CXX Error Handling: PNG Example
A prototype of a PNG decoder illustrates what can be done when the successful result cannot be passed across the FFI boundary:
#[cxx::bridge(namespace = "gfx::rust_bindings")]
mod ffi {
extern "Rust" {
/// This returns an FFI-friendly equivalent of `Result<PngReader<'a>,
/// ()>`.
fn new_png_reader<'a>(input: &'a [u8]) -> Box<ResultOfPngReader<'a>>;
/// C++ bindings for the `crate::png::ResultOfPngReader` type.
type ResultOfPngReader<'a>;
fn is_err(self: &ResultOfPngReader) -> bool;
fn unwrap_as_mut<'a, 'b>(
self: &'b mut ResultOfPngReader<'a>,
) -> &'b mut PngReader<'a>;
/// C++ bindings for the `crate::png::PngReader` type.
type PngReader<'a>;
fn height(self: &PngReader) -> u32;
fn width(self: &PngReader) -> u32;
fn read_rgba8(self: &mut PngReader, output: &mut [u8]) -> bool;
}
}
PngReader
and ResultOfPngReader
are Rust types --- objects of these types cannot cross the FFI boundary without indirection of a Box<T>
. We can't have an out_parameter: &mut PngReader
, because CXX doesn't allow C++ to store Rust objects by value.
This example illustrates that even though CXX doesn't support arbitrary generics nor templates, we can still pass them across the FFI boundary by manually specializing / monomorphizing them into a non-generic type. In the example ResultOfPngReader
is a non-generic type that forwards into appropriate methods of Result<T, E>
(e.g. into is_err
, unwrap
, and/or as_mut
).