MATLAB: Can I throw an empty MException object using THROW, RETHROW, or THROWASCALLER in MATLAB 7.12 (R2011a)

MATLAB

I want THROW, RETHROW and THROWASCALLER not displaying error message when an empty MException object is given.

When I execute:

function dummythrow
  errobj = MException.empty;
  throw(errobj);
end

I receive the following result:

??? Error using ==> throw

Parameter must be a scalar.

I want to THROW, RETHROW and THROWASCALLER behave like ERROR, not displaying the above error message when an empty MException object is given.

For example:

function errobj = MyIsEqual(a, b)
  errobj = MException.empty;
  if ~isequal(a, b)
    errobj = MException('CallerFunc:notIsEqual', 'Not equal.');
  end
end
a = 1;
error(MyIsEqual(a, 0));
throw(MyIsEqual(a, 0));

Is there any way to accomplish this?

Best Answer

The ability to throw an empty MException object using THROW, RETHROW and THROWASCALLER is not available in MATLAB. To work around this issue, use a condition statement to bypass the use of an empty MException Object:

errobj = MyIsEqual(params)
if isempty(errobj)
  % do nothing
else
  throwAsCaller(errobj)
end

Using condition statements whenever an empty MException object is given, however, may affect the conciseness of the code.

Related Solutions

MATLAB: Do I get unexpected behavior when I use LASTERR command after using CATCH command with an identifier in MATLAB 7.6 (R2008a)

This is the intended behavior of the LASTERR command.

When the CATCH command is used with an identifier, it does not change the status of the global LASTERR. Hence, we should use the identifier to check the error occured and not the LASTERR command.

Hence, the code should be as follows:

try
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MATLAB: How to replace the LASTERROR function in the MATLAB code with the try-catch exception method

This change has been incorporated into the documentation in Release 2010a (R2010a). For previous releases, read below for any additional information:

The use of the functions LASTERR and LASTERROR is no longer recommended for capturing the state of the last error. It is suggested to replace the use of these functions with the new style of try-catch statement that captures an MException object to represent the error. This is because LASTERROR and LASTERR functions kept track of only the one most recently thrown error. The state of any errors thrown before that was overwritten by newer errors. Also, this error state was globally accessible, which means that it could be unintentionally modified or destroyed either during an interactive session or by another function. As in the example below,

% This code will NOT work in future releases





try
aUserDefinedFunction;
catch
someFunction;
rethrow(lasterror);
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if someFunction did not guard against modification of lasterror as seen above, and doSomeStuff threw an error, then the rethrow(lasterror) would

rethrow the wrong error. In the following document, the function LASTERROR is meant to include both LASTERROR and LASTERR functions.

Below are some suggestions of how to replace some of the most common uses of the functions LASTERR and LASTERROR with equivalent TRY-CATCH Exception errors:

1. The data that is available from the functions LASTERROR and LASTERR can also be accessed using the properties of the MException object created with the TRY-Catch block. These properties can then be used to:

a. rethrow the last error message. As an example, replace this code

% This code will NOT work in future releases
try
    ...
catch
    rethrow(lasterr) % or rethrow(lasterror)
end

with

% This code will work




try
    ...
catch myException
    rethrow(myException) % or throw(myException), throwAsCaller(myException)
end

b. modify the appearance of the displayed error message (like remove "Error using ==>" ) or the message string. As an example, replace this code

% This code will NOT work in future releases
try
    ...
catch
    ...
    msg = strrep(lasterr,xlate('Error using ==>'));
    error(msg);
    ...
end

with

% This code will work
try
    ...
catch myException
    ...
    throw(myException)
end

c. use the last error message to throw a warning. As an example, replace this code:

% This code will NOT work in future releases
[prev, ID] = lasterr;
try
    ...
catch
    ...
    lasterr('reset') % OR lasterr('') OR  lasterr(prev,ID)
end

with

% This code will work
try
    ...
catch myException
    warning(myException.identifier, '%s', myException.getReport('basic')); 
end

2. The special case when the LASTERROR function is being used to get the error message thrown from a statement in the EVAL or the EVALC command can be replaced with the following equivalent try-catch syntax. As an example, replace this code

% This code will NOT work in future releases
eval('doStuffThatErrors;', 'myError=lasterror; doErrorHandlingStuff(myError)');

with

% This code will work
try
      eval('doStuffThatErrors;');
catch myError
      doErrorHandlingStuff(myError)
end

3. When used in a PARFOR loop, the name of the MException variable needs to be preallocated before the PARFOR. As an example, replace this code

% This code will NOT work in future releases
function testParfor 
parfor i = 1:10 
     try
         error('oups');
     catch err
         disp(err.getReport);
     end
end

with

% This code will work
   err = [ ];
parfor i = 1:10 
     try
         error('oups');
     catch err
         disp(err.getReport);
     end
end

Best Answer

Related Solutions

MATLAB: Do I get unexpected behavior when I use LASTERR command after using CATCH command with an identifier in MATLAB 7.6 (R2008a)

MATLAB: How to replace the LASTERROR function in the MATLAB code with the try-catch exception method

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