Errors¶

SDK functions that can fail return their error as a value, not as an exception. Modules are separate libraries, and an exception must not cross from one to another, so every fallible call returns a rex::Result from rex/result.h.

Result¶

Result<T> is std::expected<T, Error>. It holds either the value of type T or an Error. Result<void> holds no value on success. Test a result with if (result), read the value with *result and the error with result.error().

The header provides two helpers to build results:

Ok() returns a success with no value, and Ok(value) a success with a value.
Err(error) returns a failure. Err(category, message) and Err(category, message, code) build the Error in place.

Three macros return early from a function that itself returns a Result:

REX_TRY(decl, expr) evaluates expr, returns its error on failure, and otherwise moves its value into the declaration decl.
REX_TRY_VOID(expr) returns the error of expr on failure, and discards any value.
REX_ERROR_IF(cond, error) returns error when cond is true.

rex::Result<void> LoadAndLaunch(rex::kernel::TitleRuntime& runtime) {
  REX_TRY_VOID(runtime.Load());
  return runtime.Launch();
}

Error¶

An Error holds three fields:

category, an ErrorCategory that says what type of failure it is.
message, the error text.
code, a code from the platform or a library, or the value of an error enum. It is 0 when there is none.

GetDescription returns Success for the category kNoError, and otherwise the message, with the code appended when it is not 0. Error::FromErrno builds an error from an errno value or another platform code.

The categories are kIO, kMemory, kFormat, kCrypto, kCompression, kRuntime, kPlatform, kConfig, kValidation, kNotFound, kNotImplemented and kModule. A default constructed Error has the category kNoError, which IsSuccess reports as true.

Error Enums¶

Two parts of the SDK need finer codes than a category. Each declares an error enum, and a BuildError function that puts the enum value in Error::code. CodeOf reads the enum value back from an error.

Enum	Header	Category of the error
`FileSystemError`	`rex/filesystem/file_system_error.h`	`kNotFound` for `kNotFound`, `kPathNotFound` and `kNoMount`, `kFormat` for `kCorrupt`, `kNotImplemented` for `kUnsupported`, and `kIO` for the others.
`HIDError`	`rex/xam/input.h`	Always `kPlatform`.

if (auto opened = fs.Open(path, options); !opened &&
    rex::filesystem::CodeOf(opened.error()) == rex::filesystem::FileSystemError::kNoMount) {
  // The path is on no mount.
}