# Treating a Type as an Opaque Blob of Bytes
Sometimes a type definition is simply not translatable to Rust, for example it
uses
[C++'s SFINAE](https://en.wikipedia.org/wiki/Substitution_failure_is_not_an_error) for
which Rust has no equivalent. In these cases, it is best to treat all
occurrences of the type as an opaque blob of bytes with a size and
alignment. `bindgen` will attempt to detect such cases and do this
automatically, but other times it needs some explicit help from you.
### Library
* [`bindgen::Builder::opaque_type`](https://docs.rs/bindgen/0.23.1/bindgen/struct.Builder.html#method.opaque_type)
### Command Line
* `--opaque-type <type>`
### Annotation
```cpp
/// <div rustbindgen opaque></div>
class Foo {
// ...
};
```