In computer programming, several programming language mechanisms exist for exception handling. The term exception is typically used to denote a data structure storing information about an exceptional condition. One mechanism to transfer control, or raise an exception, is known as a throw; the exception is said to be thrown. Execution is transferred to a catch.

Usage

Programming languages differ substantially in their notion of what an exception is. Exceptions can be used to represent and handle abnormal, unpredictable, erroneous situations, but also as flow control structures to handle normal situations. For example, Python's iterators throw StopIteration exceptions to signal that there are no further items produced by the iterator. There is disagreement within many languages as to what constitutes idiomatic usage of exceptions. For example, Joshua Bloch states that Java's exceptions should only be used for exceptional situations, but Kiniry observes that Java's class is not at all an exceptional event. Similarly, Bjarne Stroustrup, author of C++, states that C++ exceptions should only be used for error handling, as this is what they were designed for, but Kiniry observes that many modern languages such as Ada, C++, Modula-3, ML and OCaml, Python, and Ruby use exceptions for flow control. Some languages such as Eiffel, C#, Common Lisp, and Modula-2 have made a concerted effort to restrict their usage of exceptions, although this is done on a social rather than technical level.

History The earliest IBM Fortran compilers had statements for testing exceptional conditions. These included the IF ACCUMULATOR OVERFLOW, IF QUOTIENT OVERFLOW, and IF DIVIDE CHECK statements. In the interest of machine independence, they were not included in FORTRAN IV nor the Fortran 66 Standard. However since Fortran 2003 it is possible to test for numerical issues via calls to functions in the IEEE_EXCEPTIONS module.

Software exception handling continued to be developed in the 1960s and 1970s. LISP 1.5 (1961) allowed exceptions to be raised by the ERROR pseudo-function, similarly to errors raised by the interpreter or compiler. Exceptions were caught by the ERRORSET keyword, which returned NIL in case of an error, instead of terminating the program or entering the debugger. PL/I introduced its own form of exception handling circa 1964, allowing interrupts to be handled with ON units. MacLisp observed that ERRSET and ERR were used not only for error raising, but for non-local control flow, and thus added two new keywords, CATCH and THROW (June 1972). The cleanup behavior now generally called "finally" was introduced in NIL (New Implementation of LISP) in the mid- to late-1970s as UNWIND-PROTECT. This was then adopted by Common Lisp. Contemporary with this was dynamic-wind in Scheme, which handled exceptions in closures. The first papers on structured exception handling were and . Exception handling was subsequently widely adopted by many programming languages from the 1980s onward.

Syntax

Many computer languages have built-in syntactic support for exceptions and exception handling. This includes Ada, BlitzMax, C++, C#, Clojure, COBOL, D, ECMAScript (e.g., ActionScript, JavaScript), Eiffel, Java, ML, Object Pascal (e.g., Delphi, Free Pascal), PowerBuilder, Objective-C, OCaml, Perl, PHP (as of version 5), PL/I, PL/SQL, Prolog, Python, REALbasic, Ruby, Scala, Smalltalk, Tcl, Visual Prolog, and most .NET languages.

Excluding minor syntactic differences, only a few exception handling styles are in use. In the most popular style, an exception is initiated by a special statement (throw or raise) with an exception object (e.g., with Java or Object Pascal) or a value of a special extendable enumerated type (e.g., with Ada or SML). The scope for exception handlers starts with a marker clause (try or the language's block starter such as begin) and ends in the start of the first handler clause (catch, except, rescue). Several handler clauses can follow, and each can specify which exception types it handles and what name it uses for the exception object. As a minor variation, some languages use a single handler clause, which deals with the class of the exception internally.

Also common is a related clause (finally or ensure) that is executed whether an exception occurred or not, typically to release resources acquired within the body of the exception-handling block. Notably, C++ does not provide this construct, recommending instead the resource acquisition is initialization (RAII) technique which frees resources using destructors. According to a 2008 paper by Westley Weimer and George Necula, the syntax of the try...finally blocks in Java is a contributing factor to software defects. When a method needs to handle the acquisition and release of 3–5 resources, programmers are apparently unwilling to nest enough blocks due to readability concerns, even when this would be a correct solution. It is possible to use a single try...finally block even when dealing with multiple resources, but that requires a correct use of sentinel values, which is another common source of bugs for this type of problem.

Python and Ruby also permit a clause (else) that is used in case no exception occurred before the end of the handler's scope was reached.

In its whole, exception handling code might look like this (in Java-style exception-handling syntax):

import java.io.IOException; import java.util.Scanner;

try { Scanner stdin = new Scanner(System.in); String line = stdin.nextLine();

if (line.length() == 0) { throw new IOException("The line read from console was empty!");

System.out.printf("Hello %s!%n", line); System.out.println("The task executed successfully."); } catch (IOException e) { System.out.println("Hello!"); } catch (Exception e) { System.out.printf("Error: %s%n", e.getMessage()); } finally { System.out.println("The program is now terminating."); }

C does not have try-catch exception handling, but uses return codes for error checking. The setjmp and longjmp standard library functions can be used to implement try-catch handling via macros.

Perl 5 uses die for throw and for try-catch. It has CPAN modules that offer try-catch semantics.

Termination and resumption semantics

When an exception is thrown, the program searches back through the stack of function calls until an exception handler is found. Some languages call for unwinding the stack as this search progresses. That is, if function , containing a handler for exception , calls function , which in turn calls function , and an exception occurs in , then functions and may be terminated, and in will handle . This is said to be termination semantics. Alternately, the exception handling mechanisms may not unwind the stack on entryIn, e.g., PL/I, a normal exit from an exception handler unwinds the stack. to an exception handler, giving the exception handler the option to restart the computation, resume or unwind. This allows the program to continue the computation at exactly the same place where the error occurred (for example when a previously missing file has become available) or to implement notifications, logging, queries and fluid variables on top of the exception handling mechanism (as done in Smalltalk). Allowing the computation to resume where it left off is termed resumption semantics.

There are theoretical and design arguments in favor of either decision. C++ standardization discussions in 1989–1991 resulted in a definitive decision to use termination semantics in C++. Bjarne Stroustrup cites a presentation by Jim Mitchell as a key data point:

Exception-handling languages with resumption include Common Lisp with its Condition System, PL/I, Dylan, R, and Smalltalk. However, the majority of newer programming languages follow C++ and use termination semantics.

In C++, std::uncaught_exceptions() is offered to count the number of exceptions in the current thread which have been thrown/rethrown and not yet entered a matching catch block. Prior to C++20, another function std::uncaught_exception() which determined whether stack unwinding was occurring or not was used.

Exception handling implementation The implementation of exception handling in programming languages typically involves a fair amount of support from both a code generator and the runtime system accompanying a compiler. (It was the addition of exception handling to C++ that ended the useful lifetime of the original C++ compiler, Cfront.Scott Meyers, The Most Important C++ Software...Ever , 2006) Two schemes are most common. The first, , generates code that continually updates structures about the program state in terms of exception handling.D. Cameron, P. Faust, D. Lenkov, M. Mehta, "A portable implementation of C++ exception handling", Proceedings of the C++ Conference (August 1992) USENIX. Typically, this adds a new element to the stack frame layout that knows what handlers are available for the function or method associated with that frame; if an exception is thrown, a pointer in the layout directs the runtime to the appropriate handler code. This approach is compact in terms of space, but adds execution overhead on frame entry and exit. It was commonly used in many Ada implementations, for example, where complex generation and runtime support was already needed for many other language features. Microsoft's 32-bit Structured Exception Handling (SEH) uses this approach with a separate exception stack., Compiler based Structured Exception Handling section Dynamic registration, being fairly straightforward to define, is amenable to proof of correctness.Graham Hutton, Joel Wright, "Compiling Exceptions Correctly ". Proceedings of the 7th International Conference on Mathematics of Program Construction, 2004.