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 must not be a coroutine.

(since C++20)

Syntax

~ class-name (); (1)
virtual ~ class-name (); (2)
decl-specifier-seq (optional) ~ class-name () = default; (3) (since C++11)
decl-specifier-seq (optional) ~ class-name () = delete; (4) (since C++11)
attr (optional) decl-specifier-seq (optional) id-expression ( void(optional) )
except (optional) attr (optional) requires-clause (optional); 		(5)
1) Typical declaration of a prospective (since C++20) destructor.
2) Virtual destructor is usually required in a base class.
3) Forcing a destructor to be generated by the compiler.
4) Disabling the implicit destructor.
5) Formal syntax of a prospective (since C++20) destructor declaration.
decl-specifier-seq - friend, inline, virtual, constexpr, consteval (since C++20)
id-expression - within a class definition, the symbol ~ followed by the class-name. Within a class template, the symbol ~ followed by the name of the current instantiation of the template. At namespace scope or in a friend declaration within a different class, nested-name-specifier followed by the symbol ~ followed by the class-name which is the same class as the one named by the nested-name-specifier. In any case, the name must be the actual name of the class or template, and not a typedef. The entire id-expression may be surrounded by parentheses which do not change its meaning.
attr - (since C++11) sequence of any number of attributes
except - exception specification as in any function declaration

If no exception specification is explicitly provided, the exception specification is considered to be one that would be used by the implicitly-declared destructor (see below). In most cases, this is noexcept(true). Thus a throwing destructor must be explicitly declared noexcept(false).

(since C++11)
requires-clause - (since C++20) requires-clause that declares the associated constraints for the prospective destructor, which must be satisfied in order for the prospective destructor to be selected as the destructor

Explanation

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

  • thread exit, for objects with thread-local storage duration
(since C++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 may also be called directly, e.g. to destroy an object that was constructed using placement-new or through an allocator member function such as std::allocator::destroy(), to destroy an object that was constructed through the allocator. Note that 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.

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 C++20)

Implicitly-declared destructor

If no user-declared prospective (since C++20) destructor is provided for a class type (struct, class, or union), 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 C++17)implicit definition would directly invoke a function with a different exception specification (until C++17). In practice, implicit destructors are noexcept unless the class is "poisoned" by a base or member whose destructor is noexcept(false).

Deleted implicitly-declared destructor

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

  • T has a non-static data member that cannot be destructed (has deleted or inaccessible destructor).
  • T has direct or virtual base class that cannot be destructed (has deleted or inaccessible destructors).
  • T is a union and has a variant member with non-trivial destructor.
(since C++11)
  • The implicitly-declared destructor is virtual (because the base class has a virtual destructor) and the lookup for the deallocation function (operator delete()) results in a call to ambiguous, deleted, or inaccessible function.

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

(since C++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.

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 C++23)constexpr function (since C++23), the generated destructor is constexpr.

(since C++20)

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 C++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 C++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.

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 C++ standards.

DR Applied to Behavior as published Correct behavior
CWG 193 C++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 C++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 C++98 static members might be destroyed
right after destructor execution 		only destroy non-
static members
CWG 2180 C++98 a destructor for class X calls the destructors
for X's virtual direct base classes 		those destructors are not called

See also

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