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I'm currently adapting some example Arduino code to fit my needs. The following snippet confuses me:
// Dont put this on the stack:
uint8_t buf[RH_RF95_MAX_MESSAGE_LEN];
What does it mean to put the buf variable on the stack? How can I avoid doing this? What bad things could happen if I did it?
The program stack has a limited size (even on desktop computers, it's typically capped in megabytes, and on an Arduino, it may be much smaller).
All function local variables for functions are stored there, in a LIFO manner; the variables of your main method are at the bottom of the stack, the variables of the functions called in main on top of that, and so on; space is (typically) reserved on entering a function, and not reclaimed until a function returns. If a function allocates a truly huge buffer (or multiple functions in a call chain allocate slightly smaller buffers) you can rapidly approach the stack limit, which will cause your program to crash.
It sounds like your array is being allocated outside of a function, putting it at global scope. The downside to this is there is only one shared buffer (so two functions can't use it simultaneously without coordinating access, while a stack buffer would be independently reserved for each function), but the upside is that it doesn't cost stack to use it; it's allocated from a separate section of program memory (a section that's usually unbounded, or at least has limits in the gigabyte, rather than megabyte range).
So to answer your questions:
What does it mean to put the buf variable on the stack?
It would be on the stack if it:
Is declared in function scope rather than global scope, and
Is not declared as static (or thread_local, though that's more complicated than you should care about right now); if it's declared static at function scope, it's basically global memory that can only be referenced directly in that specific function
How can I avoid doing this?
Don't declare huge non-static arrays at function scope.
What bad things could happen if I did it?
If the array is large enough, you could suffer a stack overflow from running out of available stack space, crashing your program.
I am new to c++ and have one question to global variables. I see in many examples that global variables are pointers with addresses of the heap. So the pointers are in the memory for global/static variables and the data behind the addresses is on the heap, right?
Instead of this you can declare global (no-pointer) variables that are stored the data. So the data is stored in the memory for global/static variables and not on the heap.
Has this solution any disadvantages over the first solution with the pointers and the heap?
Edit:
First solution:
//global
Sport *sport;
//somewhere
sport = new Sport;
Second solution:
//global
Sport sport;
A disadvantage of storing your data in a global/static variable is that the size is fixed at compile time and can't be changed as opposed to heap storage where the size can be determined at runtime and grow or shrink repeatedly over the run. The lifetime is also fixed as the complete run of the program from start to finish for global/static variables as opposed to heap storage where it can be acquired and released (even repeatedly) all through the runtime of the program. On the other hand, global and static storage management is all handled for you by the compiler where as heap storage has to be explicitly managed by your code. So in summary, global/static storage is easier but not as flexible as heap storage.
You are right in your hypothesis of where the objects are located. About usage,
It's horses for courses. There is no definite rule, it depends on the design & the type of functionality you want to implement. For example:
One may choose the pointer version to achieve lazy initialization or polymorphic behavior, neither of which is possible with global non pointer object approach.
Right. Declared variables go in the DataSegment. And they sit there for the life of the program. You cannot free them. You cannot reallocate them. In Windows, the DataSegment is a fixed size....if you put everything there you may run out of memory (at least it used to be this way).
I'm aware that questions about the stack vs. the heap have been asked several times, but I'm confused about one small aspect of choosing how to declare objects in C++.
I understand that the heap--accessed with the "new" operator--is used for dynamic memory allocation. According to an answer to another question on Stack Overflow, "the heap is for storage of data where the lifetime of the storage cannot be determined ahead of time". The stack is faster than the heap, and seems to be used for variables of local scope, i.e., the variables are automatically deleted when the relevant section of code is completed. The stack also has a relatively limited amount of available space.
In my case, I know prior to runtime that I will need an array of pointers to exactly 500 objects of a particular class, and I know I will need to store the pointers and the objects throughout the duration of runtime. The heap doesn't make sense because I know beforehand how long I will need the memory and I know exactly how man objects I will need. The stack also doesn't make sense if it is limited in scope; plus, I don't know if it can actually hold all of my objects/pointers.
What would be the best way to approach this situation and why? Thanks!
Objects allocated on the stack in main() have a lifetime of the entire run of the program, so that's an option. An array of 500 pointers is either 2000 or 4000 bytes depending on whether your pointers are 32 or 64 bits wide -- if you were programming in an environment whose stack limit was that small, you would know it (such environments do exist: for instance, kernel mode stacks are often 8192 bytes or smaller in total) so I wouldn't hesitate to put the array there.
Depending on how big your objects are, it might also be reasonable to put them on the stack -- the typical stack limit in user space nowadays is order of 8 megabytes, which is not so large that you can totally ignore it, but is not peanuts, either.
If they are too big for the stack, I would seriously consider making a global variable that was an array of the objects themselves. The major downside of this is you can't control precisely when they are initialized. If the objects have nontrivial constructors this is very likely to be a problem. An alternative is to allocate storage for the objects as a global variable, initialize them at the appropriate point within main using placement new, and explicitly call their destructors on the way out. This requires care in the presence of exceptions; I'd write a one-off RAII class that encapsulated the job.
It is not a matter of stack or heap (which to be accurate do not mean what you think in c++: they are just data structures like vector, set or queue). It is a matter of storage duration.
You most likely need here static duration objects, which can be either global, or members of a class. Automatic variables declared inside the main function could also do the job, if you design a way to access them from your other code.
There is some information about the different storage durations of C++ (automatic, static, dynamic) there. The accepted answer however uses the confusing stack/heap terminology, but the explanation is correct.
the heap is for storage of data where the lifetime of the storage cannot be determined ahead of time
While that is correct, it's also incomplete.
The stack unwinds when you exit its scope, so using it for global scoped variables is unfeasible, like you said. This however is where you stop being on the right track. While you know the lifetime (or more accurately the scope since that's the most important factor here), you also know it's above the stack frame, so given only two choices, you put it on the heap.
There is a third option, an actual static variable declared at the top scope, but this will only work if your objects have default constructors.
TL;DR: use global (static) storage for either a pointer to the array (dynamic allocation) or just the actual array (static allocation).
Note: Your assumption that the stack is somehow "faster" than the heap is wrong, they're both backed in the same RAM, you just access it relative to different registers. Also I'd like to mention yet again how much I dislike the use of the terms stack and heap.
The stack, as you mention, often has size limits. That leaves you with two choices - dynamically allocate your objects, or make them global. The time cost to allocate all of your objects once at application startup is almost certainly not of significant concern. So just pick whichever method you prefer and do it.
I think you are confusing the use of the stack (storage for local variables and parameters) and unscoped data (static class variables and data allocated via new or malloc). One appropriate solution based on your description would be a static class that has your array of pointers declared as a static class member. This would be allocated on a heap like structure (maybe even the heap depending on your C++ implementation). a "quick and dirty" solution would be to declare the array as a static variable (basically a global variable), however it isn't the best approach from a maintainability perspective.
The best concept to go by is "Use the stack when you can and the heap when you must." I don't see why the stack wouldn't be able to hold all of your objects unless they're large or you're working with a limited resources system. Try the stack and if it can't handle it, the time it would take to allocate the memory on the heap can all be done early in the program's execution and wouldn't be a significant problem.
Unless speed is an issue or you can't afford the overhead, you should stick the objects in a std::vector. If copy semantics aren't defined for the objects, you should use a std::vector of std::shared_ptrs.
I understand that this may be construed as one of those "what's your preference" questions, but I really want to know why you would choose one of the following methods over the other.
Suppose you had a super complex class, such as:
class CDoSomthing {
public:
CDoSomthing::CDoSomthing(char *sUserName, char *sPassword)
{
//Do somthing...
}
CDoSomthing::~CDoSomthing()
{
//Do somthing...
}
};
How should I declare a local instance within a global function?
int main(void)
{
CDoSomthing *pDoSomthing = new CDoSomthing("UserName", "Password");
//Do somthing...
delete pDoSomthing;
}
-- or --
int main(void)
{
CDoSomthing DoSomthing("UserName", "Password");
//Do somthing...
return 0;
}
Prefer local variables, unless you need the object's lifetime to extend beyond the current block. (Local variables are the second option). It's just easier than worrying about memory management.
P.S. If you need a pointer, because you need it to pass to another function, just use the address-of operator:
SomeFunction(&DoSomthing);
There are two main considerations when you declare a variable on the stack vs. in the heap - lifetime control and resource management.
Allocating on the stack works really well when you have tight control over the lifetime of the object. That means you are not going to pass a pointer or a reference of that object to code outside of the scope of the local function. This means, no out parameters, no COM calls, no new threads. Quite a lot of limitations, but you get the object cleaned up properly for you on normal or exceptional exit from the current scope (Though, you might want to read up on stack unwinding rules with virtual destructors). The biggest drawback of the stack allocation - the stack is usually limited to 4K or 8K, so you might want to be careful what you put on it.
Allocating on the heap on the other hand would require you to cleanup the instance manually. That also means that you have a lot of freedom how to control the lifetime of the instance. You need to do this in two scenarios: a) you are going to pass that object out of scope; or b) the object is too big and allocating it on the stack could cause stack overflow.
BTW, a nice compromise between these two is allocating the object on the heap and allocating a smart pointer to it on the stack. This ensures that you are not wasting precious stack memory, while still getting the automatic cleanup on scope exit.
The second form is the so called RAII (Resource Acquisition Is Initialization) pattern. It has many advantages over the first.
When use new, you have to use delete yourself, and guarantee it will always be deleted, even if an exception is thrown. You must guarantee all that yourself.
If you use the second form, when the variable goes out of scope, it is always cleaned up automatically. And if an exception is thrown, the stack unwinds and it is also cleaned up.
So, you should prefer RAII (the second option).
In addition to what has been said so far, but there are additional performance considerations to be taken into account, particularly in memory-allocation-intensive applications:
Using new will allocate memory from the heap. In the case of intense (extremely frequent) allocation and deallocation, you will be paying a high price in:
locking: the heap is a resource shared by all threads in your process. Operations on the heap may require locking in the heap manager (done for you in the runtime library), which may slow things down significantly.
fragmentation: heap fragments. You may see the time it takes malloc/new and free/delete to return increase 10-fold. This compounds with the locking problem above, as it takes more time to manage a fragmented heap and more threads queue up waiting for the heal lock. (On Windows there is a special flag you can set for the heap manager so it heuristically attempts to reduce fragmentation.)
Using the RAII pattern, memory is simply taken off the stack. Stack is a per-thread resource, it does not fragment, there is no locking involved, and may turn out to play in your advantage in terms of memory locality (i.e. memory caching at the CPU level.)
So, when you need objects for a brief (or scoped) period of time, definitely use the second approach (local variable, on the stack.) If you need to share data between threads, use new/malloc (on one hand you have to, on the second hand these objects are typically long-lived enough so you pay essentially 0-cost vis-a-vis the heap manager.)
The second version will unwind the stack if an exception is thrown. The first will not. I don't see much difference otherwise.
The biggest difference between the two is that the new initiates a pointer to the object.
By creating the object without new, the object initiated is stored on the stack. If it is initiated with new, it returns a pointer to the new object that has been created on the heap. It actually returns a memory address that points to the new object. When this happens, you need to memory manage the variable. When you are done using the variable, you would need to call delete on it to avoid a memory leak. Without the new operator, when the variable goes out of scope the memory will be freed automatically.
So if you need to pass the variable outside of the current scope, using new is more efficient. However, if you need to make a temporary variable, or something that will only be used temporarily, having the object on the stack is going to be better, since you don't have to worry about memory management.
Mark Ransom is right, also you'll need to instantiate with new if you are going to pass the variable as a parameter to the CreateThread-esque function.
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 problems of static and stack? Could I write an entire application without allocating variables in the heap?
I heard that others languages incorporate a "garbage collector" so you don't have to worry about memory. What does the garbage collector do?
What could you do manipulating the memory by yourself that you couldn't do using this garbage collector?
Once someone said to me that with this declaration:
int * asafe=new int;
I have a "pointer to a pointer". What does it mean? It is different of:
asafe=new int;
?
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.
The following is of course all not quite precise. Take it with a grain of salt when you read it :)
Well, the three things you refer to are automatic, static and dynamic storage duration, which has something to do with how long objects live and when they begin life.
Automatic storage duration
You use automatic storage duration for short lived and small data, that is needed only locally within some block:
if(some condition) {
int a[3]; // array a has automatic storage duration
fill_it(a);
print_it(a);
}
The lifetime ends as soon as we exit the block, and it starts as soon as the object is defined. They are the most simple kind of storage duration, and are way faster than in particular dynamic storage duration.
Static storage duration
You use static storage duration for free variables, which might be accessed by any code all times, if their scope allows such usage (namespace scope), and for local variables that need extend their lifetime across exit of their scope (local scope), and for member variables that need to be shared by all objects of their class (classs scope). Their lifetime depends on the scope they are in. They can have namespace scope and local scope and class scope. What is true about both of them is, once their life begins, lifetime ends at the end of the program. Here are two examples:
// static storage duration. in global namespace scope
string globalA;
int main() {
foo();
foo();
}
void foo() {
// static storage duration. in local scope
static string localA;
localA += "ab"
cout << localA;
}
The program prints ababab, because localA is not destroyed upon exit of its block. You can say that objects that have local scope begin lifetime when control reaches their definition. For localA, it happens when the function's body is entered. For objects in namespace scope, lifetime begins at program startup. The same is true for static objects of class scope:
class A {
static string classScopeA;
};
string A::classScopeA;
A a, b; &a.classScopeA == &b.classScopeA == &A::classScopeA;
As you see, classScopeA is not bound to particular objects of its class, but to the class itself. The address of all three names above is the same, and all denote the same object. There are special rule about when and how static objects are initialized, but let's not concern about that now. That's meant by the term static initialization order fiasco.
Dynamic storage duration
The last storage duration is dynamic. You use it if you want to have objects live on another isle, and you want to put pointers around that reference them. You also use them if your objects are big, and if you want to create arrays of size only known at runtime. Because of this flexibility, objects having dynamic storage duration are complicated and slow to manage. Objects having that dynamic duration begin lifetime when an appropriate new operator invocation happens:
int main() {
// the object that s points to has dynamic storage
// duration
string *s = new string;
// pass a pointer pointing to the object around.
// the object itself isn't touched
foo(s);
delete s;
}
void foo(string *s) {
cout << s->size();
}
Its lifetime ends only when you call delete for them. If you forget that, those objects never end lifetime. And class objects that define a user declared constructor won't have their destructors called. Objects having dynamic storage duration requires manual handling of their lifetime and associated memory resource. Libraries exist to ease use of them. Explicit garbage collection for particular objects can be established by using a smart pointer:
int main() {
shared_ptr<string> s(new string);
foo(s);
}
void foo(shared_ptr<string> s) {
cout << s->size();
}
You don't have to care about calling delete: The shared ptr does it for you, if the last pointer that references the object goes out of scope. The shared ptr itself has automatic storage duration. So its lifetime is automatically managed, allowing it to check whether it should delete the pointed to dynamic object in its destructor. For shared_ptr reference, see boost documents: http://www.boost.org/doc/libs/1_37_0/libs/smart_ptr/shared_ptr.htm
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
(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.
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 in deep trouble. 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 lead 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.
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)
Stack memory allocation (function variables, local variables) can be problematic when your stack is too "deep" and you overflow the memory available to stack allocations. The heap is for objects that need to be accessed from multiple threads or throughout the program lifecycle. You can write an entire program without using the heap.
You can leak memory quite easily without a garbage collector, but you can also dictate when objects and memory is freed. I have run in to issues with Java when it runs the GC and I have a real time process, because the GC is an exclusive thread (nothing else can run). So if performance is critical and you can guarantee there are no leaked objects, not using a GC is very helpful. Otherwise it just makes you hate life when your application consumes memory and you have to track down the source of a leak.
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.
An advantage of GC in some situations is an annoyance in others; reliance on GC encourages not thinking much about it. In theory, waits until 'idle' period or until it absolutely must, when it will steal bandwidth and cause response latency in your app.
But you don't have to 'not think about it.' Just as with everything else in multithreaded apps, when you can yield, you can yield. So for example, in .Net, it is possible to request a GC; by doing this, instead of less frequent longer running GC, you can have more frequent shorter running GC, and spread out the latency associated with this overhead.
But this defeats the primary attraction of GC which appears to be "encouraged to not have to think much about it because it is auto-mat-ic."
If you were first exposed to programming before GC became prevalent and were comfortable with malloc/free and new/delete, then it might even be the case that you find GC a little annoying and/or are distrustful(as one might be distrustful of 'optimization,' which has had a checkered history.) Many apps tolerate random latency. But for apps that don't, where random latency is less acceptable, a common reaction is to eschew GC environments and move in the direction of purely unmanaged code (or god forbid, a long dying art, assembly language.)
I had a summer student here a while back, an intern, smart kid, who was weaned on GC; he was so adament about the superiorty of GC that even when programming in unmanaged C/C++ he refused to follow the malloc/free new/delete model because, quote, "you shouldn't have to do this in a modern programming language." And you know? For tiny, short running apps, you can indeed get away with that, but not for long running performant apps.
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.