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 = default; (1)
Primary template specializations
template< class U > (2) (since C++11)
default_delete( const default_delete<U>& d ) noexcept; (constexpr since C++23)
Array specializations
template< class U > (3) (since C++11)
default_delete( const default_delete<U[]>& d ) noexcept; (constexpr since C++23)

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

Primary template specializations
void operator()( T* ptr ) const; (1) (since C++11)
(constexpr since C++23)
Array specializations
template< class U > (2) (since C++11)
void operator()( U* ptr ) const; (constexpr since C++23)

1) Calls delete on ptr.
2) Calls delete[] on ptr.
This overload participates in overload resolution only if U(*)[] is implicitly
convertible to T(*)[].
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 Value Std Feature
__cpp_lib_constexpr_memory 202202L (C++23) constexpr constructor and operator()

Example¶

// Run this code

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

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[]>());
} // OK: the destructor calls delete[]

{
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 the array specializations accept
rejected qualification conversions

See also¶

unique_ptr smart pointer with unique object ownership semantics
(C++11) (class template)