shared_ptr到数组：应该使用它吗？ [英] shared_ptr to an array : should it be used?
问题描述
只是关于 shared_ptr 的小型查询。
Just a small query regarding shared_ptr.
使用 shared_ptr 指向数组是个好习惯吗？
例如 shared_ptr< int> sp（new int [10]）;
Is it a good practice to use shared_ptr pointing to an array?
e.g shared_ptr<int> sp(new int[10]);
如果没有，那么任何人都可以告诉我为什么？我已经知道的一个原因是不能增加/减少 shared_ptr 。
If not then can anyone please tell me why? One reason i am already aware of is one can not increment/decrement the shared_ptr. Hence it can not be used like a normal pointer to an array.
推荐答案
默认情况下， shared_ptr 将在没有更多引用保留到托管对象上时调用 delete 。但是，当使用 new [] 分配时，需要调用 delete [] ，而不是 delete ，释放资源。
By default, shared_ptr will call delete on the managed object when no more references remain to it. However, when you allocate using new[] you need to call delete[], and not delete, to free the resource.
为了使用数组正确使用 shared_ptr ，必须提供自定义删除程序。
In order to correctly use shared_ptr with an array, you must supply a custom deleter.
template< typename T >
struct array_deleter
{
void operator ()( T const * p)
{
delete[] p;
}
};
创建shared_ptr如下：
Create the shared_ptr as follows:
std::shared_ptr<int> sp( new int[10], array_deleter<int>() );
现在 shared_ptr 将正确调用 delete [] 销毁受管对象。
Now shared_ptr will correctly call delete[] when destroying the managed object.
，使用C ++ 11，您可以使用 std :: default_delete 部分专业化数组类型，而不是上面的 array_deleter 。
As shown in the comments, with C++11, you can use the std::default_delete partial specialization for array types instead of array_deleter above.
std::shared_ptr<int> sp( new int[10], std::default_delete<int[]>() );
您也可以使用lambda表达式。
You can also use a lambda expression instead of functors.
std::shared_ptr<int> sp( new int[10], []( int *p ) { delete[] p; } );
此外，除非您确实需要共享对象， unique_ptr 更适合此任务，因为它对数组类型有部分专门化。
Also, unless you actually need to share the managed object, a unique_ptr is better suited for this task, since it has a partial specialization for array types.
std::unique_ptr<int[]> up( new int[10] ); // this will correctly call delete[]
由C + +扩展库引入的更改基础库
shared_ptr 通过图书馆基础技术规范（TS）增加，以允许它在拥有对象数组的情况下工作;将不需要明确提供删除器。此TS的 shared_ptr 更改的当前草案可以在 N4082 。这些更改将通过 std :: experimental 命名空间访问，并包含在< experimental / memory> 。支持数组的 shared_ptr 的一些相关更改是：
Changes introduced by the C++ Extensions for Library Fundamentals
shared_ptr is being augmented by the Library Fundamentals Technical Specification (TS) to allow it to work out of the box for the cases when it owns an array of objects; there'll be no need to explicitly supply a deleter. The current draft of the shared_ptr changes slated for this TS can be found in N4082. These changes will be accessible via the std::experimental namespace, and included in the <experimental/memory> header. A few of the relevant changes to support shared_ptr for arrays are:
—成员类型的定义 element_type 更改
— The definition of the member type element_type changes
typedef T element_type;
typedef T element_type;
typedef typename remove_extent<T>::type element_type;
—正在添加会员运算符[]
— Member operator[] is being added
element_type& operator[](ptrdiff_t i) const noexcept;
—与 unique_ptr 数组的部分专门化不同， shared_ptr< T []> 和 shared_ptr< ; T [N]> 将有效，并且将导致在被管理的对象数组上调用 delete [] 。 >
— Unlike the unique_ptr partial specialization for arrays, both shared_ptr<T[]> and shared_ptr<T[N]> will be valid and both will result in delete[] being called on the managed array of objects.
template<class Y> explicit shared_ptr(Y* p);
需要： Y 应为完整类型。当 T 是数组类型或 delete p / code>，当 T 不是一个数组类型，应该是良好的形式，应该有良好的定义行为，不会抛出异常。当 T 为 U [N] ， Y（*）[N] 可转换为 T * ;当 T 是 U [] ， Y（*）[] 应可转换为 T * ;否则 Y * 可转换为 T * 。
Requires: Y shall be a complete type. The expression delete[] p, when T is an array type, or delete p, when T is not an array type, shall be well-formed, shall have well defined behavior, and shall not throw exceptions. When T is U[N], Y(*)[N] shall be convertible to T*; when T is U[], Y(*)[] shall be convertible to T*; otherwise, Y* shall be convertible to T*.
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