Cairo: A Vector Graphics Library: Error handling

## Error handling

The error handling approach in C for Cairo has multiple elements:

- When a method on an object fails, the object is put into an error state. Subsequent operations on the object do nothing. The status of the object can be queried with a function like `status()`.

- Constructors, rather than returning `NULL` on out-of-memory failure, return a special singleton object on which all operations do nothing. Retrieving the status of the singleton object returns `CAIRO_STATUS_NO_MEMORY`

  Is this going to apply to cairo_surface_t as well?

  What about cairo_copy_path_data()? It's probably going to have to return `NULL`.

- Errors propagate from object to object. Setting a pattern in an out-of-memory state as the source of a cairo_t puts the type into an error state.

Much of the above is not yet implemented at the time of this writing

A language binding could copy the C approach, and for a language without exceptions, this is likely the right thing to do. However, for a language with exceptions, exposing a completely different style of error handling for cairo would be strange. So, instead, status should be checked after every call to cairo, and exceptions thrown as necessary.

One problem that can arise with this, in languages where handling exceptions is mandatory (like Java), is that almost every cairo function can result in a status being set, usually because of an out-of-memory condition. This could make cairo hard to use. To resolve this problem, let's classify then cairo status codes:

    /* Memory */
    CAIRO_STATUS_NO_MEMORY,

    /* Programmer error */
    CAIRO_STATUS_INVALID_RESTORE
    CAIRO_STATUS_INVALID_POP_GROUP
    CAIRO_STATUS_NO_CURRENT_POINT
    CAIRO_STATUS_INVALID_MATRIX
    CAIRO_STATUS_NO_TARGET_SURFACE
    CAIRO_STATUS_INVALID_STRING
    CAIRO_STATUS_SURFACE_FINISHED
    CAIRO_STATUS_BAD_NESTING

    /* Language binding implementation */
    CAIRO_STATUS_NULL_POINTER
    CAIRO_STATUS_INVALID_PATH_DATA
    CAIRO_STATUS_SURFACE_TYPE_MISMATCH

    /* Other */
    CAIRO_STATUS_READ_ERROR
    CAIRO_STATUS_WRITE_ERROR

If we look at these, the `CAIRO_STATUS_NO_MEMORY` should map to the native out-of-memory exception, which could happen at any point in any case. Most of the others indicate programmer error, and handling them in user code would be silly. These should be mapped into whatever the language uses for assertion failures, rather than errors that are normally handled. (In Java, a subclass of Error rather than Exception, perhaps.) And `CAIRO_STATUS_READ_ERROR`, and `CAIRO_STATUS_WRITE_ERROR` can occur only in very specific places. (In fact, as described in the section called “Streams and File I/O”, these errors may be mapped into the language's native I/O error types.) So, there really aren't exceptions that the programmer must handle at most points in the Cairo API.