MException

Capture error information

Description

Any MATLAB^® code that detects an error and throws an exception must construct an MException object. This class contains retrievable information about errors. MATLAB can throw either predefined exceptions or exceptions that you construct.

Creation

Syntax

ME = MException(msgID,msgtext)

ME = MException(msgID,msgtext,A1,...,An)

Description

ME = MException(msgID,msgtext) captures information about a specific error and stores it in the MException object, ME. The MException object is constructed with a message identifier, msgID, and an error message, msgtext.

ME = MException(msgID,msgtext,A1,...,An) allows formatting of the error message using text or numeric values, A1,...,An, to replace conversion specifiers in msgtext at run time.

Input Arguments

msgID — Identifier for error

character vector | string scalar

Identifier for the error, specified as a character vector or string scalar. Use the message identifier with exception handling to better identify the source of the error or to control a selected subset of the exceptions in your program.

The message identifier includes a component and mnemonic. The identifier must always contain a colon and follows a simple format: component:mnemonic. The component and mnemonic fields must each begin with a letter. The remaining characters can be alphanumerics (A–Z, a–z, 0–9) and underscores. No white-space characters can appear anywhere in msgID. For more information, see Message Identifiers.

Example: 'MyComponent:noSuchVariable'

msgtext — Information about cause of error

character vector | string scalar

Information about the cause of the error and how you might correct it, specified as a character vector or string scalar. To format the text, use escape sequences, such as \t or \n. You also can use any format specifiers supported by the sprintf function, such as %s or %d. Specify values for the conversion specifiers via the A1,...,An input arguments. For more information, see Formatting Text.

Example: 'Error opening file.'

Example: 'Error on line %d.'

A1,...,An — Values

character vector | string scalar | numeric scalar

Values that replace the conversion specifiers in msgtext, specified as a character vector, string scalar, or numeric scalar.

Properties

identifier — Unique identifier of error

character vector

Character vector that uniquely identifies the error, specified as a character vector by the msgID input argument. This property is read-only. For more information, see Message Identifiers.

Example: 'MATLAB:test'

Data Types: char

message — Error message

character vector

Character vector that contains the error message that is displayed when MATLAB throws the exception, specified by the msgtext and A1,...,An input arguments. This property is read only. For more information, see Text of the Error Message.

Example: 'Variable x not found'

Data Types: char

stack — Stack trace information

array of structures

Structure array that contains stack trace information including the file name (file), function name (name), and line number (line) where MATLAB throws the exception. If the error occurs in a called function, the stack property also contains the file name, function name, and line number for each of the called functions. MATLAB generates the stack only when it throws the exception.

stack is an N-by-1 struct array, where N represents the depth of the call stack. This property is read-only. For more information, see The Call Stack.

Data Types: struct

cause — Cause of the exception

cell array of MException objects

Cell array of MException objects that caused MATLAB to create this exception. Use the addCause method to add an exception to the cause field of another exception. For more information, see The Cause Array.

Correction — Suggested fix for the exception

matlab.lang.correction.AppendArgumentsCorrection

Suggested fix for this exception, specified as a matlab.lang.correction.AppendArgumentsCorrection object. When an exception is thrown and not caught, MATLAB uses the Correction property to suggest a fix for the exception.

Example:

Object Functions

throw	Throw exception
MException.last	Return last uncaught exception
rethrow	Rethrow previously caught exception
throwAsCaller	Throw exception as if occurs within calling function
addCause	Record additional causes of exception
addCorrection	Provide suggested fix for exception
getReport	Get error message for exception

Examples

Create MException Object

msgID = 'myComponent:inputError';
msgtext = 'Input does not have the expected format.';

ME = MException(msgID,msgtext)

ME = 
  MException with properties:

    identifier: 'myComponent:inputError'
       message: 'Input does not have the expected format.'
         cause: {}
         stack: [0x1 struct]
    Correction: []

Create MException with Formatted Error Message

msgID = 'MATLAB:test';
msgtext = 'There are %d errors on this page';
A1 = 10;

ME = MException(msgID,msgtext,A1)

ME = 
  MException with properties:

    identifier: 'MATLAB:test'
       message: 'There are 10 errors on this page'
         cause: {}
         stack: [0x1 struct]
    Correction: []

Create and Throw MException Object

Throw an exception if an input variable name does not exist in the workspace.

str = input('Type a variable name: ','s');
if ~exist(str,'var')
    ME = MException('MyComponent:noSuchVariable', ...
        'Variable %s not found',str);
    throw(ME)
end

At the input prompt, enter any variable that does not exist in your workspace. For example, enter notaVariable.

Variable notaVariable not found

Since notVariable doesn’t exist in your workspace, MATLAB creates an MException object, and then throws it.

Use try/catch to Capture Exception

Catch the exception generated by calling a nonexistent function, notaFunction. If the function is not defined, issue a warning and assign the output a value of 0.

try
    a = notaFunction(5,6);
catch ME
    if strcmp(ME.identifier,'MATLAB:UndefinedFunction')
        warning('Function is undefined.  Assigning a value of 0.');
    else
        rethrow(ME)
    end
end

Warning: Function is undefined.  Assigning a value of 0. 

By itself, the call to notaFunction results in an error. Using try and catch, this code catches the undefined function exception and repackages it as a warning, allowing MATLAB to continue executing subsequent commands. If the caught exception has a different error identifier, MATLAB rethrows the exception.