Magick::Exception Classes
Exception
represents the base class of objects thrown when
Magick++reports an error. Magick++ throws C++ exceptions synchronous
with the operation where the error occurred. This allows errors to be
trapped within the enclosing code (perhaps the code to process a
single image) while allowing the code to be written with a simple
coding style.
A try/catch block should be placed around any sequence of
operations which can be considered an important body of work. For
example, if your program processes lists of images and some of these
images may be defective, by placing the try/catch block around the
entire sequence of code that processes one image (including
instantiating the image object), you can minimize the overhead of
error checking while ensuring that all objects created to deal with
that object are safely destroyed (C++ exceptions unroll the stack
until the enclosing try block, destroying any created objects).
The pseudo code for the main loop of your program may look like:
for infile in list
{
try {
// Construct an image instance first so that we don't have to worry
// about object construction failure due to a minor warning exception
// being thrown.
Magick::Image image;
try {
// Try reading image file
image.read(infile);
}
catch( Magick::WarningCoder &warning )
{
// Process coder warning while loading file (e.g. TIFF warning)
// Maybe the user will be interested in these warnings (or not).
// If a warning is produced while loading an image, the image
// can normally still be used (but not if the warning was about
// something important!)
cerr << “Coder Warning: “ << warning.what() << endl;
}
catch( Magick::Warning &warning )
{
// Handle any other Magick++ warning.
cerr << “Warning: “ << warning.what() << endl;
}
catch( Magick::ErrorFileOpen &error )
{
// Process Magick++ file open error
cerr << “Error: “ << error.what() << endl;
continue; // Try next image.
}
try {
image.rotate(90);
image.write(“outfile”);
}
catch ( MagickExeption & error)
{
// Handle problem while rotating or writing outfile.
cerr << “Caught Magick++ exception: “ << error.what() << endl;
}
}
catch( std::exception &error )
{
// P
rocess any other exceptions derived from standard C++ exception
err << “Caught C++ STD exception: “ << error.what() << endl;
}
catch( ... )
{
// P
rocess *any* exception (last-ditch effort). There is not a lot
// you can do here other to retry the operation that failed, or exit
// the program.
}
}
The desired location and number of try/catch blocks in your program
depends how sophisticated its error handling must be. Very simple
programs may use just one try/catch block.
The
Exception
class is derived from the C++ standard
exception class. This means that it contains a C++ string containing
additional information about the error (e.g to display to the user).
Obtain access to this string via the what() method.  For
example:
catch( Exception &error_ )
{
cout << "Caught exception: " << error_.what()
<< endl;
}
The classes
Warning
and
Error
derive from the
Exception
class. Exceptions derived from
Warning
are
thrown to represent non-fatal errors which may effect the
completeness or quality of the result (e.g. one image provided as an
argument to montage is defective). In most cases, a
Warning
exception may be ignored by catching it immediately, processing it
(e.g. printing a diagnostic) and continuing on. Exceptions derived
from
Error
are thrown to represent fatal errors that can not
produce a valid result (e.g. attempting to read a file which does not
exist).
The specific derived exception classes
are shown in the following tables:
Warning Sub-Classes
Warning
Warning Description
WarningUndefined
Unspecified warning type.
WarningBlob
NOT
CURRENTLY USED
WarningCache
NOT
CURRENTLY USED
WarningCoder
Warnings issued by some coders.
WarningConfigure
NOT
CURRENTLY USED
WarningCorruptImage
Warning issued when an image is determined to be
corrupt.
WarningDelegate
Warnings reported by the delegate (interface to
external programs) subsystem.
WarningDraw
Warnings reported by the rendering subsystem.
WarningFileOpen
Warning reported when The image file could not be
opened (permission problem, wrong file type, or does not exist).
WarningImage
NOT CURRENTLY USED
WarningMissingDelegate
NOT CURRENTLY USED
WarningModule
NOT CURRENTLY USED
WarningMonitor
NOT CURRENTLY USED
WarningOption
Warning reported when an option is malformed or
out of range.
WarningRegistry
NOT CURRENTLY USED
WarningResourceLimit
Warning reported when a program resource is
exhausted (e.g. not enough memory).
WarningStream
NOT CURRENTLY USED
WarningType
NOT CURRENTLY USED
WarningXServer
Warnings reported by the X11 subsystem.
Error Sub-Classes
Error
Error Description
ErrorUndefined
Unspecified error type.
ErrorBlob
Error reported by BLOB I/O subsystem.
ErrorCache
Error reported by the pixel cache subsystem.
ErrorCoder
Error reported by coders (image format support).
ErrorConfigure
Errors reported while loading configuration files.
ErrorCorruptImage
Error reported when the image file is corrupt.
ErrorDelegate
Errors reported by the delegate (interface to
external programs) subsystem.
ErrorDraw
Error reported while drawing on image.
ErrorFileOpen
Error reported when the image file can not be
opened.
ErrorImage
Errors reported while drawing.
ErrorMissingDelegate
Error reported when an add-on library or program
is necessary in order to support the requested operation.
ErrorModule
Errors reported by the module loader subsystem.
ErrorMonitor
NOT CURRENTLY USED
ErrorOption
Error reported when an option is malformed or out
of range.
ErrorRegistry
Errors reported by the image/BLOB registry
subsystem.
ErrorResourceLimit
Error reported when a program resource is
exhausted (e.g. not enough memory).
ErrorStream
Errors reported by the pixel stream subsystem.
ErrorType
Errors reported by the type (font) rendering
subsystem.
ErrorXServer
Errors reported by the X11 subsystem.
