Stack, Static, and Heap in C++

Stack, Static, and Heap in C++

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.

Memory allocation areas in C++ (Stack vs heap vs Static)

A couple of side notes first. The correct terminology is automatic rather than stack, dynamic rather than heap. The other is that with C++11 there are now four rather than three types of memory. C++11 adds thread local memory to the mix.

Automatic memory is fast because it is implemented using the call stack on most machines. All it takes is to adjust the stack pointer by the right amount and voila! memory is allocated. Dynamic memory requires a whole lot more work underneath the hood. The requisite memory might not be attached to the process, and getting that to happen requires going through the OS. Even if memory is available, the dynamic memory management tools still have to find it and mark it as in use.

Static memory is "allocated" as a part of the compilation and linking process. When you define a static variable in some source file, the compiled code contains special instructions for the linker to reserve space for that variable. The compiler also converts your C/C++ code to machine code. The linker combines all of those different chunks of data and code and resolves addresses to form the executable binary image. When you run your program, that binary image is loaded into (virtual) memory. The memory for that static variable exists as soon as the program starts executing.

As far as performance is concerned, it's best not to worry too much about performance ahead of time. While static memory is fast, there are lots and lots of drawbacks. The last thing you want to do is to make all of your data static.

C++ variables and where they are stored in memory (stack, heap, static)

Mostly right.

Any variable that is accessed with a pointer is stored on the heap.

This isn't true. You can have pointers to stack-based or global variables.

Also it's worth pointing out that global variables are generally unified by the linker (i.e. if two modules have "int i" at global scope, you'll only have one global variable called "i"). Dynamic libraries complicate that slightly; on Windows, DLLs don't have that behaviour (i.e. an "int i" in a Windows DLL will not be the same "int i" as in another DLL in the same process, or as the main executable), while most other platforms dynamic libraries do. There are some additional complications on Darwin (iOS/macOS) which has a hierarchical namespace for symbols; as long as you're linking with the flat_namespace option, what I just said will hold.

Additionally, it's worth talking about initialisation behaviour; global variables are initialised automatically by the runtime (typically either using special linker features or by means of a call that is inserted into the code for your main function). The order of initialisation of globals isn't guaranteed. However, static variables declared at function scope are initialised when that function is first executed, and not at program start-up as you might suppose, and that feature is commonly used by C++ programmers to do lazy initialisation.

(Similar concerns apply to destructors for global objects; those are best avoided entirely IMO, not least because on some platforms there are fast termination features that simply won't call them.)

const keyword means you can't change the variable.

Almost. const affects the type, and there is a difference depending on where you write it exactly. For example

const char *foo;

should be read as foo is a pointer to a const char, i.e. foo itself is not const, but the thing it points at is. Contrast with

char * const foo;

which says that foo is a const pointer to char.

Finally, you've missed out volatile, the point of which is to tell the compiler not to make assumptions about the thing to which it applies (e.g. it can't assume that it's safe to cache a volatile value in a register, or to optimise away accesses, or in general to optimise across any operation that affects a volatile value). Hopefully you'll never need to use volatile; it's most often useful if you're doing really low-level things that frankly a lot of people have no need to go anywhere near.

access speed to static and heap memory

There is no difference. Absolutely. Once your program is loaded, CPU simply does not know what sort of memory (heap or static) it is dealing with.

The above statement is true for 98% of most common CPU architectures/implementations. Although some computers may have different areas of memory that work with different speeds. If this is the case you need to check this. How this special memory is mapped - this depends on a particular platform/configuration.

Depending on the compiler/environment programs with big static areas may load somewhat slower. But this is not an absolute rule.

It would be better to think about locality of your data (is your pieces of data stay close to each other or not) and how one value will kick out other value out of the CPU cache. Loading something to cache is 10-100 times slower than accessing something that is already in the cache. This will make VERY noticeable difference.

Address ordering in static, stack and heap memory allocation?

What you've misunderstood is simply that while the stack grows downward on x86, x86-64, and many other modern processors, arrays are allocated as single blocks, and hence increasing indices increase addresses. Once that is borne in mind your results are what one would expect.

Are variables on the stack statically allocated?

The stack itself is statically allocated. Variables allocated in the stack come and go, as control flow enters and leaves their scope.

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