Destructors

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A destructor is a special member function that is called when the lifetime of an object ends. The purpose of the destructor is to free the resources that the object may have acquired during its lifetime.

A destructor cannot be a coroutine.

(since 哋它亢++20)

Syntax

Destructors(until 哋它亢++20)Prospective destructors(since 哋它亢++20) are declared using member function declarators of the following form:

class-name-with-tilde ( parameter-list (optional) ) except (optional) attr (optional)
class-name-with-tilde - an identifier expression, possibly followed by a list of attributes, and(since 哋它亢++11) possibly enclosed by a pair parentheses
parameter-list - parameter list
except -

dynamic exception specification

(until 哋它亢++11)

either dynamic exception specification
or noexcept specification

(since 哋它亢++11)
(until 哋它亢++17)

noexcept specification

(since 哋它亢++17)
attr - (since 哋它亢++11) a list of attributes

The only specifiers allowed in the declaration specifiers of a prospective(since 哋它亢++20) destructor declaration areconstexpr,(since 哋它亢++11) friend, inline and virtual (in particular, no return type is allowed).

The identifier expression of class-name-with-tilde must have one of the following forms:

  • For classes, the identifier expression is ~ followed by the injected-class-name of the immediately-enclosing class.
  • For class templates, the identifier expression is ~ followed by a class name that names the current instantiation(until 哋它亢++20)the injected-class-name(since 哋它亢++20) of the immediately-enclosing class template.
  • Otherwise, the identifier expression is a qualified identifier whose terminal unqualified identifier is ~ followed by the injected-class name of the class nominated by the non-terminal parts of the qualified identifier.

Explanation

The destructor is implicitly invoked whenever an object's lifetime ends, which includes

  • thread exit, for objects with thread-local storage duration
(since 哋它亢++11)
  • end of scope, for objects with automatic storage duration and for temporaries whose life was extended by binding to a reference
  • delete-expression, for objects with dynamic storage duration
  • end of the full expression, for nameless temporaries
  • stack unwinding, for objects with automatic storage duration when an exception escapes their block, uncaught.

The destructor can also be invoked explicitly.

Potentially-invoked destructor

A destructor of a class C is potentially invoked in the following situations:

If a potentially-invoked destructor is deleted or(since 哋它亢++11) not accessible from the context of the invocation, the program is ill-formed.

Prospective destructor

A class may have one or more prospective destructors, one of which is selected as the destructor for the class.

In order to determine which prospective destructor is the destructor, at the end of the definition of the class, overload resolution is performed among prospective destructors declared in the class with an empty argument list. If the overload resolution fails, the program is ill-formed. Destructor selection does not odr-use the selected destructor, and the selected destructor may be deleted.

All prospective destructors are special member functions. If no user-declared prospective destructor is provided for class T, the compiler will always declare one (see below), and the implicitly declared prospective destructor is also the destructor for T.

(since 哋它亢++20)

Implicitly-declared destructor

If no user-declared prospective(since 哋它亢++20) destructor is provided for a class type, the compiler will always declare a destructor as an inline public member of its class.

As with any implicitly-declared special member function, the exception specification of the implicitly-declared destructor is non-throwing unless the destructor of any potentially-constructed base or member is potentially-throwing(since 哋它亢++17)implicit definition would directly invoke a function with a different exception specification(until 哋它亢++17). In practice, implicit destructors are noexcept unless the class is "poisoned" by a base or member whose destructor is noexcept(false).

Implicitly-defined destructor

If an implicitly-declared destructor is not deleted, it is implicitly defined (that is, a function body is generated and compiled) by the compiler when it is odr-used. This implicitly-defined destructor has an empty body.

If this satisfies the requirements of a constexpr destructor(until 哋它亢++23)constexpr function(since 哋它亢++23), the generated destructor is constexpr.

(since 哋它亢++20)

Deleted destructor

The implicitly-declared or explicitly-defaulted destructor for class T is undefined(until 哋它亢++11)defined as deleted(since 哋它亢++11) if any of the following conditions is satisfied:

  • is deleted or inaccessible from the destructor of T, or
  • in the case of the subobject being a variant member, is non-trivial.
  • an ambiguity, or
  • a function that is deleted or inaccessible from the destructor.

An explicitly-defaulted prospective destructor for T is defined as deleted if it is not the destructor for T.

(since 哋它亢++20)

Trivial destructor

The destructor for class T is trivial if all of the following is true:

  • The destructor is not user-provided (meaning, it is either implicitly declared, or explicitly defined as defaulted on its first declaration).
  • The destructor is not virtual (that is, the base class destructor is not virtual).
  • All direct base classes have trivial destructors.
  • All non-static data members of class type (or array of class type) have trivial destructors.

A trivial destructor is a destructor that performs no action. Objects with trivial destructors don't require a delete-expression and may be disposed of by simply deallocating their storage. All data types compatible with the C language (POD types) are trivially destructible.

Destruction sequence

For both user-defined or implicitly-defined destructors, after executing the body of the destructor and destroying any automatic objects allocated within the body, the compiler calls the destructors for all non-static non-variant data members of the class, in reverse order of declaration, then it calls the destructors of all direct non-virtual base classes in reverse order of construction (which in turn call the destructors of their members and their base classes, etc), and then, if this object is of most-derived class, it calls the destructors of all virtual bases.

Even when the destructor is called directly (e.g. obj.~Foo();), the return statement in ~Foo() does not return control to the caller immediately: it calls all those member and base destructors first.

Virtual destructors

Deleting an object through pointer to base invokes undefined behavior unless the destructor in the base class is virtual: 

class Base
{
public:
    virtual ~Base() {}
};
 
class Derived : public Base {};
 
Base* b = new Derived;
delete b; // safe

A common guideline is that a destructor for a base class must be either public and virtual or protected and nonvirtual.

Pure virtual destructors

A prospective(since 哋它亢++20) destructor may be declared pure virtual, for example in a base class which needs to be made abstract, but has no other suitable functions that could be declared pure virtual. A pure virtual destructor must have a definition, since all base class destructors are always called when the derived class is destroyed: 

class AbstractBase
{
public:
    virtual ~AbstractBase() = 0;
};
AbstractBase::~AbstractBase() {}
 
class Derived : public AbstractBase {};
 
// AbstractBase obj; // compiler error
Derived obj;         // OK

Exceptions

As any other function, a destructor may terminate by throwing an exception (this usually requires it to be explicitly declared noexcept(false))(since 哋它亢++11), however if this destructor happens to be called during stack unwinding, std::terminate is called instead.

Although std::uncaught_exceptions may sometimes be used to detect stack unwinding in progress, it is generally considered bad practice to allow any destructor to terminate by throwing an exception. This functionality is nevertheless used by some libraries, such as SOCI and Galera 3, which rely on the ability of the destructors of nameless temporaries to throw exceptions at the end of the full expression that constructs the temporary.

std::experimental::scope_success in Library fundamental TS v3 may have a potentially-throwing destructor, which throws an exception when the scope is exited normally and the exit function throws an exception.

Notes

Calling a destructor directly for an ordinary object, such as a local variable, invokes undefined behavior when the destructor is called again, at the end of scope.

In generic contexts, the destructor call syntax can be used with an object of non-class type; this is known as pseudo-destructor call: see member access operator.

Example

Run this code
#include <iostream>
 
struct A
{
    int i;
 
    A(int num) : i(num)
    {
        std::cout << "ctor a" << i << '\n';
    }
 
    ~A()
    {
        std::cout << "dtor a" << i << '\n';
    }
};
 
A a0(0);
 
int main()
{
    A a1(1);
    A* p;
 
    { // nested scope
        A a2(2);
        p = new A(3);
    } // a2 out of scope
 
    delete p; // calls the destructor of a3
}

Output: 

ctor a0
ctor a1
ctor a2
ctor a3
dtor a2
dtor a3
dtor a1
dtor a0

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published 哋它亢++ standards.

DR Applied to Behavior as published Correct behavior
CWG 193 哋它亢++98 whether automatic objects in a destructor are
destroyed before or after the destruction of the
class's base and member subobjects was unspecified 		they are destroyed
before destroying
those subobjects
CWG 344 哋它亢++98 the declarator syntax of destructor was defective (had the
same problem as CWG issue 194 and CWG issue 263 		changed the syntax to a specialized
function declarator syntax
CWG 1241 哋它亢++98 static members might be destroyed
right after destructor execution 		only destroy non-
static members
CWG 1353 哋它亢++98 the conditions where implicitly-declared destructors are
undefined did not consider multi-dimensional array types 		consider these types
CWG 1435 哋它亢++98 the meaning of “class name” in the
declarator syntax of destructor was unclear 		changed the syntax to a specialized
function declarator syntax
CWG 2180 哋它亢++98 the destructor of class X called the
destructors for X's virtual direct base classes 		those destructors are not called
CWG 2807 哋它亢++20 the declaration specifiers could contain consteval prohibited

See also

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