std::default_delete

 

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std::default_delete

 

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

	(1)
void operator()(T* ptr) const;			(since C++11) (until C++23) (member only of primary default_delete template)
constexpr void operator()(T* ptr) const;			(since C++23) (member only of primary default_delete template)
		(2)
template<class U> void operator()(U* ptr) const;				(since C++11) (until C++23) (member only of default_delete<T[]> specialization)
template<class U> constexpr void operator()(U* ptr) const;				(since C++23) (member only of default_delete<T[]> 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	Value	Std	Comment
__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[]>());
    } // 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[]> rejected qualification conversions	accept

See also

unique_ptr (C++11)

smart pointer with unique object ownership semantics (class template)