Raising and Handling Exceptions

To raise an exception object, use an instance of the exception class with a raise statement. For example, raise EMathError.Create;

In general, the form of a raise statement is raiseobjectataddress where object and at address are both optional. When an address is specified, it can be any expression that evaluates to a pointer type, but is usually a pointer to a procedure or function. For example:

raise Exception.Create('Missing parameter') at @MyFunction;

Use this option to raise the exception from an earlier point in the stack than the one where the error actually occurred.

When an exception is raised - that is, referenced in a raise statement - it is governed by special exception-handling logic. A raise statement never returns control in the normal way. Instead, it transfers control to the innermost exception handler that can handle exceptions of the given class. (The innermost handler is the one whose try...except block was most recently entered but has not yet exited.)

For example, the function below converts a string to an integer, raising an ERangeError exception if the resulting value is outside a specified range.

function StrToIntRange(const S: string; Min, Max: Longint): Longint; begin

Result := StrToInt(S); // StrToInt is declared in SysUtils if (Result < Min) or (Result > Max) then raise ERangeError.CreateFmt('%d is not within the valid range of %d.. %d', [Result, Min, Max]); end;

Notice the CreateFmt method called in the raise statement. Exception and its descendants have special constructors that provide alternative ways to create exception messages and context IDs.

A raised exception is destroyed automatically after it is handled. Never attempt to destroy a raised exception manually.

Note: Raising an exception in the initialization section of a unit may not produce the intended result. Normal exception support comes from the SysUtils unit, which must be initialized before such support is available. If an exception occurs during initialization, all initialized units - including SysUtils - are finalized and the exception is re-raised. Then the exception is caught and handled, usually by interrupting the program. Similarly, raising an exception in the finalization section of a unit may not lead to the intended result if SysUtils has already been finalized when the exception has been raised.

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Project Management Made Easy

Project Management Made Easy

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