将内存分配给std :: shared_ptr的正确方法 [英] Correct way to allocate memory to std::shared_ptr

问题描述

我实现了一个功能，在该功能中，身份被授予我，而我无法控制.它返回std::shared_ptr<const void>.在该函数中，我分配了任意数量的内存，并通过shared_ptr返回对其的访问.

I have implemented a function where the identity is given to me and out of my control. It returns std::shared_ptr<const void>. In the function i allocate an arbitrary amount of memory, and return access to it though the shared_ptr.

我的内存分配是通过new unsigned char[123]完成的.问题是valgrind检测到new和delete变体的用法之间不匹配.当我使用new[](unsigned)分配内存时，shared_ptr析构函数使用delete(void*)进行分配，并且当您使用不正确"的分配器进行分配时，valgrind会发出警告.

My memory allocation is done with new unsigned char[123]. The problem is that valgrind detects a mismatch between the usage of new and delete variants. While i use new[](unsigned) to allocate memory, the shared_ptr destructor uses delete(void*) to deallocate it, and valgrind warns whenever you are using the "incorrect" deallocator for an allocation.

更实际地说，我编写了这个测试用例来说明我的意思:

In more practical terms, i wrote this test case to show what i mean:

TEST(Example, Test1)
{
  unsigned char* mem = new unsigned char[123];
  std::shared_ptr<const void> ptr(mem);
}

valgrind报告说

The valgrind report says

==45794== Mismatched free() / delete / delete []
==45794==    at 0x4C2A64B: operator delete(void*) (vg_replace_malloc.c:576)
==45794==    by 0x40B7B5: _M_release (shared_ptr_base.h:150)
==45794==    by 0x40B7B5: ~__shared_count (shared_ptr_base.h:659)
==45794==    by 0x40B7B5: ~__shared_ptr (shared_ptr_base.h:925)
==45794==    by 0x40B7B5: ~shared_ptr (shared_ptr.h:93)
==45794==    by 0x40B7B5: Example_Test1_Test::TestBody() (test.cc:108)
==45794==  Address 0x5cb6290 is 0 bytes inside a block of size 123 alloc'd
==45794==    at 0x4C29CAF: operator new[](unsigned long) (vg_replace_malloc.c:423)
==45794==    by 0x40B72E: Example_Test1_Test::TestBody() (test.cc:107)

如果可能，我想避免使用valgrind过滤器.

I want to avoid valgrind filters if possible.

分配任意数量的数据并作为std::shared_ptr<const void>返回的正确方法是什么?

What is the correct way to allocate an arbitrary amount of data and return as a std::shared_ptr<const void>?

推荐答案

如果为shared_ptr<T>提供T指针，则它假定您创建了一个对象，并暗示了删除器delete p;.如果您的分配实际上是使用new-array执行的，则需要传递执行delete[] p;的适当删除器.您可以根据需要重复使用std::default_delete

If you give shared_ptr<T> a T pointer, it assumes that you created a single object and implies the deleter delete p;. If your allocation was actually performed with array-new, you need to pass an appropriate deleter that performs delete[] p;. You can reuse std::default_delete if you like:

return static_pointer_cast<const void>(
    std::shared_ptr<unsigned char>(
        new unsigned char[N],
        std::default_delete<unsigned char[]>()));

(由于隐含转换，您甚至不需要外部演员表.)

(You don't even need the outer cast, since the conversion is implied.)

在C ++ 17中，shared_ptr支持数组，因此可以说

In C++17, shared_ptr supports arrays, so you can say

shared_ptr<unsigned char[]>(new unsigned char[N])

存在并获取正确的删除器(然后转换为空).

there and get the correct deleter (and then convert to void).

这篇关于将内存分配给std :: shared_ptr的正确方法的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！