# error

© 2005,2008,2010 John Abbott
GNU Free Documentation License, Version 1.2

- Examples
- User documentation
  - Debugging
  - Recommended way of reporting errors
  - Adding a New Error ID and its Default Message
  - Information about errors -- for the more advanced
  - Choosing the language for error messages
- Maintainer documentation for files error.H and error.C
  - To Add a New Error Code and Message
  - To Add a New Language for Error Messages
- Bugs, Shortcomings, and other ideas
  - new improved list of errors
- Main changes

CoCoALib Documentation Index

## Examples

- ex-error1.C

## User documentation

### Debugging

If you get a `CoCoA ERROR` when you execute your program, you can easily intercept it with your preferred debugger tool.

For example, when debugging with gdb, type

      break CoCoA::ThrowError

and then `run`. When it stops in the call of `ThrowError`, type `up` to reach the line which originally caused the error.

### Recommended way of reporting errors

Usually if you have detected an error in your program, you want to report it immediately. We recommend that you use the macro `CoCoA_ERROR` to do this. Here's an example:

          value_t operator/(const value_t& num, const value_t& den)
          {
            if (IsZero(den))
              CoCoA_ERROR(ERR::DivByZero, "operator/ for value_t");
            ....
          }

The first argument should be an error ID (i.e. ERR::something); you can find a list of the IDs in the file (CoCoA_ROOT)/include/CoCoA/error.H. If no ID is suitable, you can just put a string instead. The second argument should be an indication of the function in which the error occurred.

### Adding a New Error ID and its Default Message

If you are a CoCoALib contributor and want to add a new error ID and message (or even a new language for error messages), please read the maintainer documentation for what to do.

### Information about errors -- for the more advanced

The macro `CoCoA_ERROR` does two things:

- (1) it creates a `CoCoA::ErrorInfo` object with the parameters given to the macro, and also with the filename and line number;
- (2) it calls the function `CoCoA::ThrowError` on the `ErrorInfo` object just created.

Below we explain these two stages in more detail.

The class `CoCoA::ErrorInfo` is intended to be used for creating exception objects -- indeed, it derives from `std::exception`. There are two things you are likely to want to do with exception objects:

- (A) create and throw the exception object
- (B) catch the exception object

- Case (A) Rather than using using the C++ `throw` command directly, we recommend that you pass the error object to the function `CoCoA::ThrowError` as it makes debugging easier (see above).

  We also recommend that you use the constructor which takes a `CoCoA::ERR::ID` and a string; the string should indicate where the error was detected, e.g. the name of the C++ function which found the error. Look through the list of `CoCoA::ERR::ID`s (in the file `error.H`) to find the one best suited to the type of error you wish to signal.

  If no error `CoCoA::ERR::ID` is suitable then you can use the constructor which accepts two string arguments: the first should be a description of the error (e.g. "Incompatible hypermatrix dimensions"), and the second should indicate where the error was detected. If you are a CoCoALib contributor, see the notes below about how to add a new error ID and message.

NOTE: if you set the C++ preprocessor symbol `CoCoA_DEBUG` to a value greater than 1 then a log message is produced each time `CoCoA::ThrowError` is called; the log message is printed on `std::clog`.

- Case (B) After catching a `CoCoA::ErrorInfo` object in the variable `err` you can make the following queries:

          err == ERR::ErrorID  -- returns true iff err is of type ERR::ErrorID
                           (replace ErrorID by the ID of the error you want!)
          err.what()  -- returns a C string being the error message;

  EXAMPLE (of handling a CoCoA Error):

          try { ... }
          catch (const CoCoA::ErrorInfo& err)
          {
            if (err != ERR::DivByZero) throw; // rethrow unexpected error
            // code to handle the "expected" division by zero error
          }

  If you have caught a `CoCoA::ErrorInfo` object and want to announce it as an error then call the procedure `CoCoA::ANNOUNCE` with the ostream on which to make the announcement and the ErrorInfo object as second argument. This will print an eye-catching error announcement, and then return to the caller. Note that `CoCoA::ANNOUNCE` does not cause the program to exit/abort, it merely prints out an eye-catching announcement.

  To facilitate debugging, an `ErrorInfo` object may be printed in the usual way; this produces a modest message, clearly different from an error announcement.

Recall that, as for any other "exception object", simply creating a `CoCoA::ErrorInfo` object does not cause the error condition to be signalled. To signal the error it must be thrown -- we recommend passing the error object to the function `CoCoA::ThrowError` (see above).

### Choosing the language for error messages

You may choose the language for CoCoALib error messages: the default is English. If an error message has not yet been translated into the chosen language then it is replaced by the default english message. Currently there are only two choices:

       ErrorLanguage::english();

       ErrorLanguage::italian();

EXAMPLE:

        int main()
        {
          CoCoA::ErrorLanguage::italian(); // vogliamo messaggi d'errore in italiano
          ....
        }

The language for error messages may be changed any number of times: the last chosen language is the one used when creating an `ErrorInfo` object.

## Maintainer documentation for files error.H and error.C

`CoCoA::ErrorInfo` is derived from `std::exception` for compatibility with the rest of C++. How this might be useful I cannot yet say, but it does not measurably complicate the code (though it does force the implementation of a member function called `what`).

The preferred constructors for `ErrorInfo` are those accepting an `ERR::ID` and a C string indicating context (with or without filename and line number information); the other constructors should be used only when no suitable `ERR::ID` exists. The `ERR::ID` object indicates the general nature of the error, and is used for selecting the error message to be printed.

Note that the conversion from an `ERR::ID` to a string is slightly convoluted: this is to allow the possibility of selecting at run-time a language other than English for the error messages.

I chose not to offer an `ErrorInfo` constructor which accepts natively `const char*` args because the potential extra copying of strings (to construct a `std::string`) is hardly likely to be important, and `std::string`s feel much cleaner.

The nature and context of the error are kept separate in an `ErrorInfo` object since it is possible that we may wish to propagate the nature of the error to top level in an interactive system where it would be unhelpful or confusing to refer to some C++ function irrelevant to the user.

The definition of the function `CoCoA::ThrowError` is quite straightforward. The function is deliberately not inline: efficiency is wholly unimportant whereas the ability to set a breakpoint in the function is (some debuggers may be unable to set a breakpoint in an inline function).

Each CoCoA error ID object is in reality a constant global variable containing two pointers to constant C strings called `myName` and `myDefaultMesg`: the latter contains the associated default error message (which must be in English), and the former contains the name of the error ID object. The identity of the error ID actually resides in the address of the specific string constant in the data member `myName` -- this implies that copying the ID object does not change its identity. Since the different objects necessarily have different names, the string literals containing those names are surely distinct, and so we are guaranteed that these addresses are distinct. There are comparison operators (equal, not-equal, and less-than) for `ERR::ID`; less-than is needed for using C++ maps when implementing error messages in languages other than english. These comparison operators merely conduct the required comparison on the addresses of the strings in `myName`; this is quick and simple, and sufficient for our purposes -- the actual values of strings pointed to are not taken into consideration!

### To Add a New Error Code and Message

Invent a new name for the error code, and insert it in the list of names of "error variables" (in the file error.H). Make sure you insert it in the right place respecting alphabetical order -- this way it is easy to check whether a name is already present in the list. Add a short comment indicating what sort of error that code is to be used for.

Next you must add a message corresponding to that code. In the file error.C you will find a long list of "error variable" initializations. Add an initialization for your new "error variable" -- the syntax is quite obvious from the other initializations there (which use the macro `DEFINE_ERROR`). You may wish to add translations of your new error message into the other languages present in error.C.

### To Add a New Language for Error Messages

You must write a function analogous to the function italian() which resides inside the namespace `CoCoA::ErrorLanguage`. The new function must have a name different from the other functions there: I suggest the english name of the language. Inside the function you will have to fill a `MsgTable_t` object with the translated messages associated to each possible error code. At the end you should check to make sure that you have a message for each possible code: it should suffice simply to count them. The code will still compile and run even if some translated messages are missing: if an error occurs for which the current error language has no translation then the default (english) message is printed.

EXAMPLE: Suppose we want to add german error messages. We choose to use the name "german" for the function which activates german error messages. Here is what we do:

(1) edit error.H;
  immediately after the line containing "void italian();" insert "void german();"

(2) edit error.C;
  make a copy of the function italian(){...} and change its name to "german" -- make sure you stay inside namespace `ErrorLanguage`; translate all the error messages in the strings.

## Bugs, Shortcomings, and other ideas

The throw specifications on the destructor and `what` member function are needed for compatibility with `std::exception` -- I regard this as a nuisance. I wonder if `std::string::c_str` can throw?

What about parameter values? In some cases it would be handy to give the bad value which caused the error: e.g. "Bad characteristic: 33". A problem is that a parameter value could be very large. We could simply allow up to 100 (say) characters of parameter information in a suitable string.

Only very few error messages have been translated into italian so far.

Perhaps allow the user to specify which ostream to print the logging message in `ThrowError`?

### new improved list of errors

work in progress

- SERIOUS
- NYI
- nonstandard (specified by a string) -- equality test makes sense for these?
- system/LowLevel (about GlobalMgr, MemPool, I/O, AssertFail)
  - InsuffPrecision???
- BadArg (UnsuitableValue)
  - NotNonZero RENAME ExpectedNonZero
  - IndexOutOfRange (incl iterators?)
  - ValueOutOfRange
    - NotNonNegative (RENAME ExpectedNonNegative?)
    - NotPositive (RENAME ExpectedNonNegative?)
    - TooBig (*not* system independent!!!)
- BadArgType
- IncompatibleArgs (op not def'd for the given combination of args)
  - IncompatibleSizes
  - IncompatibleTypes

## Main changes

2013

- July (v0.995??):
  - major re-write in progress -