NAME

std::default_delete - std::default_delete

Synopsis

Defined in header <memory>
template< class T > struct default_delete; (1) (since C++11)
template< class T > struct default_delete<T[]>; (2) (since C++11)

std::default_delete is the default destruction policy used by std::unique_ptr when
no deleter is specified. Specializations of default_delete are empty classes on
typical implementations, and used in the empty base class optimization.

1) The non-specialized default_delete uses delete to deallocate memory for a single
object.
2) A partial specialization for array types that uses delete[] is also provided.

Member functions

constructor constructs a default_delete object
(public member function)
operator() deletes the object or array
(public member function)

std::default_delete::default_delete

constexpr default_delete() noexcept = (1)
default;
(since C++11)
template <class U> (until C++23)
default_delete( const default_delete<U>& (member only of
d ) noexcept; primary
default_delete
template)
(since C++23)
template <class U> (member only of
constexpr default_delete( const primary
default_delete<U>& d ) noexcept; (2) default_delete
template)
(since C++11)
template<class U> (until C++23)
default_delete( const (member only of
default_delete<U[]>& d ) noexcept; default_delete<T[]>
(3) specialization)
template<class U> (since C++23)
constexpr default_delete( const (member only of
default_delete<U[]>& d ) noexcept; default_delete<T[]>
specialization)

1) Constructs a std::default_delete object.
2) Constructs a std::default_delete<T> object from another std::default_delete
object. This constructor will only participate in overload resolution if U* is
implicitly convertible to T*.
3) Constructs a std::default_delete<T[]> object from another
std::default_delete<U[]> object. This constructor will only participate in overload
resolution if U(*)[] is implicitly convertible to T(*)[].

Parameters

d - a deleter to copy from

Notes

The converting constructor template of std::default_delete makes possible the
implicit conversion from std::unique_ptr<Derived> to std::unique_ptr<Base>.

std::default_delete::operator()

(since C++11)
void operator()(T* (until C++23)
ptr) const; (member only of primary
default_delete
template)
constexpr void (since C++23)
operator()(T* ptr) (member only of primary
const; default_delete
(1) template)
(since C++11)
template <class U> (until C++23)
void operator()(U* (member only of
ptr) const; default_delete<T[]>
(2) specialization)
template <class U> (since C++23)
constexpr void (member only of
operator()(U* ptr) default_delete<T[]>
const; specialization)

1) Calls delete on ptr
2) Calls delete[] on ptr. This function will only participate in overload resolution
if U(*)[] is implicitly convertible to T(*)[].

In any case, if U is an incomplete type, the program is ill-formed.

Parameters

ptr - an object or array to delete

Exceptions

No exception guarantees.

Invoking over Incomplete Types

At the point in the code the operator() is called, the type must be complete. In
some implementations a static_assert is used to make sure this is the case. The
reason for this requirement is that calling delete on an incomplete type is
undefined behavior in C++ if the complete class type has a nontrivial destructor or
a deallocation function, as the compiler has no way of knowing whether such
functions exist and must be invoked.

Notes

Feature-test macro: __cpp_lib_constexpr_memory ((C++23) constexpr)

Example

// Run this code

#include <memory>
#include <vector>
#include <algorithm>

int main()
{
// {
// std::shared_ptr<int> shared_bad(new int[10]);
// } // the destructor calls delete, undefined behavior

{
std::shared_ptr<int> shared_good(new int[10], std::default_delete<int[]>());
} // the destructor calls delete[], ok

{
std::unique_ptr<int> ptr(new int(5));
} // unique_ptr<int> uses default_delete<int>

{
std::unique_ptr<int[]> ptr(new int[10]);
} // unique_ptr<int[]> uses default_delete<int[]>

// default_delete can be used anywhere a delete functor is needed
std::vector<int*> v;
for(int n = 0; n < 100; ++n)
v.push_back(new int(n));
std::for_each(v.begin(), v.end(), std::default_delete<int>());
}

Defect reports

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

DR Applied to Behavior as published Correct behavior
LWG 2118 C++11 member functions of default_delete<T[]> accept
rejected qualification conversions