on the Julia side, so the regular Julia export statement can be used.

• safe_cfunction is now a macro, just like cfunction became a macro in Julia.

• Precompilation: add this function after the @wrapmodule macro:

  function __init__()
    @initcxx
  end

Breaking changes in v0.9

• No automatic conversion between Julia String and std::string, but StdString (which maps std::string) implements the Julia AbstractStringinterface.
• No automatic dereference of const ref
• ArrayRef no longer supports boxed values
• Custom smart pointer: use jlcxx::add_smart_pointer<MySmartPointer>(module, "MySmartPointer")
• IsMirroredType instead of IsImmutable and IsBits, added using map_type. By default, IsMirroredType is true for trivial standard layout types, so if you want to wrap these normally (i.e. you get an unexpected error Mirrored types (marked with IsMirroredType) can't be added using add_type, map them directly to a struct instead and use map_type) then you have to explicitly disable the mirroring for that type:

  template<> struct IsMirroredType<Foo> : std::false_type { };

• box C++ function takes an explicit template argument
• Introduction of specific integer types, such as CxxBool, that map to the C++ equivalent (should be transparent except for template parameters)
• Defining SuperType on the C++ side is now necessary for any kind of casting to base class, because the previous implementation was wrong in the case of multiple inheritance.
• Use Ref(CxxPtr(x)) for pointer or reference to pointer
• Use CxxPtr{MyData}(C_NULL) instead of nullptr(MyData)
• Defining a C++ supertype in C++ must now be done using the jlcxx::julia_base_type<T>() function instead of jlcxx::julia_type<T>()

Breaking changes in v0.10

• Requires Julia 1.3 for the use of JLL packages
• Reorganized integer types so the fixed-size types always map to built-in Julia types

Breaking changes in v0.13

• Automatic dereferencing of smart pointers was removed, so some code may require adding the dereferencing operator [] explicitly. See PR #338.

Breaking changes in v0.15

• This release is based on libcxxwrap-julia 0.12, which is binary incompatible with previous versions, so JLLs should be rebuilt to use CxxWrap 0.15
• The constructor method now takes a jlcxx::finalize_policy instead of a bool, e.g. .constructor<Foo>(false) becomes .constructor<Foo>(jlcxx::finalize_policy::no)

Breaking changes in v0.16

There was no change in the API, but because of a change in the way the mapping between C++ and Julia types is implemented the C++ modules need to be recompiled against libcxxwrap-julia 0.13. The reason for this change is that the old method caused crahses on macOS with Apple CPUs (M1, ...).

References

• JuliaCon 2020 Talk: Julia and C++: a technical overview of CxxWrap.jl
• JuliaCon 2020 Workshop: Wrapping a C++ library using CxxWrap.jl