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C++ bindings for Rust enums

A Rust enum is mapped to an opaque C++ type.

To receive C++ bindings, the enum must be movable in C++. See Movable Types.

Example

Given the following Rust crate:

{{ #include ../../examples/rust/enum/example.rs }}

Crubit will generate the following bindings:

{{ #include ../../examples/rust/enum/example_generated.h }}

Why isn't it a C++ enum? {#cpp_enum}

A repr(i32) or fieldless repr(C) enum is very similar to a C++ enum. However, Rust enums are exhaustive: any value not explicitly listed in the enum declaration does not exist, and it is undefined behavior to attempt to create one.

C++ enums, in contrast, are "non-exhaustive": a C++ enum can have any value supported by the underlying type, even one not listed in the enumerators. For example, if the above example were a C++ enum, static_cast<Color>(42) would be a valid instance of Color, even though neither Red, Blue, nor Green have that value.

In order to prevent invalid Rust values from being produced by C++, a C++ enum cannot be used to represent a Rust enum. Instead, the C++ bindings are a struct, even for fieldless enums.

C++ movable {#cpp_movable}

To receive C++ bindings, the enum must be movable in C++. See Movable Types.

Constructing Rust enums from C++

C++ bindings for Rust enums provide a static Make<variant name> method for each of enum variants. These methods can be used to construct an enum value with the corresponding variant.

Example

Given the following Rust crate:

{{ #include ../../examples/rust/enum_with_payload/example.rs }}

Crubit will generate the following bindings:

{{ #include ../../examples/rust/enum_with_payload/example_generated.h }}

Known issues

The following bugs track future work in this area:

  • b/487357254: Constructing variants with a struct payload
  • b/489085607: Bindings for constructing enums should be constexpr