How to properly free the memory allocated by placement new?
Using the new
expression does two things, it calls the function operator new
which allocates memory, and then it uses placement new, to create the object in that memory. The delete
expression calls the object's destructor, and then calls operator delete
. Yeah, the names are confusing.
//normal version calls these two functions
MyClass* pMemory = new MyClass; void* pMemory = operator new(sizeof(MyClass));
MyClass* pMyClass = new( pMemory ) MyClass();
//normal version calls these two functions
delete pMemory; pMyClass->~MyClass();
operator delete(pMemory);
Since in your case, you used placement new manually, you also need to call the destructor manually. Since you allocated the memory manually, you need to release it manually.
However, placement new is designed to work with internal buffers as well (and other scenarios), where the buffers were not allocated with operator new
, which is why you shouldn't call operator delete
on them.
#include <type_traits>
struct buffer_struct {
std::aligned_storage_t<sizeof(MyClass), alignof(MyClass)> buffer;
};
int main() {
buffer_struct a;
MyClass* pMyClass = new (&a.buffer) MyClass(); //created inside buffer_struct a
//stuff
pMyClass->~MyClass(); //can't use delete, because there's no `new`.
return 0;
}
The purpose of the buffer_struct
class is to create and destroy the storage in whatever way, while main
takes care of the construction/destruction of MyClass
, note how the two are (almost*) completely separate from each other.
*we have to be sure the storage has to be big enough
One reason this is wrong:
delete pMyClass;
is that you must delete pMemory
with delete[]
since it is an array:
delete[] pMemory;
You can't do both of the above.
Similarly, you might ask why you can't use malloc()
to allocate memory, placement new to construct an object, and then delete
to delete and free the memory. The reason is that you must match malloc()
and free()
, not malloc()
and delete
.
In the real world, placement new and explicit destructor calls are almost never used. They might be used internally by the Standard Library implementation (or for other systems-level programming as noted in the comments), but normal programmers don't use them. I have never used such tricks for production code in many years of doing C++.