Exception handling syntax: Difference between revisions

Revision as of 10:46, 28 June 2022

Latest revision as of 23:17, 3 July 2025

(32 intermediate revisions by 21 users not shown)

Line 1:

{{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.

Line 45 ⟶ 47:

=== Assembly language ===

{{Further|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 ===

Line 186 ⟶ 202:

{{Further|C++}}

<syntaxhighlight lang="cpp">

import std;

#include <exception>

int main() {

try {

// do something (might throw an exception)

} catch (const std::exception&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>

Line 253 ⟶ 269:

try {

// code which could result in an exception

} catch (any e){

retry;

Line 416 ⟶ 431:

<syntaxhighlight lang="cpp">

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>

Line 453 ⟶ 468:

} 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>

Line 465 ⟶ 493:

// Statements in which exceptions might be thrown

throw new Error("error");

} catch (error) {

// Statements that execute in the event of an exception

} finally {

Line 483 ⟶ 511:

try {

throw 12345; // primitive number

} catch (error) {

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

}

Line 492 ⟶ 520:

// 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>

Line 506 ⟶ 534:

var example = null;

example.toString();

} catch (error) {

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

// Variable might be null

Line 521 ⟶ 549:

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

Line 558 ⟶ 586:

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>

Line 688 ⟶ 726:

<syntaxhighlight lang="objc">

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

reason:@"whateveryourReason" userInfo:nil];

</syntaxhighlight>

Line 783 ⟶ 821:

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]{{dead link|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">

Line 796 ⟶ 834:

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.

Line 961 ⟶ 999:

=== S-Lang ===

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

try

Line 982 ⟶ 1,020:

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 ===

Line 1,090 ⟶ 1,128:

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 ===

Line 1,102 ⟶ 1,140:

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

'...

Line 1,120 ⟶ 1,158:

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>

Line 1,128 ⟶ 1,166:

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 SOME_ERRORNUMBER

'exception handling

Line 1,201 ⟶ 1,239:

{{Further|Visual Prolog}}

<syntaxhighlight lang="prologvisualprolog">

try

% Block to protect

Line 1,213 ⟶ 1,251:

=== X++ ===

{{Further|Microsoft Dynamics AX}}

<syntaxhighlight lang="javax++">

public static void Main(Args _args)

{

Line 1,244 ⟶ 1,282:

[[Category:Programming language syntax]]

[[Category:Control flow]]

[[Category:Articles with example code]]

[[Category:Programming language comparisons]]

<!-- Hidden categories below -->

[[Category:Articles with example Ada code]]

[[Category:Articles with example BASIC code]]

[[Category:Articles with example C code]]

[[Category:Articles with example C++ code]]

[[Category:Articles with example C Sharp code]]

[[Category:Articles with example Haskell code]]

[[Category:Articles with example Java code]]

[[Category:Articles with example JavaScript code]]

[[Category:Articles with example Lisp (programming language) code]]

[[Category:Articles with example Objective-C code]]

[[Category:Articles with example OCaml code]]

[[Category:Articles with example Pascal code]]

[[Category:Articles with example Perl code]]

[[Category:Articles with example PHP code]]

[[Category:Articles with example Python (programming language) code]]

[[Category:Articles with example R code]]

[[Category:Articles with example Ruby code]]

[[Category:Articles with example Smalltalk code]]

[[Category:Articles with example Swift code]]

[[Category:Articles with example Tcl code]]