Tryexcept Statements

Exceptions are handled within try...except statements. For example, try

X := Y/Z; except on EZeroDivide do HandleZeroDivide;


This statement attempts to divide y by z, but calls a routine named HandleZeroDivide if an EZeroDivide exception is raised.

The syntax of a try...except statement is try statementsexceptexceptionBlockend where statements is a sequence of statements (delimited by semicolons) and exceptionBlock is either

  • another sequence of statements or
  • a sequence of exception handlers, optionally followed by elsestatements

An exception handler has the form onidentifier: typedostatement where identifier: is optional (if included, identifier can be any valid identifier), type is a type used to represent exceptions, and statement is any statement.

A try...except statement executes the statements in the initial statements list. If no exceptions are raised, the exception block (exceptionBlock) is ignored and control passes to the next part of the program.

If an exception is raised during execution of the initial statements list, either by a raise statement in the statements list or by a procedure or function called from the statements list, an attempt is made to 'handle' the exception:

  • If any of the handlers in the exception block matches the exception, control passes to the first such handler. An exception handler 'matches' an exception just in case the type in the handler is the class of the exception or an ancestor of that class.
  • If no such handler is found, control passes to the statement in the else clause, if there is one.
  • If the exception block is just a sequence of statements without any exception handlers, control passes to the first statement in the list.

If none of the conditions above is satisfied, the search continues in the exception block of the next-most-recently entered try...except statement that has not yet exited. If no appropriate handler, else clause, or statement list is found there, the search propagates to the next-most-recently entered try...

except statement, and so forth. If the outermost try...except statement is reached and the exception is still not handled, the program terminates.

When an exception is handled, the stack is traced back to the procedure or function containing the try...except statement where the handling occurs, and control is transferred to the executed exception handler, else clause, or statement list. This process discards all procedure and function calls that occurred after entering the try...except statement where the exception is handled. The exception object is then automatically destroyed through a call to its Destroy destructor and control is passed to the statement following the try...except statement. (If a call to the Exit, Break, or Continue standard procedure causes control to leave the exception handler, the exception object is still automatically destroyed.)

In the example below, the first exception handler handles division-by-zero exceptions, the second one handles overflow exceptions, and the final one handles all other math exceptions. EMathError appears last in the exception block because it is the ancestor of the other two exception classes; if it appeared first, the other two handlers would never be invoked.

try except on EZeroDivide do HandleZeroDivide; on EOverflow do HandleOverflow; on EMathError do HandleMathError; end;

An exception handler can specify an identifier before the name of the exception class. This declares the identifier to represent the exception object during execution of the statement that follows The scope of the identifier is limited to that statement. For example, try except on E: Exception do ErrorDialog(E.Message, E.HelpContext); end;

If the exception block specifies an else clause, the else clause handles any exceptions that aren't handled by the block's exception handlers. For example, try except on EZeroDivide do HandleZeroDivide; on EOverflow do HandleOverflow; on EMathError do HandleMathError; else

HandleAllOthers; end;

Here, the else clause handles any exception that isn't an EMathError.

An exception block that contains no exception handlers, but instead consists only of a list of statements, handles all exceptions. For example, try except

HandleException; end;

Here, the HandleException routine handles any exception that occurs as a result of executing the statements between try and except.

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Project Management Made Easy

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