An experienced C++ user told me that I should strive for using heap variables, i.e.:
A* obj = new A("A");
as opposed to:
A obj("A");
Aside from all that stuff about using pointers being nice and flexible, he said it's better to put things on the heap rather than the stack (something about the stack being smaller than the heap?). Is it true? If so why?
NB: I know about issues with lifetime. Let's assume I have managed the lifetime of these variables appropriately. (i.e. the only criteria of concern is heap vs. stack storage with no lifetime concern)
Depending on the context we can consider heap or stack. Every thread gets a stack and the thread executes instructions by invoking functions. When a function is called, the function variables are pushed to stack. And when the function returns the stack rollbacks and memory is reclaimed. Now there is a size limitation for the thread local stack, it varies and can be tweaked to some extent. Considering this if every object is created on stack and the object requires large memory, then the stack space will exhaust resulting to stackoverflow error. Besides this if the object is to be accessed by multiple threads then storing such object on stack makes no sense.
Thus small variables, small objects who's size can be determine at compile time and pointers should be stored on stack. The concern of storing objects on heap or free store is, memory management becomes difficult. There are chances of memory leak, which is bad. Also if application tries to access an object which is already deleted, then access violation can happen which can cause application crash.
C++11 introduces smart pointers (shared, unique) to make memory management with heap easier. The actual referenced object is on heap but is encapsulation by the smart pointer which is always on the stack. Hence when the stack rollbacks during function return event or during exception the destructor of smart pointer deletes the actual object on heap. In case of shared pointer the reference count is maintained and the actually object is deleted when the reference count is zero.
http://en.wikipedia.org/wiki/Smart_pointer
There are no general rules regarding use of stack allocated vs heap allocated variables. There are only guidelines, depending on what you are trying to do.
Here are some pros and cons:
Heap Allocation:
Pros:
more flexible - in case you have a lot of information that is not available at compile-time
bigger in size - you can allocate more - however, it's not infinite, so at some point your program might run out of memory if allocations/deallocations are not handled correctly
Cons:
slower - dynamic allocation is usually slower than stack allocation
may cause memory fragmentation - allocating and deallocating objects of different sizes will make the memory look like Swiss cheese :) causing some allocations to fail if there is no memory block of the required size available
harder to maintain - as you know each dynamic allocation must be followed by a deallocation, which should be done by the user - this is error prone as there are a lot of cases where people forget to match every malloc() call with a free() call or new() with delete()
Stack allocation:
Pros:
faster - which is important mostly on embedded systems (I believe that for embedded there is a MISRA rule which forbids dynamic allocation)
does not cause memory fragmentation
makes the behavior of applications more deterministic - e.g. removes the possibility to run out of memory at some point
less error prone - as the user is not needed to handle deallocation
Cons:
less flexible - you have to have all information available at compile-time (data size, data structure, etc.)
smaller in size - however there are ways to calculate total stack size of an application, so running out of stack can be avoided
I think this captures a few of the pros and cons. I'm sure there are more.
In the end it depends on what your application needs.
The stack should be prefered to the heap, as stack allocated variables are automatic variables: their destruction is done automatically when the program goes out of their context.
In fact, the lifespan of object created on the stack and on the heap is different:
The local variables of a function or a code block {} (not allocated by new), are on the stack. They are automatically destroyed when you are returning from the function. (their destructors are called and their memory is freed).
But, if you need something an object to be used outside of the the function, you will have to allocate in on the heap (using new) or return a copy.
Example:
void myFun()
{
A onStack; // On the stack
A* onHeap = new A(); // On the heap
// Do things...
} // End of the function onStack is destroyed, but the &onHeap is still alive
In this example, onHeap will still have its memory allocated when the function ends. Such that if you don't have a pointer to onHeap somewhere, you won't be able to delete it and free the memory. It's a memory leak as the memory will be lost until the program end.
However if you were to return a pointer on onStack, since onStack was destroyed when exiting the function, using the pointer could cause undefined behaviour. While using onHeap is still perfectly valid.
To better understand how stack variables are working, you should search information about the call stack such as this article on Wikipedia. It explains how the variables are stacked to be used in a function.
It is always better to avoid using new as much as possible in C++.
However, there are times when you cannot avoid it.
For ex:
Wanting variables to exist beyond their scopes.
So it should be horses for courses really, but if you have a choice always avoid heap allocated variables.
The answer is not as clear cut as some would make you believe.
In general, you should prefer automatic variables (on the stack) because it's just plain easier. However some situations call for dynamic allocations (on the heap):
unknown size at compile time
extensible (containers use heap allocation internally)
large objects
The latter is a bit tricky. In theory, the automatic variables could get allocated infinitely, but computers are finite and worse all, most of the times the size of the stack is finite too (which is an implementation issue).
Personally, I use the following guideline:
local objects are allocated automatically
local arrays are deferred to std::vector<T> which internally allocates them dynamically
it has served me well (which is just anecdotal evidence, obviously).
Note: you can (and probably should) tie the life of the dynamically allocated object to that of a stack variable using RAII: smart pointers or containers.
C++ has no mention of the Heap or the Stack. As far as the language is concerned they do not exist/are not separate things.
As for a practical answer - use what works best - do you need fast - do you need guarantees. Application A might be much better with everything on the Heap, App B might fragment OS memory so badly it kills the machine - there is no right answer :-(
Simply put, don't manage your own memory unless you need to. ;)
Stack = Static Data allocated during compile time. (not dynamic)
Heap = Dyanamic Data allocated during run time. (Very dynamic)
Although pointers are on the Stack...Those pointers are beautiful because they open the doors for dynamic, spontaneous creation of data (depending on how you code your program).
(But I'm just a savage, so why does it matter what i say)
Related
An experienced C++ user told me that I should strive for using heap variables, i.e.:
A* obj = new A("A");
as opposed to:
A obj("A");
Aside from all that stuff about using pointers being nice and flexible, he said it's better to put things on the heap rather than the stack (something about the stack being smaller than the heap?). Is it true? If so why?
NB: I know about issues with lifetime. Let's assume I have managed the lifetime of these variables appropriately. (i.e. the only criteria of concern is heap vs. stack storage with no lifetime concern)
Depending on the context we can consider heap or stack. Every thread gets a stack and the thread executes instructions by invoking functions. When a function is called, the function variables are pushed to stack. And when the function returns the stack rollbacks and memory is reclaimed. Now there is a size limitation for the thread local stack, it varies and can be tweaked to some extent. Considering this if every object is created on stack and the object requires large memory, then the stack space will exhaust resulting to stackoverflow error. Besides this if the object is to be accessed by multiple threads then storing such object on stack makes no sense.
Thus small variables, small objects who's size can be determine at compile time and pointers should be stored on stack. The concern of storing objects on heap or free store is, memory management becomes difficult. There are chances of memory leak, which is bad. Also if application tries to access an object which is already deleted, then access violation can happen which can cause application crash.
C++11 introduces smart pointers (shared, unique) to make memory management with heap easier. The actual referenced object is on heap but is encapsulation by the smart pointer which is always on the stack. Hence when the stack rollbacks during function return event or during exception the destructor of smart pointer deletes the actual object on heap. In case of shared pointer the reference count is maintained and the actually object is deleted when the reference count is zero.
http://en.wikipedia.org/wiki/Smart_pointer
There are no general rules regarding use of stack allocated vs heap allocated variables. There are only guidelines, depending on what you are trying to do.
Here are some pros and cons:
Heap Allocation:
Pros:
more flexible - in case you have a lot of information that is not available at compile-time
bigger in size - you can allocate more - however, it's not infinite, so at some point your program might run out of memory if allocations/deallocations are not handled correctly
Cons:
slower - dynamic allocation is usually slower than stack allocation
may cause memory fragmentation - allocating and deallocating objects of different sizes will make the memory look like Swiss cheese :) causing some allocations to fail if there is no memory block of the required size available
harder to maintain - as you know each dynamic allocation must be followed by a deallocation, which should be done by the user - this is error prone as there are a lot of cases where people forget to match every malloc() call with a free() call or new() with delete()
Stack allocation:
Pros:
faster - which is important mostly on embedded systems (I believe that for embedded there is a MISRA rule which forbids dynamic allocation)
does not cause memory fragmentation
makes the behavior of applications more deterministic - e.g. removes the possibility to run out of memory at some point
less error prone - as the user is not needed to handle deallocation
Cons:
less flexible - you have to have all information available at compile-time (data size, data structure, etc.)
smaller in size - however there are ways to calculate total stack size of an application, so running out of stack can be avoided
I think this captures a few of the pros and cons. I'm sure there are more.
In the end it depends on what your application needs.
The stack should be prefered to the heap, as stack allocated variables are automatic variables: their destruction is done automatically when the program goes out of their context.
In fact, the lifespan of object created on the stack and on the heap is different:
The local variables of a function or a code block {} (not allocated by new), are on the stack. They are automatically destroyed when you are returning from the function. (their destructors are called and their memory is freed).
But, if you need something an object to be used outside of the the function, you will have to allocate in on the heap (using new) or return a copy.
Example:
void myFun()
{
A onStack; // On the stack
A* onHeap = new A(); // On the heap
// Do things...
} // End of the function onStack is destroyed, but the &onHeap is still alive
In this example, onHeap will still have its memory allocated when the function ends. Such that if you don't have a pointer to onHeap somewhere, you won't be able to delete it and free the memory. It's a memory leak as the memory will be lost until the program end.
However if you were to return a pointer on onStack, since onStack was destroyed when exiting the function, using the pointer could cause undefined behaviour. While using onHeap is still perfectly valid.
To better understand how stack variables are working, you should search information about the call stack such as this article on Wikipedia. It explains how the variables are stacked to be used in a function.
It is always better to avoid using new as much as possible in C++.
However, there are times when you cannot avoid it.
For ex:
Wanting variables to exist beyond their scopes.
So it should be horses for courses really, but if you have a choice always avoid heap allocated variables.
The answer is not as clear cut as some would make you believe.
In general, you should prefer automatic variables (on the stack) because it's just plain easier. However some situations call for dynamic allocations (on the heap):
unknown size at compile time
extensible (containers use heap allocation internally)
large objects
The latter is a bit tricky. In theory, the automatic variables could get allocated infinitely, but computers are finite and worse all, most of the times the size of the stack is finite too (which is an implementation issue).
Personally, I use the following guideline:
local objects are allocated automatically
local arrays are deferred to std::vector<T> which internally allocates them dynamically
it has served me well (which is just anecdotal evidence, obviously).
Note: you can (and probably should) tie the life of the dynamically allocated object to that of a stack variable using RAII: smart pointers or containers.
C++ has no mention of the Heap or the Stack. As far as the language is concerned they do not exist/are not separate things.
As for a practical answer - use what works best - do you need fast - do you need guarantees. Application A might be much better with everything on the Heap, App B might fragment OS memory so badly it kills the machine - there is no right answer :-(
Simply put, don't manage your own memory unless you need to. ;)
Stack = Static Data allocated during compile time. (not dynamic)
Heap = Dyanamic Data allocated during run time. (Very dynamic)
Although pointers are on the Stack...Those pointers are beautiful because they open the doors for dynamic, spontaneous creation of data (depending on how you code your program).
(But I'm just a savage, so why does it matter what i say)
What is the difference between
Kwadrat* k1 = new Kwadrat(1,2,3);
k1->field = 0;
Kwadrat k2(1,2,3);
k2.field = 0;
The first one is the pointer to the allocated memory, the second one is the object(where is it, on system stack?) why the second is worse? When we use first, whend the second one?
Dynamic allocation on the heap (uses new):
Kwadrat* k1 = new Kwadrat(1,2,3);
Object creation on the stack (without new):
Kwadrat k2(1,2,3);
Check out this Stack Overflow question for extended discussion about the stack and heap. Brian R. Bondy's answer does a nice job of comparing the two, while Jeff Hill's answer gives you some more nitty gritty details.
For a dangerously small summary:
you must delete objects you create with new, or else your code will suffer from memory leaks
you don't have to worry about manually deleting objects created on the stack because C++ Resource Allocation is Initialization (RAII) takes care of that for you
The stack can overflow if you attempt very large allocations.
As for which one you should use, that depends on the intended scope of the object you are creating
Using new dynamically creates an object on the heap meaning it will persist even if the pointer (k1*) goes out of scope.
This can be handy, but if it goes out of scope and you don't keep a copy of the pointer around it is permanently lost and causes a memory leak. This means you will lose the space used by that resource as long as the program executes. This is the downside of dynamically allocating memory with new, you have to track it and manually free it with the delete operator which takes extra work.
Doing it the other way creates a stack object which will be destroyed once it leaves scope, which is usually preferable.
Often when using dynamic memory creation with new, people work to gain this stack-like functionality by wrapping the dynamically created object in another object which will destroy it when it goes out of scope. This pattern is called Resource Acquisition is Initialization (RAII). This is most useful with reference counting so your objects can still be persisted out of scope, but will be destroyed when nothing refers to them any longer.
new allocates an object on the heap. Your second example allocates memory on the stack. As soon as the function where you allocate k2 returns the memory won't be valid anymore (the destructor of k2 will of course be called first). You need to use new if you want the object to live longer than the function that creates it.
new lets you specify the storage for the allocation you request. allocations via new/new[] are generally on the heap (e.g. wrapping malloc), but ultimately implementation defined. as well, the compiler could have enough information to bypass this in some cases.
where is it, on system stack?
Technically, you would typically write C++ to the specification -- of an abstract machine.
But typically, yes - the object is allocated on the thread's stack.
Why the second is worse? When we use first, when the second one?
The second should be your default because it is very clear, the compiler manages its lifetime and allocation for you, and also very fast. Some exceptions to this include:
when you have a large allocation (stack sizes are relatively small)
sometimes when you want to share the allocation with another thread
when you want to pass the allocation to another thread
when you want the object to live beyond the scope of the method, and you will typically use a smart pointer or other container which will delete the object properly.
in short, there's a lot less that could go wrong (using the second), and far less time is spent creating allocations via the general purpose system allocators.
I've been programming for a while but It's been mostly Java and C#. I've never actually had to manage memory on my own. I recently began programming in C++ and I'm a little confused as to when I should store things on the stack and when to store them on the heap.
My understanding is that variables which are accessed very frequently should be stored on the stack and objects, rarely used variables, and large data structures should all be stored on the heap. Is this correct or am I incorrect?
No, the difference between stack and heap isn't performance. It's lifespan: any local variable inside a function (anything you do not malloc() or new) lives on the stack. It goes away when you return from the function. If you want something to live longer than the function that declared it, you must allocate it on the heap.
class Thingy;
Thingy* foo( )
{
int a; // this int lives on the stack
Thingy B; // this thingy lives on the stack and will be deleted when we return from foo
Thingy *pointerToB = &B; // this points to an address on the stack
Thingy *pointerToC = new Thingy(); // this makes a Thingy on the heap.
// pointerToC contains its address.
// this is safe: C lives on the heap and outlives foo().
// Whoever you pass this to must remember to delete it!
return pointerToC;
// this is NOT SAFE: B lives on the stack and will be deleted when foo() returns.
// whoever uses this returned pointer will probably cause a crash!
return pointerToB;
}
For a clearer understanding of what the stack is, come at it from the other end -- rather than try to understand what the stack does in terms of a high level language, look up "call stack" and "calling convention" and see what the machine really does when you call a function. Computer memory is just a series of addresses; "heap" and "stack" are inventions of the compiler.
I would say:
Store it on the stack, if you CAN.
Store it on the heap, if you NEED TO.
Therefore, prefer the stack to the heap. Some possible reasons that you can't store something on the stack are:
It's too big - on multithreaded programs on 32-bit OS, the stack has a small and fixed (at thread-creation time at least) size (typically just a few megs. This is so that you can create lots of threads without exhausting address space. For 64-bit programs, or single threaded (Linux anyway) programs, this is not a major issue. Under 32-bit Linux, single threaded programs usually use dynamic stacks which can keep growing until they reach the top of the heap.
You need to access it outside the scope of the original stack frame - this is really the main reason.
It is possible, with sensible compilers, to allocate non-fixed size objects on the heap (usually arrays whose size is not known at compile time).
It's more subtle than the other answers suggest. There is no absolute divide between data on the stack and data on the heap based on how you declare it. For example:
std::vector<int> v(10);
In the body of a function, that declares a vector (dynamic array) of ten integers on the stack. But the storage managed by the vector is not on the stack.
Ah, but (the other answers suggest) the lifetime of that storage is bounded by the lifetime of the vector itself, which here is stack-based, so it makes no difference how it's implemented - we can only treat it as a stack-based object with value semantics.
Not so. Suppose the function was:
void GetSomeNumbers(std::vector<int> &result)
{
std::vector<int> v(10);
// fill v with numbers
result.swap(v);
}
So anything with a swap function (and any complex value type should have one) can serve as a kind of rebindable reference to some heap data, under a system which guarantees a single owner of that data.
Therefore the modern C++ approach is to never store the address of heap data in naked local pointer variables. All heap allocations must be hidden inside classes.
If you do that, you can think of all variables in your program as if they were simple value types, and forget about the heap altogether (except when writing a new value-like wrapper class for some heap data, which ought to be unusual).
You merely have to retain one special bit of knowledge to help you optimise: where possible, instead of assigning one variable to another like this:
a = b;
swap them like this:
a.swap(b);
because it's much faster and it doesn't throw exceptions. The only requirement is that you don't need b to continue to hold the same value (it's going to get a's value instead, which would be trashed in a = b).
The downside is that this approach forces you to return values from functions via output parameters instead of the actual return value. But they're fixing that in C++0x with rvalue references.
In the most complicated situations of all, you would take this idea to the general extreme and use a smart pointer class such as shared_ptr which is already in tr1. (Although I'd argue that if you seem to need it, you've possibly moved outside Standard C++'s sweet spot of applicability.)
You also would store an item on the heap if it needs to be used outside the scope of the function in which it is created. One idiom used with stack objects is called RAII - this involves using the stack based object as a wrapper for a resource, when the object is destroyed, the resource would be cleaned up. Stack based objects are easier to keep track of when you might be throwing exceptions - you don't need to concern yourself with deleting a heap based object in an exception handler. This is why raw pointers are not normally used in modern C++, you would use a smart pointer which can be a stack based wrapper for a raw pointer to a heap based object.
To add to the other answers, it can also be about performance, at least a little bit. Not that you should worry about this unless it's relevant for you, but:
Allocating in the heap requires finding a tracking a block of memory, which is not a constant-time operation (and takes some cycles and overhead). This can get slower as memory becomes fragmented, and/or you're getting close to using 100% of your address space. On the other hand, stack allocations are constant-time, basically "free" operations.
Another thing to consider (again, really only important if it becomes an issue) is that typically the stack size is fixed, and can be much lower than the heap size. So if you're allocating large objects or many small objects, you probably want to use the heap; if you run out of stack space, the runtime will throw the site titular exception. Not usually a big deal, but another thing to consider.
Stack is more efficient, and easier to managed scoped data.
But heap should be used for anything larger than a few KB (it's easy in C++, just create a boost::scoped_ptr on the stack to hold a pointer to the allocated memory).
Consider a recursive algorithm that keeps calling into itself. It's Very hard to limit and or guess the total stack usage! Whereas on the heap, the allocator (malloc() or new) can indicate out-of-memory by returning NULL or throw ing.
Source: Linux Kernel whose stack is no larger than 8KB!
For completeness, you may read Miro Samek's article about the problems of using the heap in the context of embedded software.
A Heap of Problems
The choice of whether to allocate on the heap or on the stack is one that is made for you, depending on how your variable is allocated. If you allocate something dynamically, using a "new" call, you are allocating from the heap. If you allocate something as a global variable, or as a parameter in a function it is allocated on the stack.
In my opinion there are two deciding factors
1) Scope of variable
2) Performance.
I would prefer to use stack in most cases but if you need access to variable outside scope you can use heap.
To enhance performance while using heaps you can also use the functionality to create heap block and that can help in gaining performance rather than allocating each variable in different memory location.
probably this has been answered quite well. I would like to point you to the below series of articles to have a deeper understanding of low level details. Alex Darby has a series of articles, where he walks you through with a debugger. Here is Part 3 about the Stack.
http://www.altdevblogaday.com/2011/12/14/c-c-low-level-curriculum-part-3-the-stack/
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.
I've been using C++ for a short while, and I've been wondering about the new keyword. Simply, should I be using it, or not?
With the new keyword...
MyClass* myClass = new MyClass();
myClass->MyField = "Hello world!";
Without the new keyword...
MyClass myClass;
myClass.MyField = "Hello world!";
From an implementation perspective, they don't seem that different (but I'm sure they are)... However, my primary language is C#, and of course the 1st method is what I'm used to.
The difficulty seems to be that method 1 is harder to use with the std C++ classes.
Which method should I use?
Update 1:
I recently used the new keyword for heap memory (or free store) for a large array which was going out of scope (i.e. being returned from a function). Where before I was using the stack, which caused half of the elements to be corrupt outside of scope, switching to heap usage ensured that the elements were intact. Yay!
Update 2:
A friend of mine recently told me there's a simple rule for using the new keyword; every time you type new, type delete.
Foobar *foobar = new Foobar();
delete foobar; // TODO: Move this to the right place.
This helps to prevent memory leaks, as you always have to put the delete somewhere (i.e. when you cut and paste it to either a destructor or otherwise).
Method 1 (using new)
Allocates memory for the object on the free store (This is frequently the same thing as the heap)
Requires you to explicitly delete your object later. (If you don't delete it, you could create a memory leak)
Memory stays allocated until you delete it. (i.e. you could return an object that you created using new)
The example in the question will leak memory unless the pointer is deleted; and it should always be deleted, regardless of which control path is taken, or if exceptions are thrown.
Method 2 (not using new)
Allocates memory for the object on the stack (where all local variables go) There is generally less memory available for the stack; if you allocate too many objects, you risk stack overflow.
You won't need to delete it later.
Memory is no longer allocated when it goes out of scope. (i.e. you shouldn't return a pointer to an object on the stack)
As far as which one to use; you choose the method that works best for you, given the above constraints.
Some easy cases:
If you don't want to worry about calling delete, (and the potential to cause memory leaks) you shouldn't use new.
If you'd like to return a pointer to your object from a function, you must use new
There is an important difference between the two.
Everything not allocated with new behaves much like value types in C# (and people often say that those objects are allocated on the stack, which is probably the most common/obvious case, but not always true). More precisely, objects allocated without using new have automatic storage duration
Everything allocated with new is allocated on the heap, and a pointer to it is returned, exactly like reference types in C#.
Anything allocated on the stack has to have a constant size, determined at compile-time (the compiler has to set the stack pointer correctly, or if the object is a member of another class, it has to adjust the size of that other class). That's why arrays in C# are reference types. They have to be, because with reference types, we can decide at runtime how much memory to ask for. And the same applies here. Only arrays with constant size (a size that can be determined at compile-time) can be allocated with automatic storage duration (on the stack). Dynamically sized arrays have to be allocated on the heap, by calling new.
(And that's where any similarity to C# stops)
Now, anything allocated on the stack has "automatic" storage duration (you can actually declare a variable as auto, but this is the default if no other storage type is specified so the keyword isn't really used in practice, but this is where it comes from)
Automatic storage duration means exactly what it sounds like, the duration of the variable is handled automatically. By contrast, anything allocated on the heap has to be manually deleted by you.
Here's an example:
void foo() {
bar b;
bar* b2 = new bar();
}
This function creates three values worth considering:
On line 1, it declares a variable b of type bar on the stack (automatic duration).
On line 2, it declares a bar pointer b2 on the stack (automatic duration), and calls new, allocating a bar object on the heap. (dynamic duration)
When the function returns, the following will happen:
First, b2 goes out of scope (order of destruction is always opposite of order of construction). But b2 is just a pointer, so nothing happens, the memory it occupies is simply freed. And importantly, the memory it points to (the bar instance on the heap) is NOT touched. Only the pointer is freed, because only the pointer had automatic duration.
Second, b goes out of scope, so since it has automatic duration, its destructor is called, and the memory is freed.
And the barinstance on the heap? It's probably still there. No one bothered to delete it, so we've leaked memory.
From this example, we can see that anything with automatic duration is guaranteed to have its destructor called when it goes out of scope. That's useful. But anything allocated on the heap lasts as long as we need it to, and can be dynamically sized, as in the case of arrays. That is also useful. We can use that to manage our memory allocations. What if the Foo class allocated some memory on the heap in its constructor, and deleted that memory in its destructor. Then we could get the best of both worlds, safe memory allocations that are guaranteed to be freed again, but without the limitations of forcing everything to be on the stack.
And that is pretty much exactly how most C++ code works.
Look at the standard library's std::vector for example. That is typically allocated on the stack, but can be dynamically sized and resized. And it does this by internally allocating memory on the heap as necessary. The user of the class never sees this, so there's no chance of leaking memory, or forgetting to clean up what you allocated.
This principle is called RAII (Resource Acquisition is Initialization), and it can be extended to any resource that must be acquired and released. (network sockets, files, database connections, synchronization locks). All of them can be acquired in the constructor, and released in the destructor, so you're guaranteed that all resources you acquire will get freed again.
As a general rule, never use new/delete directly from your high level code. Always wrap it in a class that can manage the memory for you, and which will ensure it gets freed again. (Yes, there may be exceptions to this rule. In particular, smart pointers require you to call new directly, and pass the pointer to its constructor, which then takes over and ensures delete is called correctly. But this is still a very important rule of thumb)
The short answer is: if you're a beginner in C++, you should never be using new or delete yourself.
Instead, you should use smart pointers such as std::unique_ptr and std::make_unique (or less often, std::shared_ptr and std::make_shared). That way, you don't have to worry nearly as much about memory leaks. And even if you're more advanced, best practice would usually be to encapsulate the custom way you're using new and delete into a small class (such as a custom smart pointer) that is dedicated just to object lifecycle issues.
Of course, behind the scenes, these smart pointers are still performing dynamic allocation and deallocation, so code using them would still have the associated runtime overhead. Other answers here have covered these issues, and how to make design decisions on when to use smart pointers versus just creating objects on the stack or incorporating them as direct members of an object, well enough that I won't repeat them. But my executive summary would be: don't use smart pointers or dynamic allocation until something forces you to.
Which method should I use?
This is almost never determined by your typing preferences but by the context. If you need to keep the object across a few stacks or if it's too heavy for the stack you allocate it on the free store. Also, since you are allocating an object, you are also responsible for releasing the memory. Lookup the delete operator.
To ease the burden of using free-store management people have invented stuff like auto_ptr and unique_ptr. I strongly recommend you take a look at these. They might even be of help to your typing issues ;-)
If you are writing in C++ you are probably writing for performance. Using new and the free store is much slower than using the stack (especially when using threads) so only use it when you need it.
As others have said, you need new when your object needs to live outside the function or object scope, the object is really large or when you don't know the size of an array at compile time.
Also, try to avoid ever using delete. Wrap your new into a smart pointer instead. Let the smart pointer call delete for you.
There are some cases where a smart pointer isn't smart. Never store std::auto_ptr<> inside a STL container. It will delete the pointer too soon because of copy operations inside the container. Another case is when you have a really large STL container of pointers to objects. boost::shared_ptr<> will have a ton of speed overhead as it bumps the reference counts up and down. The better way to go in that case is to put the STL container into another object and give that object a destructor that will call delete on every pointer in the container.
Without the new keyword you're storing that on call stack. Storing excessively large variables on stack will lead to stack overflow.
If your variable is used only within the context of a single function, you're better off using a stack variable, i.e., Option 2. As others have said, you do not have to manage the lifetime of stack variables - they are constructed and destructed automatically. Also, allocating/deallocating a variable on the heap is slow by comparison. If your function is called often enough, you'll see a tremendous performance improvement if use stack variables versus heap variables.
That said, there are a couple of obvious instances where stack variables are insufficient.
If the stack variable has a large memory footprint, then you run the risk of overflowing the stack. By default, the stack size of each thread is 1 MB on Windows. It is unlikely that you'll create a stack variable that is 1 MB in size, but you have to keep in mind that stack utilization is cumulative. If your function calls a function which calls another function which calls another function which..., the stack variables in all of these functions take up space on the same stack. Recursive functions can run into this problem quickly, depending on how deep the recursion is. If this is a problem, you can increase the size of the stack (not recommended) or allocate the variable on the heap using the new operator (recommended).
The other, more likely condition is that your variable needs to "live" beyond the scope of your function. In this case, you'd allocate the variable on the heap so that it can be reached outside the scope of any given function.
The simple answer is yes - new() creates an object on the heap (with the unfortunate side effect that you have to manage its lifetime (by explicitly calling delete on it), whereas the second form creates an object in the stack in the current scope and that object will be destroyed when it goes out of scope.
Are you passing myClass out of a function, or expecting it to exist outside that function? As some others said, it is all about scope when you aren't allocating on the heap. When you leave the function, it goes away (eventually). One of the classic mistakes made by beginners is the attempt to create a local object of some class in a function and return it without allocating it on the heap. I can remember debugging this kind of thing back in my earlier days doing c++.
C++ Core Guidelines R.11: Avoid using new and delete explicitly.
Things have changed significantly since most answers to this question were written. Specifically, C++ has evolved as a language, and the standard library is now richer. Why does this matter? Because of a combination of two factors:
Using new and delete is potentially dangerous: Memory might leak if you don't keep a very strong discipline of delete'ing everything you've allocated when it's no longer used; and never deleteing what's not currently allocated.
The standard library now offers smart pointers which encapsulate the new and delete calls, so that you don't have to take care of managing allocations on the free store/heap yourself. So do other containers, in the standard library and elsewhere.
This has evolved into one of the C++ community's "core guidelines" for writing better C++ code, as the linked document shows. Of course, there exceptions to this rule: Somebody needs to write those encapsulating classes which do use new and delete; but that someone is rarely yourself.
Adding to #DanielSchepler's valid answer:
The second method creates the instance on the stack, along with such things as something declared int and the list of parameters that are passed into the function.
The first method makes room for a pointer on the stack, which you've set to the location in memory where a new MyClass has been allocated on the heap - or free store.
The first method also requires that you delete what you create with new, whereas in the second method, the class is automatically destructed and freed when it falls out of scope (the next closing brace, usually).
The short answer is yes the "new" keyword is incredibly important as when you use it the object data is stored on the heap as opposed to the stack, which is most important!