# std::default_delete

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< cpp‎ | memory

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.

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) |
template <class U> default_delete( const default_delete<U>& d ) noexcept; | (2) | (member only of primary `default_delete` template)
template<class U> default_delete( const default_delete<U[]>& d) noexcept; | (3) | (since C++17) (member only of the array `default_delete` specialization)

1) Constructs a `std::default_delete` object.
2) Constructs a `std::default_delete` 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<U[]>` 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()

void operator()(T* ptr) const; | (1) | (as of C++17, no longer a member of the `default_delete<T[]>` template specialization)
template <class U> void operator()(U* ptr) const; | (2) | (member only of `default_delete<T[]>` template specialization, but defined as deleted prior to C++17)

1) Calls `delete` (primary template) or `delete[]` (array specialization) on `ptr` 2) Defined as deleted | (until C++17)
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(*)[]. | (since C++17)

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.

### 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>());
    }

### See also

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

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