Exception handling syntax: Difference between revisions

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{{Short description|Keywords provided by a programming language}}

'''Exception handling syntax''' is the set of keywords and/or structures provided by a computer [[programming language]] to allow [[exception handling]], which separates the handling of errors that arise during a program's operation from its ordinary processes. Syntax for exception handling varies between [[programming language]]s, partly to cover semantic differences but largely to fit into each language's overall [[syntax#Syntax in computer science|syntactic structure]]. Some languages do not call the relevant concept "[[exception handling]]"; others may not have direct facilities for it, but can still provide means to implement it.

{{Cleanup bare URLs|date=August 2022}}

'''Exception handling syntax''' is the set of keywords and/or structures provided by a computer [[programming language]] to allow [[exception handling (programming)|exception handling]], which separates the handling of errors that arise during a program's operation from its ordinary processes. Syntax for exception handling varies between [[programming language]]s, partly to cover semantic differences but largely to fit into each language's overall [[syntax#Syntax in computer science|syntactic structure]]. Some languages do not call the relevant concept "[[exception handling]]"; others may not have direct facilities for it, but can still provide means to implement it.

Most commonly, error handling uses a <code>try...[catch...][finally...]</code> block, and errors are created via a <code>throw</code> statement, but there is significant variation in naming and syntax.

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=== Ada ===

{{Further information|Ada (programming language)}}

{{Wikibooks|Ada Programming|Exceptions}}

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=== Assembly language ===

{{Further information|Assembly language}}

Most assembly languages will have a macro instruction or an interrupt address available for the particular system to intercept events such as illegal op codes, program check, data errors, overflow, [[divide by zero]], and other such. IBM and Univac mainframes had the [[STXIT]] macro. [[Digital Equipment Corporation]] [[RT11]] systems had trap vectors for program errors, i/o interrupts, and such. [[DOS]] has certain interrupt addresses. [[Microsoft Windows]] has specific module calls to trap program errors.

=== ATS ===

{{Further|ATS (programming language)}}

<syntaxhighlight lang="ocaml">

exception MyException of (string, int) (* exceptions can carry a value *)

implement main0 (): void =

try $raise MyException("not enough food", 2) with

| ~MyException(s, i) => begin

$extfcall(void, "fprintf", stderr_ref, "%s: %d", s, i);

fileref_close(stderr_ref);

end

</syntaxhighlight>

=== Bash ===

{{Further information|Bash (Unix shell)}}

<syntaxhighlight lang="bash">

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=== BASIC ===

{{Further information|BASIC}}

An ''On Error goto/gosub'' structure is used in BASIC and is quite different from modern exception handling; in BASIC there is only one global handler whereas in modern exception handling, exception handlers are stacked.

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=== C ===

{{Further information|C (programming language)}}

TheC mostdoes commonnot wayprovide todirect implementsupport to exception handling: init standardis Cthe isprogrammer's responsibility to useprevent [[setjmp.h|setjmp/longjmp]]errors in the first place and test return values from the functions:.

In any case, a possible way to implement exception handling in standard C is to use [[setjmp.h|setjmp/longjmp]] functions:

<syntaxhighlight lang="c">

#include <setjmp.h>

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==== Microsoft-specific ====

{{Further information|Microsoft-specific exception handling mechanisms}}

Two types exist:

* Structured Exception Handling (SEH)

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=== C# ===

{{Further information|C Sharp (programming language)}}

A <code>try</code> block must have at least one <code>catch</code> or <code>finally</code> clause and at most one <code>finally</code> clause.

<syntaxhighlight lang="csharp">

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=== C++ ===

{{Further information|C++}}

<syntaxhighlight lang="cpp">

import std;

#include <exception>

int main() {

try {

// do something (might throw an exception)

} catch (const std::runtime_error e) {

}

// handle a runtime_error e

catch (const std::exception& e) {

} catch (const // handle std::exception& e) {

// catches all exceptions as e

}

} catch (...) {

// catches all exceptionsthrown types (including primitives or objects that do nit extend exception), not already caught by a catch block before

}

// can be used to catch exception of unknown or irrelevant type

}

}

</syntaxhighlight>

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=== ColdFusion Markup Language (CFML) ===

{{Further information|ColdFusion Markup Language}}

==== Script syntax ====

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==== Railo-Lucee specific syntax ====

{{Further information|Railo|Lucee}}

Added to the standard syntax above, CFML dialects of [[Railo]] and [[Lucee]] allow a <code>retry</code> statement.<ref>https://issues.jboss.org/browse/RAILO-2176 # JBoss Community issue tracker ticket for adding <code>retry</code></ref>

This statement returns processing to the start of the prior <code>try</code> block.

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<syntaxhighlight lang="javascript">

try {

// code which could result in an exception

} catch (any e){

retry;

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=== D ===

{{Further information|D (programming language)}}

<syntaxhighlight lang="d">

import std.stdio; // for writefln()

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=== Delphi ===

{{Further information|Delphi (programming language)}}

; Exception declarations

<syntaxhighlight lang="delphi">

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raise Exception.Create('Message');

raise Exception.CreateFmt('Message with values: %d, %d',[value1, value2]); // See SysUtils.Format() for parameters.

raise ECustom.CreateCustom(X);

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=== Erlang ===

{{Further information|Erlang (programming language)}}

<syntaxhighlight lang="erlang">

try

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=== F# ===

{{Further information|F_Sharp_(programming_language)}}

In addition to the OCaml-based <code>try...with</code>, F# also has the separate <code>try...finally</code> construct, which has the same behavior as a try block with a <code>finally</code> clause in other .NET languages.

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=== Haskell ===

{{Further information|Haskell (programming language)}}

Haskell does not have special syntax for exceptions. Instead, a {{Haskell|try}}/{{Haskell|catch}}/{{Haskell|finally}}/{{Haskell|etc}}. interface is provided by functions.

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in analogy with this C++

<syntaxhighlight lang="ccpp">

import std;

#include <iostream>

using namespace std;

int main() {

try {

{

throw static_cast<int>(42);

try

{throw} catch(intdouble e)42;} {

std::println("(0,{})", e);

catch(double e)

{cout} << "catch(0," <<int e << ")" << endl;}{

std::println("(1,{})", e);

catch(int e)

}

{cout << "(1," << e << ")" << endl;}

}

</syntaxhighlight>

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=== Java ===

{{Further information|Java (programming language)}}

{{Wikibooks|Java Programming|Exceptions}}

A <code>try</code> block must have at least one <code>catch</code> or <code>finally</code> clause and at most one <code>finally</code> clause.

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} finally {

// Always run when leaving the try block (including finally clauses), regardless of whether any exceptions were thrown or whether they were handled.

// Often used to cleanCleans up and closecloses resources suchacquired in athe filetry handlesblock.

// May nonot be run when System.exit() is called and in other system-wide exceptional conditions (e.g. power loss).

// Rarely used after try-with-resources was added to the language (see below).

}

</syntaxhighlight>

If multiple resources are acquired, the correct way to deal with them is with nested try blocks.<ref>Bloch, Joshua (2018). ''Effective Java, Third Edition''. Addison-Wesley. Item 9, p. 54. {{ISBN|978-0-13-468599-1}}</ref> For this reason and others, ''try-with-resources'' was added to the language to almost entirely replace finally clauses. Resources acquired in a parentheses after the try keyword will be cleaned up automatically. Classes used in these statements must implement an interface called AutoCloseable.<ref>{{cite web | url=https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html | title=The try-with-resources Statement (The Java™ Tutorials > Essential Java Classes > Exceptions) }}</ref>

<syntaxhighlight lang="java">

try (FileReader fr = new FileReader(path);

BufferedReader br = new BufferedReader(fr)) {

// Normal execution path.

} catch (IOException ioe) {

// Deal with exception.

} // Resources in the try statement are automatically closed afterwards.

finally {

// A finally clause can be included, and will run after the resources in the try statements are closed.

}

</syntaxhighlight>

=== JavaScript ===

{{Further information|JavaScript}}

The design of JavaScript makes loud/hard errors very uncommon. [[Soft_errorSoft error|Soft/quiet errors]] are much more prevalent. Hard errors propagate to the nearest <code>try</code> statement, which must be followed by either a single <code>catch</code> clause, a single <code>finally</code> clause, or both.

<syntaxhighlight lang="javascript">

try {

// Statements in which exceptions might be thrown

throw new Error("error");

} catch (error) {

// Statements that execute in the event of an exception

} finally {

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try {

throw 12345; // primitive number

} catch (error) {

console.log(error); // logs 12345 as a primitive number to the console

}

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// Example in Java

try {

Integer i = null;

i.intValue(); // throws a NullPointerException

} catch (NullPointerException error) {

// Variable might be null

} catch (ArithmeticException error) {

// Handle problems with numbers

}

</syntaxhighlight>

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var example = null;

example.toString();

} catch (error) {

if (error.type === "TypeError") {

// Variable might be null

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var example = null;

example.toString();

} catch (error) {

if (error.type !== "TypeError") throw error;

// Variable might be null

}

} catch (error) {

if (error.type !== "RangeError") throw error;

// Handle problems with numbers

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obj.selfPropExample = obj; // circular reference

throw obj;

} catch (error) {

// Statements that execute in the event of an exception

}

</syntaxhighlight>

=== Kotlin ===

{{Further|Kotlin (programming language)}}

<syntaxhighlight lang="kotlin">

try {

// Code that may throw an exception

} catch (e: SomeException) {

// Code for handling the exception

}

</syntaxhighlight>

=== Lisp ===

{{Further information|Lisp (programming language)}}

==== Common Lisp ====

{{Further information|Common Lisp}}

<syntaxhighlight lang="lisp">

(ignore-errors (/ 1 0))

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=== Lua ===

{{Further information|Lua (programming language)}}

Lua uses the <code>pcall</code> and <code>xpcall</code> functions, with <code>xpcall</code> taking a function to act as a <code>catch</code> block.

; Predefined function

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=== Next Generation Shell ===

{{Further information|Next Generation Shell}}

; Defining custom exception type

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=== Objective-C ===

{{Further information|Objective-C}}

; Exception declarations

<syntaxhighlight lang="objc">

NSException *exception = [NSException exceptionWithName:@"myException"

reason:@"whateveryourReason" userInfo:nil];

</syntaxhighlight>

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=== OCaml ===

{{Further information|OCaml}}

<syntaxhighlight lang="ocaml">

exception MyException of string * int (* exceptions can carry a value *)

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=== Perl 5===

{{Further information|Perl}}

The [[Perl]] mechanism for exception handling uses {{Perl2|die}} to throw an exception when wrapped inside an {{Perl2|eval { ... };}} block. After the {{Perl2|eval}}, the special variable {{Perl2|$@}} contains the value passed from {{Perl2|die}}. However, scoping issues can make doing this correctly quite ugly:

<syntaxhighlight lang="perl">

my ($error, $failed);

{

local $@;

$failed = not eval {

# Code that could throw an exception (using 'die')

open(FILE, $file) || die "Could not open file: $!";

while (<FILE>) {

process_line($_);

}

close(FILE) || die "Could not close $file: $!";

return 1;

};

$error = $@;

}

if ($failed) {

warn "got error: $error";

}

</syntaxhighlight>

Perl 5.005 added the ability to throw objects as well as strings. This allows better introspection and handling of types of exceptions.

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The forms shown above can sometimes fail if the global variable {{Perl2|$@}} is changed between when the exception is thrown and when it is checked in the {{Perl2|if ($@)}} statement. This can happen in multi-threaded environments, or even in single-threaded environments when other code (typically

called in the destruction of some object) resets the global variable before the checking code.

The following example shows a way to avoid this problem (see [https://archive.today/20130415214802/http://www.perlfoundation.org/perl5/index.cgi?exception_handling]{{deadlink|date=October 2020}} or [https://stackoverflow.com/a/10343025]; ''cf''. [http://mvp.kablamo.org/essentials/die-eval/]). But at the cost of not being able to use return values:

<syntaxhighlight lang="perl">

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Several modules in the Comprehensive Perl Archive Network ([[CPAN]]) expand on the basic mechanism:

* {{Perl2|Error}} provides a set of exception classes and allows use of the try/throw/catch/finally syntax.

* {{Perl2|TryCatch}} and, {{Perl2|Try::Tiny}} bothand {{Perl2|Nice::Try}} all allow the use of try/catch/finally syntax instead of boilerplate to handle exceptions correctly.

* {{Perl2|Exception::Class}} is a base class and class-maker for derived exception classes. It provides a full structured [[stack trace]] in {{Perl2|$@->trace}} and {{Perl2|$@->trace->as_string}}.

* {{Perl2|Fatal}} overloads previously defined functions that return true/false e.g., {{Perl2|open}}, {{Perl2|close}}, {{Perl2|read}}, {{Perl2|write}}, etc. This allows built-in functions and others to be used as if they threw exceptions.

=== PHP ===

{{Further information|PHP}}

<syntaxhighlight lang="php">

// Exception handling is only available in PHP versions 5 and greater.

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=== PowerBuilder ===

{{Further information|PowerBuilder}}

Exception handling is available in PowerBuilder versions 8.0 and above.

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=== PowerShell ===

{{Further information|PowerShell}}

==== Version 1.0 ====

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=== Python ===

{{Further information|Python (programming language)}}

<syntaxhighlight lang="python">

f = None

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=== R ===

{{Further information|R (programming language)}}

<syntaxhighlight lang="splus">

tryCatch({

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=== Rebol ===

{{Further information|Rebol}}

<syntaxhighlight lang="rebol">

REBOL [

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=== Rexx ===

{{Further information|Rexx}}

<syntaxhighlight lang="rexx">

signal on halt;

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=== Ruby ===

{{Further information|Ruby (programming language)}}

<syntaxhighlight lang="ruby">

begin

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=== S-Lang ===

{{Further information|S-Lang (programming library)}}

try

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New exceptions may be created using the {{S-Lang|new_exception}} function, e.g.,

new_exception ("MyIOError", IOError, "My I/O Error");

will create an exception called {{S-Lang|MyIOError}} as a subclass of {{S-Lang|IOError}}. Exceptions may be generated using the throw statement, which can throw arbitrary [[S-Lang (programming language)|S-Lang]] objects.

=== Smalltalk ===

{{Further information|Smalltalk}}

<syntaxhighlight lang="smalltalk">

[ "code that might throw an exception" ]

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=== Swift ===

{{Further information|Swift (programming language)}}

Exception handling is supported since Swift 2.

<syntaxhighlight lang="swift">

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=== Tcl ===

{{Further information|Tcl}}

<syntaxhighlight lang="tcl">

if { [ catch {

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=== VBScript ===

{{Further information|VBScript}}

<syntaxhighlight lang="vbnet">

With New Try: On Error Resume Next

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Case 0 'this line is required when using 'Case Else' clause because of the lack of "Is" keyword in VBScript Case statement

'no exception

Case ERRORNUMBERSOME_ERRORNUMBER

'exception handling

Case Else

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End Property

End Class

</syntaxhighlight><ref name="VBexhandle">[{{Cite web |url=https://sites.google.com/site/truetryforvisualbasic/ |title=Try-Catch for VB] |access-date=2012-03-17 |archive-date=2016-04-16 |archive-url=https://web.archive.org/web/20160416093023/https://sites.google.com/site/truetryforvisualbasic/ |url-status=dead }}</ref>

=== Visual Basic 6 ===

{{Further information|Visual Basic}}

Exception handling syntax is very similar to Basic. Error handling is local on each procedure.

<syntaxhighlight lang="vbnet">

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On Error Resume Next 'Object Err is set, but execution continues on next command. You can still use Err object to check error state.

'...

Err.Raise 6 ' Generate an "Overflow" error using buildbuilt-in object Err. If there is no error handler, calling procedure can catch exception by same syntax

'...

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MsgBox Err.Number & " " & Err.Source & " " & Erl & " " & Err.Description & " " & Err.LastDllError 'show message box with important error properties

'Erl is VB6 buildbuilt-in line number global variable (if used). Typically is used some kind of IDE Add-In, which labels every code line with number before compilation

Resume FinallyLabel

</syntaxhighlight>

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With New Try: On Error Resume Next 'Create new object of class "Try" and use it. Then set this object as default. Can be "Dim T As New Try: ... ... T.Catch

'do Something (only one statement recommended)

.Catch: On Error GoTo 0: Select Case .Number 'Call Try.Catch() procedure. Then switch off error handling. Then use "switch-like" statement on result of Try.Number property (value of property Err.Number of buildbuilt-in Err object)

Case ERRORNUMBERSOME_ERRORNUMBER

'exception handling

Case Is <> 0 'When Err.Number is zero, no error has occurred

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==== Visual Basic .NET ====

{{Further information|Visual Basic .NET}}

A <code>Try</code> block must have at least one clause <code>Catch</code> or <code>Finally</code> clause and at most one <code>Finally</code> clause.

<syntaxhighlight lang="vbnet">

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=== Visual Prolog ===

{{Further information|Visual Prolog}}

http://wiki.visual-prolog.com/index.php?title=Language_Reference/Terms#Try-catch-finally

<syntaxhighlight lang="prologvisualprolog">

try

% Block to protect

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% Code will be executed regardles however the other parts behave

end try

</syntaxhighlight><ref>http://wiki.visual-prolog.com/index.php?title=Language_Reference/Terms#Try-catch-finally</ref>

</syntaxhighlight>

=== X++ ===

{{Further information|Microsoft Dynamics AX}}

<syntaxhighlight lang="javax++">

public static void Main(Args _args)

{

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