In C++, when you make a new variable on the heap like this:
int* a = new int;
you can tell C++ to reclaim the memory by using delete like this:
delete a;
However, when your program closes, does it automatically free the memory that was allocated with new?
Yes, it is automatically reclaimed, but if you intend to write a huge program that makes use of the heap extensively and not call delete anywhere, you are bound to run out of heap memory quickly, which will crash your program.
Therefore, it is a must to carefully manage your memory and free dynamically allocated data with a matching delete for every new (or delete [] if using new []), as soon as you no longer require the said variable.
When the process is terminated the memory is reclaimed back by the OS. Of course this argument shouldn't in any case be used to not perform proper memory management by the program.
Don't let people tell you yes. C++ has no concept of an OS, so to say "yes the OS will clean it up" is no longer talking about C++ but about C++ running on some environment, which may not be yours.
That is, if you dynamically allocate something but never free it you've leaked. It can only end its lifetime once you call delete/delete[] on it. On some OS's (and almost all desktop OS's), memory will be reclaimed (so other programs may use it.) But memory is not the same as resource! The OS can free all the memory it wants, if you have some socket connection to close, some file to finish writing to, etc, the OS might not do it. It's important not to let resources leak. I've heard of some embedded platforms that won't even reclaim the memory you've not freed, resulting in a leak until the platform is reset.
Instead of dynamically allocating things raw (meaning you're the one that has to explicitly delete it), wrap them into automatically allocated (stack allocated) containers; not doing so is considered bad practice, and makes your code extremely messy.
So don't use new T[N], use std::vector<T> v(N);. The latter won't let a resource leak occur. Don't use new T;, use smart_ptr p(new T);. The smart pointer will track the object and delete it when it's know longer used. This is called Scope-bound Resource Management (SBRM, also known as the dumber name Resource-Acquisition is Initialization, or RAII.)
Note there is no single "smart_ptr". You have to pick which one is best. The current standard includes std::auto_ptr, but it's quite unwieldy. (It cannot be used in standard containers.) Your best bet is to use the smart pointers part of Boost, or TR1 if your compiler supports it. Then you get shared_ptr, arguably the most useful smart pointer, but there are many others.
If every pointer to dynamically allocated memory is in an object that will destruct (i.e., not another object that is dynamically allocated), and that object knows to free the memory, that pointer is guaranteed to be freed. This question shouldn't even be a problem, since you should never be in a position to leak.
No, when the program exits ("closes") the dynamically allocated memory is left as is
EDIT:
Reading the other answers, I should be more precise. The destructors of dynamically allocated objects will not run but the memory will be reclaimed anyway by any decent OS.
PS: The first line should read
int* a = new int;
No, it's your responsibility to free it. Also, a must be a pointer, so it should be:
int *a = new int;
delete a;
This excellent answer by Brian R. Bondy details why it's good practice to free the memory allocated by a.
It is important to explicitly call
delete because you may have some code
in the destructor that you want to
execute. Like maybe writing some data
to a log file. If you let the OS free
your memory for you, your code in your
destructor will not be executed.
Most operating systems will deallocate
the memory when your program ends. But
it is good practice to deallocate it
yourself and like I said above the OS
won't call your destructor.
As for calling delete in general, yes
you always want to call delete, or
else you will have a memory leak in
your program, which will lead to new
allocations failing.
When your process terminates, the OS does regain control of all resources the process was using, including memory. However, that, of course, will not cause C++'s destructors to be necessarily run, so it's not a panacea for not explicitly freeing said resources (though it won't be a problem for int or other types with noop dtors, of course;-).
Related
When using dynamically allocated objects in C++ eg:
TGraph* A = new TGraph(...);
One should always delete these because otherwise the objects might still be in memory when
control is handed back to the parent scope. While I can see why this is true for subscopes and subroutines of a program, does the same count for the main scope?
Am I obliged to delete objects that were dynamically built inside main()? The reason why this seems a bit redudant to me is that when main ends, the program also ends, so there is no need to worry about memory leaks.
Most of the modern OS always reclaim back all memory they allocated to a program(process).
The OS doesn't really understand if your program leaked memory it merely takes back what it allocatted.
But there are bigger issues at hand than just the memory loss:
Note that if the destructor of the object whos delete needs to be called performs some non-trivial operation and your program depends on the side effects produced by it then your program falls prey to Undefined Behavior[Ref 1]. Once that happens all bets are off and your program may show any beahvior.
Also, An OS usually reclaims the allocated memory but not the other resources, So you might leak those resources indirectly. This may include operations dealing with file descriptors or state of the program itself etc.
Hence, it is a good practice to always deallocate all your allocations by calling delete or delete [] before exiting your program.
[Ref 1]C++03 Standard 3.8 Para 4:
"....if there is no explicit call to the destructor or if a delete-expression (5.3.5) is not used to release the storage, the destructor shall not be implicitly called and any program that depends on the side effects produced by the destructor has undefined behavior."
IMO it is best to always call delete properly:
to make it an automatic habit, making it less likely to forget it when it is really needed
to cover cases when non-memory resources (sockets, file handles, ...) need to be freed - these aren't automatically freed by the OS
to cater for future refactoring when the code in question might be moved out of main scope
Yes, you should call delete, at least because it's best practice. If you have important logic in your destructor, that's one extra reason that you should call delete.
Corrected: If the program depends on logic in the destructor, not calling delete explicitly results in undefined behavior.
The reason why this seems a bit redudant to me is that when main ends,
the program also ends, so there is no need to worry about memory
leaks.
You're right, but consider this: you create a class object which opens a connection to remote DB. After your program will complete, you should tell the DB "I'm done, i'm going to disconnect", but it won't happen in the case you won't call delete properly.
Its the best practice to de-allocate memory that's been allocated. You should keep in mind that Heap memory is limited and just allocating with out de-allocating while your program is running might run the heap space out for some other/or the same program(if its some kind of a daemon that is meant to run for a very long time) that needs heap.
Of course memory will be reclaimed by the operating system at the end of the program's execution.
I see you are using ROOT (CMS guy?). I think ROOT takes care of this and cleans up, doesn't it?
Best practices:
Do not use new, use automatic allocation
When dynamic allocation is necessary, use RAII to ensure automatic cleanup
You should never have to write delete in applicative code.
Here, why are you calling new for TGraph ?
TGraph A(...);
works better: less worries!
When using dynamically allocated objects in C++ eg:
TGraph* A = new TGraph(...);
One should always delete these because otherwise the objects might still be in memory when
control is handed back to the parent scope. While I can see why this is true for subscopes and subroutines of a program, does the same count for the main scope?
Am I obliged to delete objects that were dynamically built inside main()? The reason why this seems a bit redudant to me is that when main ends, the program also ends, so there is no need to worry about memory leaks.
Most of the modern OS always reclaim back all memory they allocated to a program(process).
The OS doesn't really understand if your program leaked memory it merely takes back what it allocatted.
But there are bigger issues at hand than just the memory loss:
Note that if the destructor of the object whos delete needs to be called performs some non-trivial operation and your program depends on the side effects produced by it then your program falls prey to Undefined Behavior[Ref 1]. Once that happens all bets are off and your program may show any beahvior.
Also, An OS usually reclaims the allocated memory but not the other resources, So you might leak those resources indirectly. This may include operations dealing with file descriptors or state of the program itself etc.
Hence, it is a good practice to always deallocate all your allocations by calling delete or delete [] before exiting your program.
[Ref 1]C++03 Standard 3.8 Para 4:
"....if there is no explicit call to the destructor or if a delete-expression (5.3.5) is not used to release the storage, the destructor shall not be implicitly called and any program that depends on the side effects produced by the destructor has undefined behavior."
IMO it is best to always call delete properly:
to make it an automatic habit, making it less likely to forget it when it is really needed
to cover cases when non-memory resources (sockets, file handles, ...) need to be freed - these aren't automatically freed by the OS
to cater for future refactoring when the code in question might be moved out of main scope
Yes, you should call delete, at least because it's best practice. If you have important logic in your destructor, that's one extra reason that you should call delete.
Corrected: If the program depends on logic in the destructor, not calling delete explicitly results in undefined behavior.
The reason why this seems a bit redudant to me is that when main ends,
the program also ends, so there is no need to worry about memory
leaks.
You're right, but consider this: you create a class object which opens a connection to remote DB. After your program will complete, you should tell the DB "I'm done, i'm going to disconnect", but it won't happen in the case you won't call delete properly.
Its the best practice to de-allocate memory that's been allocated. You should keep in mind that Heap memory is limited and just allocating with out de-allocating while your program is running might run the heap space out for some other/or the same program(if its some kind of a daemon that is meant to run for a very long time) that needs heap.
Of course memory will be reclaimed by the operating system at the end of the program's execution.
I see you are using ROOT (CMS guy?). I think ROOT takes care of this and cleans up, doesn't it?
Best practices:
Do not use new, use automatic allocation
When dynamic allocation is necessary, use RAII to ensure automatic cleanup
You should never have to write delete in applicative code.
Here, why are you calling new for TGraph ?
TGraph A(...);
works better: less worries!
I am here stuck with a question in my C++ book with the following:
"What does the use of new require you to also call delete?"
Maybe you have an answer for that?
Because that is the way C++ is designed & that is the intended behavior.
The intention was to provide a memory allocation which you demand and own till you reliquish it explicitly.
new gives you a dynamic memory allocation(on heap) which will continue to exist and you own it untill you explicitly deallocate it by calling delete.
Failing to call a delete on a newed buffer will lead to Undefined Behaviors usually in the form of. 1 memory leaks.
1 This was discussed here.
when you do a new, OS allocates the memory to the pointer you are assigning it. After your usage is completed you may not require it anymore. But the memory is still marked as "being used" by OS.
Now, when the pointer is declared in a scope of a function or any other block (of {}), it will be deleted (only pointer will be removed) when the execution of the block is over. In such cases the memory that was allocated using new is remained marked "being used" by OS and is not allocated to any other pointer that calls new or to a variable. This causes an orphan block of memory in RAM, that will never be used because its pointer was removed from memory but it will occupy a memory block.
This is called a memory leak. A few of such blocks may make your application unstable as well.
You use delete to free such memory blocks and relieve the OS so that it can be used well for other requests
There is no Garbage Collector in C++, and therefore you are responsible for deallocating the allocated memory. Anyway, the operating system "knows" what memory your program allocated. So when your program exits, the operating system is again responsible for the memory. But if you have a long running C++ program and never call delete noone will help you to get rid of your garbage.
Calling new has allocated memory for the object and it has also arranged for the constructor of that object to be executed.
You could free the memory by calling free(), but you should actually use delete to free memory allocated by new, since this will also cause the objects destructor to be executed.
In some classes I have an static std::map with pointers inside. My question is if I need to delete at the end of the program or this memory is automatically freed. My concern is if the pointers stored inside are correctly deleted through our destructors when std::map is deleted.
Thanks.
If the map contains pointers that were allocated with new (or new[], or malloc), then each pointer needs a corresponding delete (or delete[], or free).
The map's destructor wont know what to do with a bald pointer. Consider using a smart pointer that has appropriate move semantics like a boost smart pointer or if you've got a very new compiler, one of the C++0x smart pointers. However, do not use the current standard's std::auto_ptr, inside of STL containers. See this thread for why.
Edit:
As Billy ONeal pointed out, boost::ptr_map is also designed exactly for this purpose.
If I understand the situation correctly, you don't delete the map itself. But you probably need to delete the objects the map is pointing to. It would probably be a really good idea to use a smart pointer such as Boost shared_ptr in your map instead of native pointers. Then the objects would be cleaned up automatically.
Edit:
Using Boost ptr_map might be an even better idea.
The memory is "automatically freed", in the sense that the entire process memory is freed, but the destructors of the objects pointed to will not be called. This can cause a resource leak, if you use RAII.
std::map never calls delete on it's members. Assuming you're working with a relatively recent operating system, the OS will reclaim the memory occupied by the members on process termination, but the destructors will not run.
If you have a map of pointers, then the answer is 'no', your destructors will not be called, but 'yes', the memory will be freed at the end of process execution. All memory allocated by a process is always freed by the Operating System when a process exits (even if it crashes), but destructors might not be called.
A memory "leak" is where memory is unintentionally not deleted over a period of time, and ends up reducing as the process continues. If it is a type of process that runs for a very long period of time, eg a server that is rarely restarted, this can be a major problem.
Memory leak detectors will pick up on any memory that is allocated and not deleted by a programming call, so valgrind, etc. will report this as a leak.
It is as well to check your code with programs like valgrind, and therefore the less that "gets in the way", the easier it will be to spot real leaks. Therefore my advice is not do just let the system clean up the memory, or singletons, etc, for you when you have allocated a pointer with new (or malloc or new[]).
You can have a "clean-up" routine to do this. Just have an object in the scope of your map that has a deleter (as it will be deleted when it exits) that will clean up the pointers in the map. As you need your object to be deleted first it should be declared later than the map.
Like in any case of storage class which stores pointers: you are responsible to deallocate memory they point to. Storage class is responsible only to clean its own resources. Relying on reclaiming memory by OS at the process termination is a bad practice.
Additional thanks extend to Daniel Newby for answering my memory usage question (and Martin York for explaining it a bit more). It is definitely the answer I was looking for, but more of my other questions were answered by others.
Thanks everyone
for clearing up all of my concerns. Very pleased to see things running how I expect them to run.
I've run into something that I'm not exactly sure about.
In my program, I'm not using malloc() or free(). I'm making instances of my classes with new and I've made sure each one runs it's destructor when it's delete'd, however, there are no free() calls or even setting their pointers (to things inside a global scope, or other classes) to NULL or 0.
What I mean by "I've made sure", is not that I call each destructor. I only use delete to call on the destructor to run, but I have variables that increase by 1 everytime an object is created, and everytime it's destructor is run. This is how I've made sure the amount of objects I created are equal to the amount of destructors called.
Should I be using malloc() and free() anyway? Should I be NULLing pointers to things that I still want to exist?
A second question is why, when I look at my task manager, does my process never "drop" memory? It used to never stop gaining, and then I started deleting everything properly. Or so I thought.
Wouldn't free() or delete make the memory usage go down?
What practices should I pursue about malloc'ing and free'ing memory with linked lists?
There's rarely a reason to use malloc() and free() in a C++ program. Stick with new and delete. Note that unlike languages with garbage collection, setting a pointer to NULL or 0 in C++ has nothing to do with deallocating the memory.
you should be using delete with a new and free with a malloc. delete will call the class' destructor so you don't have to explicitly call it. The purpose of the destructor is to release and resources the class might have and delete will free the memory as well.
The only time you should explicetly use the destructor is when you have initialized your object through placement new. You should put yourself in a position where the compiler generated code releases your resources -- read this article on the C++ idiom : resource acquisition is initialization.
Also setting the pointer of a class to null does nothing, there is no garbage collector in the background cleaning up your memory. If you don't free dynamic memory in C++ it will be "leaked" memory -- i.e., there are no links to the memory and it will never be reclaimed till the process exits.
p.s., once again do not mix the pairs of the memory allocation functions.
edt: don't implement linked lists, use the containers provided by the Standard template library. If you feel you need better performance use the intrusive containers from boost.
You should use new and delete in preference to malloc()/calloc()/realloc() and free().
If you're creating linked lists you should use std::list. Also, look into std::vector
As far as the apparent memory usage of your application: it's quite likely that memory is not returned to the system until the application exits.
new and delete can more or less to be considered the C++ versions of malloc and free. So stick within one pair or another, i.e. if a pointer was created with new, it should be released with delete, which ensures the destructor call you mentioned. The malloc/free pair are not C++ aware, and just allocate and release a block of memory with no attached constructor/destructor semantics.
Yes indeed I consider it good form to set pointers to NULL or zero when they've been freed or deleted. During debugging I also sometimes set them to something characteristic like 0xdeadbeef so they stand out.
It's likely that the OS "memory usage" is reflecting the entire size of your process' heap, rather than your memory manager's idea of how much memory is allocated. When the allocator discovers that it doesn't have enough heap space, it grows the heap, and this will be reflected in the "memory usage" you're looking at. In theory it would be possible to shrink the heap accordingly on memory release, but this doesn't always happen. Thus you may only see the memory usage grow, and never shrink.
"Should I be using malloc() and free() anyway?"
No, in most cases. Stick with one form of memory management only. Trying to use both means that you will inevitably screw up and delete a malloc()ed item, or free() a newed item, giving a subtle bug. Stick with one form and you fix these bugs in advance.
"Wouldn't free() or delete make the memory usage go down?"
The OS allocates memory in pages, often 4 kiB in size. As long as a single byte of the page is still in use, it will not be returned to the OS. You are probably allocating many small items, but not deallocating all of them. (There are ways to help with a custom new/delete, but they are generally not worth the effort.)
In my program, I'm not using malloc()or free(). I'm making instances of my classes with new and I've made sure each one runs it's destructor when it's delete'd,
That is scary. You should not need to make anything run its destructor. It is allocated (new) it is destroyed (delete) the constructor is run atomatically on new and the destructor is run automatically on delete.
however, there are no free() calls or even setting their pointers (to things inside a global scope, or other classes) to NULL or 0.
There is no need to use malloc/free in C++ code (there are a few situations where you are using C libs that require malloced memory but they are explicitly documented and few).
Technically there is no need to set a pointer to NULL after you delete it.
It is good technique for a variable to go out of scope just after it is deleted so it can not accidently be re-used. If for some reason the pointer variable lives (ie does not go out of scope) for a long time after you call delete then it is usefull to set it to NULL so that it is no accidently re-used.
Should I be using malloc() and free() anyway? Should I be NULLing pointers to things that I still want to exist?
No and No.
Note: C++ unlike Java does not keep track of how many pointers point at an object.
If you have more than one pointer pointing at an object you need to use smart pointers (you should be using smart pointers anyway).
A second question is why, when I look at my task manager, does my process never "drop" memory? It used to never stop gaining, and then I started deleting everything properly. Or so I thought.
The application never releases back to the OS (on most OS's in normal situations).
So the memory will never go down (until the application exits).
Internally the memory management tracks all the frees so that the memory can be re-used.
But if it runs out it will ask the OS for more and thus in the task manager the memory allocation will go up (this will not be returned to the OS).
Wouldn't free() or delete make the memory usage go down?
No.
What practices should I pursue about malloc'ing and free'ing memory with linked lists?
You should use Smart Pointers so you don't need to worry about when to delete an object.
They also make your code exception safe.
But if you using pointers. call delete to remove an element then set is value to NULL (the pointer scope is alive long after the delete is called).
Note: A boost:shared_pointer is very similar to a Java pointer. It tracks the number of pointers and deletes the object when the last reference to the object is destroyed. No need for you to do any deleting (just like Java) and all you actually need to do is call new (just like Java).