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3f35a93 Jan 3, 2018
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Generating Bindings to C++

bindgen can handle a some C++ features, but not all of them. To set expectations: bindgen will give you the type definitions and FFI declarations you need to build an API to the C++ library, but using those types in Rust will be nowhere near as nice as using them in C++. You will have to manually call constructors, destructors, overloaded operators, etc yourself.

When passing in header files, the file will automatically be treated as C++ if it ends in .hpp. If it doesn't, adding -x c++ clang args can be used to force C++ mode. You probably also want to use -std=c++14 or similar clang args as well.

You pretty much must use whitelisting when working with C++ to avoid pulling in all of the std::* types, many of which bindgen cannot handle. Additionally, you may want to mark other types as opaque that bindgen stumbles on. It is recommended to mark all of std::* opaque, and to whitelist only precisely the functions and types you intend to use.

You should read up on the FAQs as well.

Supported Features

  • Inheritance (for the most part; there are some outstanding bugs)

  • Methods

  • Bindings to constructors and destructors (but they aren't implicitly or automatically invoked)

  • Function and method overloading

  • Templates without specialization. You should be able to access individual fields of the class or struct.

Unsupported Features

When bindgen finds a type that is too difficult or impossible to translate into Rust, it will automatically treat it as an opaque blob of bytes. The philosophy is that

  1. we should always get layout, size, and alignment correct, and

  2. just because one type uses specialization, that shouldn't cause bindgen to give up on everything else.

Without further ado, here are C++ features that bindgen does not support or cannot translate into Rust:

  • Inline functions and methods: see "Why isn't bindgen generating bindings to inline functions?"

  • Template functions, methods of template classes and structs. We don't know which monomorphizations exist, and can't create new ones because we aren't a C++ compiler.

  • Anything related to template specialization:

    • Partial template specialization
    • Traits templates
    • Specialization Failure Is Not An Error (SFINAE)
  • Cross language inheritance, for example inheriting from a Rust struct in C++.

  • Automatically calling copy and/or move constructors or destructors. Supporting this isn't possible with Rust's move semantics.

  • Exceptions: if a function called through a bindgen-generated interface raises an exception that is not caught by the function itself, this will generate undefined behaviour. See the tracking issue for exceptions for more details.