Callling object constructor/destructor with a custom allocator

c++, memory, memory-management, new-operator

Solution

In essence, when you use a new expression like: `T *t = new T;`, it's roughly equivalent to:

void *temp = operator new(sizeof(T));
T *t = new(temp) T;

So, first it allocates some raw memory using the allocation function, then it constructs an object in that memory. Likewise, when you use a delete expression like: `delete t;`, it's roughly equivalent to:

t->~T();
operator delete(t);

So, if you overload `new` and `delete` for a particular class:

class T { 
    int data; 
public:
    // I've made these static explicitly, but they'll be static even if you don't.
    static void *operator new(size_t size) { 
        return malloc(size);
    }
    static void operator delete(void *block) { 
        free(block);
    }
};

Then when you use a new expression, it'll invoke the class' `operator new` to allocate the memory, and that will call `malloc`, so `T *t = new T();` will end up allocating memory via `malloc` (and likewise, when you `delete` it, it'll use `operator delete`, which will call `free`).

At least as the term is normally used, an Allocator is quite similar, except that it's used by a container instead of other code. It also encapsulates the allocation function and deletion function into a class, so when you pass one to the container, you only have to pass one object, and there's little chance of an allocation and delete function getting mismatched.

Ignoring, for the moment, the details about what names are used for things, the Allocator class in the standard library mostly does the same, so with a little renaming of the functions in the `T` class above, you'd be about half done writing a standard allocator. To go with the allocation and deletion, it has a function to `rebind` some memory (change a block of memory to another type), create an object in place (basically just a wrapper around a placement new) and destroy an object (again, trivial wrapper around destructor invocation). Of course, it uses `operator new` and `operator delete` instead of `malloc` and `free` like I've used above.

Problem

I have been looking into custom allocators and I quite often see them using some kind of function to allocate memory. For testing purposes and further educate my self, I tried to make a "simple" example of doing so. However, there is one fundamental thing I am understand on how to do. One of the key differences in `malloc` vs `new` is that with new the constructor is called. What if I wanted to write my own allocator that was essentially replacing `new`, how would I get the constructor to be called when using `malloc`? I understand that on classes I can overload `new` and `delete` for the class, so I suppose a big part of the question is, how is `new` calling the objects constructor during allocation? Similarly, I am interested in how `delete` is calling the destructor. I created a sample test code that I was hoping to have the `SomeClass` constructor called during allocation, but I don't see how. ``` #include <malloc.h> void* SomeAllocationFunction(size_t size) { return malloc(size); } class SomeClass { public: SomeClass() { int con = 1000; } ~SomeClass() { int des = 80; } }; int main(void){ SomeClass* t = (SomeClass*)SomeAllocationFunction(sizeof(SomeClass)); return 0; } ``` (As a note, I know I can just use `new`. However, for the purposes of learning I am trying to create a custom allocator that does not just call `new` or `placement new`).

Original source