std::default_delete
Defined in header <memory>
template< class T > struct default_delete;
template< class T > struct default_delete<T[]>;
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.
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
pub | (constructor) | constructs a default_delete object (public member function) |
pub | operator() | deletes the object or array (public member function) |
std::default_delete::default_delete
// 1)
constexpr default_delete() noexcept = default;
// 2)
template <class U>
default_delete( const default_delete<U>& d ) noexcept; // (since C++11, until C++23)
template <class U>
constexpr default_delete( const default_delete<U>& d ) noexcept; // (since C++23)
// 3)
template<class U>
default_delete( const default_delete<U[]>& d ) noexcept; // (since C++11, until C++23)
template<class U>
constexpr default_delete( const default_delete<U[]>& d ) noexcept; // (since C++23)
Constructs a std::default_delete object.
2) (member only of primary default_delete template)
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) (member only of default_delete<T[]> specialization)
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)
constexpr void operator()(T* ptr) const; // (since C++23)
// 2)
template <class U>
void operator()(U* ptr) const; // (since C++11, until C++23)
template <class U>
constexpr void operator()(U* ptr) const; // (since C++23)
-
(member only of primary default_delete template)
Calls delete on ptr -
(member only of default_delete<T[]> specialization)
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
#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[]> rejected qualification conversions | accept |