Operator Reference
try (Operator)
try — Starts a program block where exceptions are detected and caught.
Signature
try( : : : )
Description
With the help of the operators try, catch, endtry,
and throw it is possible to implement a dynamic exception handling
in HDevelop, which is comparable to the exception handling in C++
and C#. By the operators try, catch, and endtry
two code blocks are formed: the first one (try .. catch)
contains the watched program lines that perform the normal program logic.
The second block (catch .. endtry) contains the code
that has is executed if an exception occurs.
The operator try enables the exception handling for the following
program lines, i.e., the following code block up to the corresponding
catch operator is watched for exceptions.
If during the execution of the subsequent program lines an error or another
exceptional state occurs, or if an exception is thrown explicitly by the
operator throw, the try block is left immediately
(or---depending on a user preference---after displaying an error message box)
and the program execution continues at the corresponding catch
operator.
If the exception is thrown within a procedure that was called from the
try block (directly or via other procedure calls), the procedure
call and all intermediate procedure calls that are on the call stack above
the try block are immediately aborted (or, if applicable, also after
displaying an error message box).
Whether an error message box is displayed before the exception is thrown
or not, is controlled by the HDevelop preference
'Suppress error message dialogs within try-catch blocks'
that can be reached via
Edit->Preferences->General Options->Experienced User.
This message box also offers the opportunity to stop the program execution
before the exception is thrown in order to edit the possibly erroneous
operator call.
The program block that is watched for exceptions ends with the corresponding
catch operator.
If within the watched try block no exception occurred, the following
catch block is ignored and the program execution continues after
the corresponding endtry operator.
try-catch-endtry blocks can be nested arbitrarily
into each other, within a procedure or over different procedure calls,
as long as any inner try-catch-endtry block lies
completely either within an outer try-catch
or a catch-endtry block.
If an exception is thrown within an inner try-catch block,
the exception handling is caught in the corresponding
catch-endtry block. Hence, the exception is not
visible for the outer try-catch blocks unless the
exception is rethrown explicitly by calling a throw operator
from the catch block.
If within a HALCON operator an error occurs, an exception tuple is created
and passed to the catch operator that is responsible for catching
the exception.
The tuple collects information about the error such as the error code and
the error text.
After catching an exception, this information can be accessed with the
help of the operator dev_get_exception_data.
For more information about the passed exception data, how to access them,
and considerations about the code export, see the description of that
operator.
The reference of the operator throw describes how to throw
user-defined exception tuples.
HDevelop offers the opportunity to disable the handling of HALCON errors.
This can be achieved by calling the operator
dev_set_check('~give_error')
or by unchecking the check box Give Error on the dialog
Edit->Preferences->Runtime Settings.
If the error handling is switched off, in case of an HALCON error no
exception is thrown but the program execution is continued as normal at the
next operator.
In contrast to that, the operator throw will always throw an
exception independently of the 'give_error' setting.
The same applies if an error occurred during the evaluation of an
parameter expression.
Attention
The export of the operators try, catch, endtry,
and throw is not supported for the language C, but only for
the languages C++, C# and VisualBasic/.NET.
Only the latter support throwing exceptions across procedures.
Example (HDevelop)
try
read_image (Image, 'may_be_not_available')
catch (Exception)
if (Exception[0] == 5200)
dev_get_exception_data (Exception, 'error_message', ErrMsg)
set_tposition (3600, 24, 12)
write_string (3600, ErrMsg)
return ()
else
* rethrow the exception
throw ([Exception,'unknown exception in myproc'])
endif
endtry
Result
try always returns 2 (
H_MSG_TRUE)
.
Alternatives
See also
catch,
endtry,
throw,
dev_get_exception_data,
dev_set_check
Module
Foundation