堆栈更喜欢堆？

Question

I recently dove into graphics programming and I noticed that many graphic engines (ie Ogre), and many coders overall, prefer to initialize class instances dynamically. Here's an example from Ogre Basic Tutorial 1

//...

Ogre::Entity* ogreHead = mSceneMgr->createEntity("Head", "ogrehead.mesh");
Ogre::SceneNode* headNode = mSceneMgr->getRootSceneNode()->createChildSceneNode("HeadNode");

//...

ogreHead and headNode data members and methods are then referred to as ogreHead->blabla .

Why mess around with object pointers instead of plain objects?

BTW, I've also read somewhere that heap memory allocation is much slower than stack memory allocation.

Answer 1

Heap allocation is, inevitably much slower than stack allocation. More on "How much slower?" later. However, in many cases, the choice is "made for you", for several reasons:

1. Stack is limited. And if you run out, the application almost always gets terminated - there is no real good recovery, even printing an error message to say "I ran out of stack" may be hard...
2. Stack allocation "goes away" when you leave the function where the allocation was made.
3. Variability is much more well defined and easy to deal with. C++ does not cope with "variable length arrays" very well, and it's certainly not guaranteed to work in all compilers.

How much slower is heap over stack?

We'll get to "and does it matter" in a bit.

For a given allocation, stack allocation is simply a subtract operation [1], where at the very minimum new or malloc will be a function call, and probably even the most simple allocator will be several dozen instructions, in complex cases thousands [because memory has to be gotten from the OS, and cleared of it's previous content]. So anything from a 10x to "infinitely" slower, give or take. Exact numbers will depend on the exact system the code is running in, size of the allocation, and often "previous calls to the allocator" (eg a long list of "freed" allocations can make allocating a new object slower, because a good fit has to be searched for). And of course, unless you do the "ostrich" method of heap management, you also need to free the object and cope with "out of memory" which adds more code/time to the execution and complexity of the code.

With some reasonably clever programming, however, this can be mostly hidden - for example, allocating something that stays allocated for a long time, over the lifetime of the object, will be "nothing to worry about". Allocating objects from the heap for every pixel or every trianle in a 3D game would CLEARLY be a bad idea. But if the lifetime of the object is many frames or even the entire game, the time to allocate and free it will be nearly nothing.

Similarly, instead of doing 10000 individual object allocations, make one for 10000 objects. Object pool is one such concept.

Further, often the allocation time isn't where the time is spent. For example, reading a triangle list from a file from a disk will take much longer than allocating the space for the same triangle list - even if you allocate each single one!

To me, the rule is:

1. Does it fit nicely on the stack? Typically a few kilobytes is fine, many kilobytes not so good, and megabytes definitely not ok.
2. Is the number (eg array of objects) known, and the maximum such that you can fit it on the stack?
3. Do you know what the object will be? In other words abstract/polymorphic classes will probably need to be allocated on the heap.
4. Is its lifetime the same as the scope it is in? If not, use the heap (or stack further down, and pass it up the stack).

[1] Or add if stack is "grows towards high addresses" - I don't know of a machine which has such an architecture, but it is conceivable and I think some have been made. C certainly makes no promises as to which way the stack grows, or anything else about how the runtime stack works.

Answer 2

The scope of the stack is limited: it only exists within a function. Now, modern user-interfacing programs are usually event driven, which means that a function of yours is invoked to handle an event, and then that function must return in order for the program to continue running. So, if your event handler function wishes to create an object which will remain in existence after the function has returned, clearly, that object cannot be allocated on the stack of that function, because it will cease to exist as soon as the function returns. That's the main reason why we allocate things on the heap.

There are other reasons, too.

Sometimes, the exact size of a class is not known during compilation time. If the exact size of a class is not known, it cannot be created on the stack, because the compiler needs to have precise knowledge of how much space it needs to allocate for each item on the stack.

Furthermore, factory methods like whatever::createEntity() are often used. If you have to invoke a separate method to create an object for you, then that object cannot be created on the stack, for the reason explained in the first paragraph of this answer.

Answer 3

Why pointers instead of objects?

Because pointers help make things fast. If you pass an object by value, to another function, for example

shoot(Orge::Entity ogre)

instead of

shoot(Orge::Entity* ogrePtr)

If ogre isn't a pointer, what happens is you are passing the whole object into the function, rather than a reference. If the compiler doesn't optimize, you are left with an inefficient program. There are other reasons too, with the pointer, you can modify the passed in object (some argue references are better but that's a different discussion). Otherwise you would be spending too much time copying modified objects back and forth.

Why heap?

1. In some sense heap is a safer type of memory to access and allows you to safely reset/recover. If you call new and don't have memory, you can flag that as an error. If you are using the stack, there is actually no good way to know you have caused stackoverflow, without some other supervising program, at which point you are already in danger zone.
2. Depends on your application. Stack has local scope so if the object goes out of scope, it will deallocate memory for the object. If you need the object in some other function, then no real way to do that.
3. Applies more to OS, heap is comparatively much larger than stack, especially in multi-threaded application where each thread can have a limited stack size.