Destructor (computer programming)

<h2 id="destructor-syntax">Destructor syntax</h2>
<ul><li><a href="/facts/C%252B%252B/Et0F2qn9">C++</a>: destructors have the same name as the class with which they are associated, but with a <a href="/facts/Tilde/wQTx6ytR">tilde</a> prefix (for example, a class X with a constructor X() has a destructor ~X()).<a class="footnote-ref" id="fnref:3" href="#fn:3">3</a></li>
<li><a href="/facts/C_Sharp_(programming_language)/UC2gxeyb">C#</a>: same syntax as C++ (prefixed with a tilde). The language historically referred to these as destructors, however they are now referred to as <a href="/facts/Finalizer/6vU0SvEb">finalizers</a> due to the confusion this caused.<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a></li>
<li><a href="/facts/D_(programming_language)/4qIzRjkF">D</a>: destructors are declared with name ~this() (whereas constructors are declared with this()).</li>
<li><a href="/facts/Object_Pascal/cuGkuISv">Object Pascal</a>: destructors have the keyword destructor and can have user-defined names, but are mostly named Destroy.</li>
<li><a href="/facts/Objective-C/5mM557BW">Objective-C</a>: the destructor method has the name dealloc.</li>
<li><a href="/facts/Perl/fx31kjlT">Perl</a>: the destructor method has the name DESTROY; in the <a href="/facts/Moose_(Perl)/Um2JSQc9">Moose object system extension</a>, it is named DEMOLISH.</li>
<li><a href="/facts/PHP/vU6YRJX0">PHP</a>: In PHP 5+, the destructor method has the name __destruct. There were no destructors in prior versions of PHP.<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a></li>
<li><a href="/facts/Python_(programming_language)/YbuGqofa">Python</a>: there are __del__ methods called destructors by the Python 2 language guide,<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a> but they are actually <a href="/facts/Finalizer/6vU0SvEb">finalizers</a> as acknowledged in Python 3.<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a></li>
<li><a href="/facts/Rust_(programming_language)/8JNMV19F">Rust</a>: the destructor method for Rust has the name drop and is provided by the Drop trait.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a></li>
<li><a href="/facts/Swift_(programming_language)/A7qHQC32">Swift</a>: the destructor method has the name deinit.</li></ul>
<h2 id="in-c">In C++</h2>
The destructor has the same name as the class, but with a <a href="/facts/Tilde/wQTx6ytR">tilde</a> (~) before it.<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a> For example, a class called foo will have the destructor ~foo(). Additionally, destructors have neither parameters nor return types.<a class="footnote-ref" id="fnref:10" href="#fn:10">10</a> As stated above, a destructor for an object is called whenever the object's lifetime ends.<a class="footnote-ref" id="fnref:11" href="#fn:11">11</a> If the object was created as an <a href="/facts/Automatic_variable/S6KdHa7p">automatic variable</a>, its lifetime ends and the destructor is called automatically when the object goes out of scope. Because C++ does not have garbage collection, if the object was created with a <a href="/facts/New_and_delete_(C%252B%252B)/nNclXwey">new</a> statement (dynamically on the <a href="/facts/Memory_management/vthLkUpz">heap</a>), then its destructor is called when the <a href="/facts/Delete_(C%252B%252B)/nNclXwey">delete</a> operator is applied to a pointer to the object. Usually that operation occurs within another destructor, typically the destructor of a <a href="/facts/Smart_pointer/XuxYXAKn">smart pointer</a> object.
In inheritance hierarchies, the declaration of a <a href="/facts/Virtual_function/cnxckyHy">virtual destructor</a> in the base class ensures that the destructors of derived classes are invoked properly when an object is deleted through a pointer-to-base-class. Objects that may be deleted in this way need to inherit a virtual destructor.
A destructor should never throw an exception.<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a>
Non-class <a href="/facts/Scalar_type/Rwma07cc">scalar types</a> have what's called a pseudo-destructor which can be accessed by using typedef or template arguments. This construct makes it possible to write code without having to know if a destructor exists for a given type.

int f() {
 int a = 123;
 using T = int;
 a.~T();
 return a; // undefined behavior
}

In older versions of the standard, pseudo-destructors were specified to have no effect, however that was changed in a defect report to make them end the lifetime of the object they are called on.<a class="footnote-ref" id="fnref:13" href="#fn:13">13</a>

<h3>Example</h3>
import std;

class Foo {
public:
    Foo(): 
        data_(new char[sizeof("Hello, World!")]) {
        std::strcpy(data_, "Hello, World!");
    }

Foo(const Foo& other) = delete;             // disable copy construction
    Foo& operator=(const Foo& other) = delete;  // disable assignment

~Foo(void) { delete[] data_; }

private:
 friend std::ostream& operator<<(std::ostream& os, const Foo& foo) {
 os << foo.data_;
 return os;
 }

char* data_;
};

int main() {
 Foo foo;
 std::cout << foo << std::endl;
}

Objects which cannot be safely copied and/or assigned should be disabled from such semantics by declaring their corresponding functions as deleted within a public encapsulation level. A detailed description of this method can be found in <a href="/facts/Scott_Meyers/7lEPrl3A">Scott Meyers</a>' popular book, Effective Modern C++ (Item 11: "Prefer deleted functions to private undefined ones."<a class="footnote-ref" id="fnref:14" href="#fn:14">14</a>).

<h2 id="in-c-with-gcc-extensions">In C with GCC extensions</h2>
The <a href="/facts/GNU_Compiler_Collection/t07jh01k">GNU Compiler Collection</a>'s <a href="/facts/C_(programming_language)/Ky2No763">C</a> compiler comes with 2 extensions that allow implementing destructors:

<ul><li>The destructor function attribute<a class="footnote-ref" id="fnref:15" href="#fn:15">15</a> allows defining global prioritized destructor functions: when main() returns, these functions are called in priority order before the process terminates. See also: Hacking the art of exploitation.<a class="footnote-ref" id="fnref:16" href="#fn:16">16</a></li>
<li>The <a href="/facts/Resource_Acquisition_Is_Initialization/mtEsE4jH">cleanup variable attribute</a> allows attaching a destructor function to a variable: the function is called when the variable goes out of scope.</li></ul>
<h2 id="xojo">Xojo</h2>
Destructors in <a href="/facts/Xojo/ColkhLN1">Xojo</a> (REALbasic) can be in one of two forms. Each form uses a regular method declaration with a special name (with no parameters and no return value). The older form uses the same name as the Class with a ~ (tilde) prefix. The newer form uses the name Destructor. The newer form is preferred because it makes <a href="/facts/Code_refactoring/gkw9oVTE">refactoring</a> the class easier.

Class Foobar
 // Old form
 Sub ~Foobar()
 End Sub

// New form
  Sub Destructor()
  End Sub
End Class

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Finalizer/6vU0SvEb">Finalizer</a></li>
<li><a href="/facts/Constructor_(computer_science)/8JNHsH5H">Constructor (computer science)</a></li>
<li><a href="/facts/Object_lifetime/QgCnhNlf">Object lifetime</a></li>
<li><a href="/facts/Resource_Acquisition_Is_Initialization/mtEsE4jH">Resource Acquisition Is Initialization</a></li>
<li><a href="/facts/Rule_of_three_(C%252B%252B_programming)/VVAvmfao">Rule of three (C++ programming)</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">"dtor". TheFreeDictionary.com. Retrieved 2018-10-14. <a href="https://acronyms.thefreedictionary.com/dtor" target="_blank">https://acronyms.thefreedictionary.com/dtor</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Sebesta, Robert W. (2012). ""11.4.2.3 Constructors and Destructors"". Concepts of Programming Languages (print) (10th ed.). Boston, MA, USA: Addison-Wesley. p. 487. ISBN 978-0-13-139531-2. <a href="978-0-13-139531-2" target="_blank">978-0-13-139531-2</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Sebesta, Robert W. (2012). ""11.4.2.3 Constructors and Destructors"". Concepts of Programming Languages (print) (10th ed.). Boston, MA, USA: Addison-Wesley. p. 487. ISBN 978-0-13-139531-2. <a href="978-0-13-139531-2" target="_blank">978-0-13-139531-2</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">"Finalizers (C# Programming Guide)". <a href="https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/finalizers" target="_blank">https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/finalizers</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Constructors and Destructors, from PHP online documentation <a href="http://www.php.net/manual/en/language.oop5.decon.php" target="_blank">http://www.php.net/manual/en/language.oop5.decon.php</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">"3. Data model — Python 2.7.18 documentation". <a href="https://docs.python.org/2/reference/datamodel.html#object.__del__" target="_blank">https://docs.python.org/2/reference/datamodel.html#object.__del__</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">"3. Data model — Python 3.10.4 documentation". <a href="https://docs.python.org/3/reference/datamodel.html#object.__del__" target="_blank">https://docs.python.org/3/reference/datamodel.html#object.__del__</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">"Destructors - the Rust Reference". <a href="https://doc.rust-lang.org/stable/reference/destructors.html" target="_blank">https://doc.rust-lang.org/stable/reference/destructors.html</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">Sebesta, Robert W. (2012). ""11.4.2.3 Constructors and Destructors"". Concepts of Programming Languages (print) (10th ed.). Boston, MA, USA: Addison-Wesley. p. 487. ISBN 978-0-13-139531-2. <a href="978-0-13-139531-2" target="_blank">978-0-13-139531-2</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">Sebesta, Robert W. (2012). ""11.4.2.3 Constructors and Destructors"". Concepts of Programming Languages (print) (10th ed.). Boston, MA, USA: Addison-Wesley. p. 487. ISBN 978-0-13-139531-2. <a href="978-0-13-139531-2" target="_blank">978-0-13-139531-2</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">Sebesta, Robert W. (2012). ""11.4.2.3 Constructors and Destructors"". Concepts of Programming Languages (print) (10th ed.). Boston, MA, USA: Addison-Wesley. p. 487. ISBN 978-0-13-139531-2. <a href="978-0-13-139531-2" target="_blank">978-0-13-139531-2</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">GotW #47: Uncaught exceptions Accessed 31 July 2011. <a href="http://www.gotw.ca/gotw/047.htm" target="_blank">http://www.gotw.ca/gotw/047.htm</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
<li id="fn:13">Smith, Richard; Voutilainen, Ville. "P0593R6:Implicit creation of objects for low-level object manipulation". open-std.org. Retrieved 2022-11-25. <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p0593r6.html" target="_blank">https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p0593r6.html</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></li>
<li id="fn:14">Scott Meyers: Effective Modern C++, O'REILLY, ISBN 9781491903995 <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></li>
<li id="fn:15">C "destructor" function attribute <a href="https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html#Function-Attributes" target="_blank">https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html#Function-Attributes</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></li>
<li id="fn:16">Erickson, Jon (2008). Hacking the art of exploitation. No Starch Press. ISBN 978-1-59327-144-2. <a href="978-1-59327-144-2" target="_blank">978-1-59327-144-2</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></li>
</ol>

Destructor (computer programming) open-in-new

Destructor (computer programming)