How can deleting a void pointer do anything other than invoke the global delete operator?

问题

The C++ standard very clearly and explicitly states that using delete or delete[] on a void-pointer is undefined behavior, as quoted in this answer:

This implies that an object cannot be deleted using a pointer of type void* because there are no objects of type void.

However, as I understand it, delete and delete[] do just two things:

• Call the appropriate destructor(s)
• Invoke the appropriate operator delete function, typically the global one

There is a single-argument operator delete (as well as operator delete[]), and that single argument is void* ptr.

So, when the compiler encounters a delete-expression with a void* operand, it of course could maliciously do some completely unrelated operation, or simply output no code for that expression. Better yet, it could emit a diagnostic message and refuse to compile, though the versions of MSVS, Clang, and GCC I've tested don't do this. (The latter two emit a warning with -Wall; MSVS with /W3 does not.)

But there's really only one sensible way to deal with each of the above steps in the delete operation:

• void* specifies no destructor, so no destructors are invoked.
• void is not a type and therefore cannot have a specific corresponding operator delete, so the global operator delete (or the [] version) must be invoked. Since the argument to the function is void*, no type conversion is necessary, and the operator function must behavior correctly.

So, can common compiler implementations (which, presumably, are not malicious, or else we could not even trust them to adhere to the standard anyway) be relied on to follow the above steps (freeing memory without invoking destructors) when encountering such delete expressions? If not, why not? If so, is it safe to use delete this way when the actual type of the data has no destructors (e.g. it's an array of primitives, like long[64])?

Can the global delete operator, void operator delete(void* ptr) (and the corresponding array version), be safely invoked directly for void* data (assuming, again, that no destructors ought to be called)?

回答1:

A void* is a pointer to an object of unknown type. If you do not know the type of something, you cannot possibly know how that something is to be destroyed. So I would argue that, no, there is not "really only one sensible way to deal with such a delete operation". The only sensible way to deal with such a delete operation, is to not deal with it. Because there is simply no way you could possibly deal with it correctly.

Therefore, as the original answer you linked to said: deleting a void* is undefined behavior ([expr.delete] §2). The footnote mentioned in that answer remains essentially unchanged to this day. I'm honestly a bit astonished that this is simply specified as undefined behavior rather than making it ill-formed, since I cannot think of any situation in which this could not be detected at compile time.

Note that, starting with C++14, a new expression does not necessarily imply a call to an allocation function. And neither does a delete expression necessarily imply a call to a deallocation function. The compiler may call an allocation function to obtain storage for an object created with a new expression. In some cases, the compiler is allowed to omit such a call and use storage allocated in other ways. This, e.g., enables the compiler to sometimes pack multiple objects created with new into one allocation.

Is it safe to call the global deallocation function on a void* instead of using a delete expression? Only if the storage was allocated with the corresponding global allocation function. In general, you can't know that for sure unless you called the allocation function yourself. If you got your pointer from a new expression, you generally don't know if that pointer would even be a valid argument to a deallocation function, since it may not even point to storage obtained from calling an allocation function. Note that knowing which allocation function must've been used by a new expression is basically equivalent to knowing the dynamic type of whatever your void* points to. And if you knew that, you could also just static_cast<> to the actual type and delete it…

Is it safe to deallocate the storage of an object with trivial destructor without explicitly calling the destructor first? Based on, [basic.life] §1.4, I would say yes. Note that, if that object is an array, you might still have to call the destructors of any array elements first. Unless they are also trivial.

Can you rely on common compiler implementations to produce the behavior you deem reasonable? No. Having a formal definition of what exactly you can rely on is literally the whole point of having a standard in the first place. Assuming you have a standard-conforming implementation, you can rely on the guarantees the standard gives you. You can also rely on any additional guarantees the documentation of a particular compiler may give you, so long as you use that particular version of that particular compiler to compile your code. Beyond that, all bets are off…

回答2:

If you want to invoke the deallocation function, then just call the deallocation function.

This is good:

void* p = ::operator new(size);

::operator delete(p);  // only requires that p was returned by ::operator new()

This is not:

void* p = new long(42);

delete p;  // forbidden: static and dynamic type of *p do not match, and static type is not polymorphic

But note, this also is not safe:

void* p = new long[42];

::operator delete(p); // p was not obtained from allocator ::operator new()

回答3:

While the Standard would allow an implementation to use the type passed to delete to decide how to clean up the object in question, it does not require that implementations do so. The Standard would also allow an alternative (and arguably superior) approach based on having the memory-allocating new store cleanup information in the space immediately preceding the returned address, and having delete implemented as a call to something like:

typedef void(*__cleanup_function)(void*);
void __delete(void*p)
{
  *(((__cleanup_function*)p)[-1])(p);
}

In most cases, the cost of implementing new/delete in such fashion would be relatively trivial, and the approach would offer some semantic benefit. The only significant downside of such an approach is that it would require that implementations that document the inner workings of their new/delete implementation, and whose implementations can't support a type-agnostic delete, would have to break any code that relies upon their documented inner workings.

Note that if passing a void* to delete were a constraint violation, that would forbid implementations from providing a type-agnostic delete even if they would be easily capable of doing so, and even if some code written for them would relies upon such ability. The fact that code relies upon such an ability would make it portable only to implementations that can provide it, of course, but allowing implementations to support such abilities if they choose to do so is more useful than making it a constraint violation.

Personally, I would have liked to see the Standard offer implementations two specific choices:

1. Allow passing a void* to delete and delete the object using whatever type had been passed to new, and define a macro indicating support for such a construct.

2. Issue a diagnostic if a void* is passed to delete, and define a macro indicating it does not support such a construct.

Programmers whose implementations supported type-agnostic delete could then decide whether the benefit they could receive from such feature would justify the portability limitations imposed by using it, and implementers could decide whether the benefits of supporting a wider range of programs would be sufficient to justify the small cost of supporting the feature.

回答4:

void* specifies no destructor, so no destructors are invoked.

That is most likely one of the reasons it's not permitted. Deallocating the memory that backs a class instance without calling the destructor for said class is just all around a really really bad idea.

Suppose, for example, the class contains a std::map that has a few hundred thousand elements in it. That represents a significant amount of memory. Doing what you're proposing would leak all of that memory.

回答5:

A void doesn't have a size, so the compiler has no way of knowing how much memory to deallocate.

How should the compiler handle the following?

struct s
{
    int arr[100];
};

void* p1 = new int;
void* p2 = new s;
delete p1;
delete p2;

来源：https://stackoverflow.com/questions/51623643/how-can-deleting-a-void-pointer-do-anything-other-than-invoke-the-global-delete