Assume I have the following code:
Object* obj = new Object();
void* mem = malloc(sizeof(obj) + 40);
*((Object*)mem) = *obj;
In that case, is it guaranteed that obj data would be stored at the beginning of the malloced memory, and can I use the rest of the block mem + sizeof(obj) without overriding obj?
You're guaranteed to have an undefined behaviour.
First, sizeof(obj) is size of a pointer, not size of Object. Use sizeof(*obj) or sizeof(Object). Second, there is no Object at the location mem, so this dereference *((Object*)mem) is undefined behaviour.
Use placement new:
new (mem) Object(*obj); // or new (mem) Object(std::move(*obj));
to put Object at the beginning of mem block. You can use 40 remaining bytes at your own discretion.
sizeof(Object) and thereby eliminate the Object* obj = new Object(); line entirely.
obj it somehow modified after first new. If it is not the case, I agree with your comment.
Objectyou are assigning to is a valid Object (i.e. one that was already constructed and in a valid state), but you are using a pointer (mem) to a bunch of uninitialized bytes instead. You might have better luck using placement-new, e.g.Object * newObj = new (mem) Object();