Alloc more than one object at the time

Question

I would like to know how to allow more than one object at the time. I am looking for something like NSSet with 50 empty Objects.

The reason for that is that I have a intensive work with a lot of allocations (1.000.000) and it is very time consuming to to it per one bases. (Alloc method is time expensive).

Answer 1

To answer your question directly, there are two runtime functions that allow you to create/destruct an object located in a pre-allocated block of memory:

/** 
 * Creates an instance of a class at the specific location provided.
 * 
 * @param cls The class that you wish to allocate an instance of.
 * @param bytes The location at which to allocate an instance of \e cls.
 *  Must point to at least \c class_getInstanceSize(cls) bytes of well-aligned,
 *  zero-filled memory.
 */
OBJC_EXPORT id objc_constructInstance(Class cls, void *bytes) 
    __OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_0)
    OBJC_ARC_UNAVAILABLE;

/** 
 * Destroys an instance of a class without freeing memory and removes any
 * associated references this instance might have had.
 */
OBJC_EXPORT void *objc_destructInstance(id obj) 
    __OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_0)
    OBJC_ARC_UNAVAILABLE;

Totally untested example usage (ARC must be disabled in the @implementation file):

+ (void)allocInPlace:(void *)buffer
{
    objc_constructInstance(self, buffer);
}

- (void)dealloc
{
    objc_destructInstance(self);
}

size_t instanceSize = class_getInstanceSize(MyClass.class);

// This may not be enough, since we need to ensure that object instances are 16-byte aligned
void *buffer = calloc(instanceSize, numObjects);

for (size_t i = 0; i < numObjects; i++) {
    // Need to ensure that this is 16-byte aligned
    void *objBuffer = buffer + (instanceSize * i); 
    [mySet addObject:[[MyClass allocInPlace:objBuffer] init]];
}

You'll need to hang onto buffer and free() it when no longer needed.

However you should probably just use a C array of structs if possible. The technique I've used here runs against standard Obj-C practice and is error-prone (if, for example, you free the buffer before all objects have been destructed then Bad Things will happen).