Stack, Static, and Heap in C++

Question

I\'ve searched, but I\'ve not understood very well these three concepts. When do I have to use dynamic allocation (in the heap) and what\'s its real advantage? What are the

Answer 1

It's been said elaborately, just as "the short answer":

• static variable (class)
  lifetime = program runtime (1)
  visibility = determined by access modifiers (private/protected/public)

• static variable (global scope)
  lifetime = program runtime (1)
  visibility = the compilation unit it is instantiated in (2)

• heap variable
  lifetime = defined by you (new to delete)
  visibility = defined by you (whatever you assign the pointer to)

• stack variable
  visibility = from declaration until scope is exited
  lifetime = from declaration until declaring scope is exited

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(1) more exactly: from initialization until deinitialization of the compilation unit (i.e. C / C++ file). Order of initialization of compilation units is not defined by the standard.

(2) Beware: if you instantiate a static variable in a header, each compilation unit gets its own copy.

Answer 2

I'm sure one of the pedants will come up with a better answer shortly, but the main difference is speed and size.

Stack

Dramatically faster to allocate. It is done in O(1) since it is allocated when setting up the stack frame so it is essentially free. The drawback is that if you run out of stack space you are boned. You can adjust the stack size, but IIRC you have ~2MB to play with. Also, as soon as you exit the function everything on the stack is cleared. So it can be problematic to refer to it later. (Pointers to stack allocated objects leads to bugs.)

Heap

Dramatically slower to allocate. But you have GB to play with, and point to.

Garbage Collector

The garbage collector is some code that runs in the background and frees memory. When you allocate memory on the heap it is very easy to forget to free it, which is known as a memory leak. Over time, the memory your application consumes grows and grows until it crashes. Having a garbage collector periodically free the memory you no longer need helps eliminate this class of bugs. Of course this comes at a price, as the garbage collector slows things down.

Answer 3

What if your program does not know upfront how much memory to allocate (hence you cannot use stack variables). Say linked lists, the lists can grow without knowing upfront what is its size. So allocating on a heap makes sense for a linked list when you are not aware of how many elements would be inserted into it.

Answer 4

Stack is a memory allocated by the compiler, when ever we compiles the program, in default compiler allocates some memory from OS ( we can change the settings from compiler settings in your IDE) and OS is the one which give you the memory, its depends on many available memory on the system and many other things, and coming to stack memory is allocate when we declare a variable they copy(ref as formals) those variables are pushed on to stack they follow some naming conventions by default its CDECL in Visual studios ex: infix notation: c=a+b; the stack pushing is done right to left PUSHING, b to stack, operator, a to stack and result of those i,e c to stack. In pre fix notation: =+cab Here all the variables are pushed to stack 1st (right to left)and then the operation are made. This memory allocated by compiler is fixed. So lets assume 1MB of memory is allocated to our application, lets say variables used 700kb of memory(all the local variables are pushed to stack unless they are dynamically allocated) so remaining 324kb memory is allocated to heap. And this stack has less life time, when the scope of the function ends these stacks gets cleared.

Answer 5

What are the problems of static and stack?

The problem with "static" allocation is that the allocation is made at compile-time: you can't use it to allocate some variable number of data, the number of which isn't known until run-time.

The problem with allocating on the "stack" is that the allocation is destroyed as soon as the subroutine which does the allocation returns.

I could write an entire application without allocate variables in the heap?

Perhaps but not a non-trivial, normal, big application (but so-called "embedded" programs might be written without the heap, using a subset of C++).

What garbage collector does ?

It keeps watching your data ("mark and sweep") to detect when your application is no longer referencing it. This is convenient for the application, because the application doesn't need to deallocate the data ... but the garbage collector might be computationally expensive.

Garbage collectors aren't a usual feature of C++ programming.

What could you do manipulating the memory by yourself that you couldn't do using this garbage collector?

Learn the C++ mechanisms for deterministic memory deallocation:

• 'static': never deallocated
• 'stack': as soon as the variable "goes out of scope"
• 'heap': when the pointer is deleted (explicitly deleted by the application, or implicitly deleted within some-or-other subroutine)

Answer 6

A similar question was asked, but it didn't ask about statics.

Summary of what static, heap, and stack memory are:

• A static variable is basically a global variable, even if you cannot access it globally. Usually there is an address for it that is in the executable itself. There is only one copy for the entire program. No matter how many times you go into a function call (or class) (and in how many threads!) the variable is referring to the same memory location.

• The heap is a bunch of memory that can be used dynamically. If you want 4kb for an object then the dynamic allocator will look through its list of free space in the heap, pick out a 4kb chunk, and give it to you. Generally, the dynamic memory allocator (malloc, new, et c.) starts at the end of memory and works backwards.

• Explaining how a stack grows and shrinks is a bit outside the scope of this answer, but suffice to say you always add and remove from the end only. Stacks usually start high and grow down to lower addresses. You run out of memory when the stack meets the dynamic allocator somewhere in the middle (but refer to physical versus virtual memory and fragmentation). Multiple threads will require multiple stacks (the process generally reserves a minimum size for the stack).

When you would want to use each one:

• Statics/globals are useful for memory that you know you will always need and you know that you don't ever want to deallocate. (By the way, embedded environments may be thought of as having only static memory... the stack and heap are part of a known address space shared by a third memory type: the program code. Programs will often do dynamic allocation out of their static memory when they need things like linked lists. But regardless, the static memory itself (the buffer) is not itself "allocated", but rather other objects are allocated out of the memory held by the buffer for this purpose. You can do this in non-embedded as well, and console games will frequently eschew the built in dynamic memory mechanisms in favor of tightly controlling the allocation process by using buffers of preset sizes for all allocations.)

• Stack variables are useful for when you know that as long as the function is in scope (on the stack somewhere), you will want the variables to remain. Stacks are nice for variables that you need for the code where they are located, but which isn't needed outside that code. They are also really nice for when you are accessing a resource, like a file, and want the resource to automatically go away when you leave that code.

• Heap allocations (dynamically allocated memory) is useful when you want to be more flexible than the above. Frequently, a function gets called to respond to an event (the user clicks the "create box" button). The proper response may require allocating a new object (a new Box object) that should stick around long after the function is exited, so it can't be on the stack. But you don't know how many boxes you would want at the start of the program, so it can't be a static.

Garbage Collection

I've heard a lot lately about how great Garbage Collectors are, so maybe a bit of a dissenting voice would be helpful.

Garbage Collection is a wonderful mechanism for when performance is not a huge issue. I hear GCs are getting better and more sophisticated, but the fact is, you may be forced to accept a performance penalty (depending upon use case). And if you're lazy, it still may not work properly. At the best of times, Garbage Collectors realize that your memory goes away when it realizes that there are no more references to it (see reference counting). But, if you have an object that refers to itself (possibly by referring to another object which refers back), then reference counting alone will not indicate that the memory can be deleted. In this case, the GC needs to look at the entire reference soup and figure out if there are any islands that are only referred to by themselves. Offhand, I'd guess that to be an O(n^2) operation, but whatever it is, it can get bad if you are at all concerned with performance. (Edit: Martin B points out that it is O(n) for reasonably efficient algorithms. That is still O(n) too much if you are concerned with performance and can deallocate in constant time without garbage collection.)

Personally, when I hear people say that C++ doesn't have garbage collection, my mind tags that as a feature of C++, but I'm probably in the minority. Probably the hardest thing for people to learn about programming in C and C++ are pointers and how to correctly handle their dynamic memory allocations. Some other languages, like Python, would be horrible without GC, so I think it comes down to what you want out of a language. If you want dependable performance, then C++ without garbage collection is the only thing this side of Fortran that I can think of. If you want ease of use and training wheels (to save you from crashing without requiring that you learn "proper" memory management), pick something with a GC. Even if you know how to manage memory well, it will save you time which you can spend optimizing other code. There really isn't much of a performance penalty anymore, but if you really need dependable performance (and the ability to know exactly what is going on, when, under the covers) then I'd stick with C++. There is a reason that every major game engine that I've ever heard of is in C++ (if not C or assembly). Python, et al are fine for scripting, but not the main game engine.