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This DIP is currently under construction. Please come back later.

Title: A new exception hierarchy
DIP: 33
Version: 1
Status: Draft
Created: 2013-04-01
Last Modified: 2013-04-01
Author: Lars T. Kyllingstad


This is a proposal for a new exception hierarchy for Druntime and Phobos.



The following is an outline of the exceptions in the hierarchy, and how they are related to each other. Deeper levels are subclasses of those above.

  • Throwable
    • Error
      • AssertError
      • InvalidArgumentError
      • RangeError
    • Exception
      • ConversionException
      • Exception
      • EncodingException
      • FilesystemException
      • IOException
        • NetworkException
      • ParseException
        • DocParseException
      • ProcessException
      • SystemException
        • ErrnoException
        • WinAPIException
      • ThreadException
    • OutOfMemory

Top-level classes

Strictly speaking, Throwable is of course the (only) top-level exception class. Here, however, we discuss its direct descendants, from which all other exception classes derive.


class Error : Throwable { }

Error and its subclasses are used to signal programming errors. If an Error is thrown, it means that there is something wrong with how the program is constructed. Examples include array index out of bounds, invalid function arguments, etc. Importantly, it should always be possible to avoid an Error by design.

In general, Errors should not be caught, primarily because they indicate that the program logic is compromised, and that the program may therefore be in an invalid state from which there is no recovery. Furthermore, one cannot rely on them being thrown at all. For example, assert statements and array bounds checks, which both trigger Errors, may be disabled by compiler switches.

If an Error must be caught, it is recommended to do so at a very high level (e.g. in main()), and then only to perform critical cleanup work before terminating the program gracefully.


class Exception : Throwable { }

Exception and its descendants are used to signal normal runtime errors. These are exceptional circumstances that the programmer cannot be expected to avoid by design. Examples include file not found, problems with parsing a document, system errors, etc.


class OutOfMemory : Throwable { }

This exception is thrown on an attempt to allocate more memory than what is currently available for the program. Strictly speaking, this is not an Error, as the programmer cannot reasonably be expected to check memory availability before each allocation. However, is not desirable to catch it along with normal Exceptions either, as an out-of-memory condition requires special treatment. Therefore, this DIP places OutOfMemory at the top level of the hierarchy, alongside Error and Exception.


Here follows a more detailed description of the various Error subclasses.


class AssertError : Error { }

This error is thrown when an assert statement fails.


class InvalidArgumentError : Error { }

This error is thrown when one or more function arguments are invalid. Since it is an Error, it should only be used to signal errors that the programmer (i.e. the user of the function in question) can reasonably be expected to avoid, and which are not too costly to check. Circumstances that are out of the programmer's control, or which are so expensive to verify that it is undesirable to have them checked by both the caller and the callee, should be signalled with an Exception instead.

void processFile(string path)
    // The following is an acceptable use of InvalidArgumentError,
    // as the function should never be given an empty path, and the
    // check is trivial.
    if (path.empty)
        throw new InvalidArgumentError("path is empty");

    // The function caller should not be expected to verify file existence.
    // Firstly, it could change between the time it is checked and the time
    // the function is called, and secondly, it requires filesystem lookup
    // which is a relatively expensive operation.
    if (!exists(path))
        throw new FilesystemException("File not found: "~path);


class RangeError : Error { }

This error is thrown on illegal range operations. Examples include when an array index is out of bounds, when front or popFront() is called on an empty range, etc.

struct MyRange(T)
    @property bool empty() { ... }
    @property T front()
        if (empty) throw new RangeError("front called on empty range");
    void popFront()
        if (empty) throw new RangeError("popFront() called on empty range");


This document has been placed in the Public Domain.