I've been trying to deal with low memory situation in my VC++ code.
I've used std::nothrow and checking returns value of new operator for NULL. Application works fine.
But problem is at very low system memory and it crashes abruptly anywhere especially inside STL containers calls (map, vector, queue etc) and the error is "Exception bad_alloc". Obviously these containers cannot allocate required memory so they simply throw bad_alloc.
Now since I've used these containers liberally in my code, I just don't want each and every function inside "try...catch" block. It would clutter the code. (And moreover, the code uses event based library. So, many of the functions are callbacks. Hence, its not like one or few parent caller function(s) I can put in try/catch block and solve this problem)
Without using try/catch, how can this problem be addressed?
At least can someone please tell which of these containers and methods throw bad_alloc (So that I will try putting only that particular code in try/catch block)
If you're not using dynamic_cast or any other features that that it gives you, you can turn off RTTI - that might save you a bit, but probably not enough.
The only other option I can offer is to profile your memory usage, and optimize your code so that you're freeing things you no longer need earlier.
You ask: "how can this problem be addressed?"
Well, what is the problem?
"I don't have enough memory to run my program" — procure more
"My program uses too much memory" — use less
You can't magically work around it any other way.
Would it be possible in C++ to create a custom allocator that works simply like this:
{
// Limit memory to 1024 KB
ScopedMemoryPool memoryPool(1024 * 1024);
// From here on all heap allocations ('new', 'malloc', ...) take memory from the pool.
// If the pool is depleted these calls result in an exception being thrown.
// Examples:
std::vector<int> integers(10);
int a * = new int [10];
}
I couldn't find something like this in the boost libraries, or anywhere else.
Is there a fundamental problem that makes this impossible?
You would need to create a custom allocator that you pass in as a template param to vector. This custom allocator would essentially wrap the access to your pool and do whatever size validations that it wants.
Yes you can make such a construct, it's used in many games, but you'll basically need to implement your own containers and call memory allocation methods of that pool that you've created.
You could also experiment with writing a custom allocator for the STL containers, although it seems that that sort of work is generally advised against. (I've done it before and it was tedious, but I don't remember any specific problems.)
Mind- writing your own memory allocator is not for the faint of heart. You could take a look at Doug Lea's malloc, which provides "memory spaces", which you could use in your scoping construct somehow.
I will answer a different question. Look at 'efficient c++' book. One of the things they discuss is implementing this kind of thing. That was for a web server
For this particular thing you can either mess at the c++ layer by overriding new and supplying custom allocators to the STL.
Or you can mess at the malloc level, start with a custom malloc and work from there (like dmalloc)
Is there a fundamental problem that makes this impossible?
Arguing about program behavior would become fundamentally impossible. All sorts of weird issues will come up. Certain sections of the code may or may not execute though this will seeminly have no effect on the next sections which may work un-hindered. Certain sections may always fail. Dealing with the standard-library or any other third party library will become extremely difficult. There may be fragmentations at run-time at times and at times not.
If intent is that all allocations within that scope occur with that allocator object, then it's essentially a thread-local variable.
So, there will be multithreading issues if you use a static or global variable to implement it. Otherwise, not a bad workaround for the statelessness of allocators.
(Of course, you'll need to pass a second template argument eg vector< int, UseScopedPool >.)
I'm creating a class for a Lua binding which holds a pointer and can be changed by the scripter. It will include a few functions such as :ReadString and :ReadBool, but I don't want the application to crash if I can tell that the address they supplied will cause an access violation.
Is the a good way to detect if an address is outside of the readable/writable memory? Thanks!
A function library that may be useful is the "Virtual" function libraries, for example VirtualQuery
I'm not really looking for a foolproof design, I just want to omit the obvious (null pointers, pointers way outside the possible memory location)
I understand how unsafe this library is, and I'm not looking for safety, just sanity.
There are ways, but they do not serve the purpose you intend. That is; yes, you can determine whether an address appears to be valid at the current moment in time. But; no, you cannot determine whether that address will be valid a few clock cycles from now. Another thread could change the virtual memory map and a formerly valid address would become invalid.
The only way to properly handle the possibility of accessing suspect pointers is using whatever native exception handling is available on your platform. This may involve handling the signal SIG_BUS or it may involve using the proprietary __try and __catch extensions.
The idiom to use is the one wherein you attempt the access, and explicitly handle the resulting exception, if any does happen to occur.
You also have the problem of ensuring that the pointers you return point to your memory or to some other memory. For that, you could make your own data structure, a tree springs to mind, which stores the valid address ranges your "pointers" can achieve. Otherwise, the script code can hand you some absolute addresses and you will change memory structures by the operating system for the process environment.
The application you write about is highly suspect and you should probably start over with a less explosive design. But I thought I would tell you how to play with fire if you really want to.
Check out Raymond Chen's blog post, which goes more deeply into why this practice is bad. Most interestingly, he points out that, once a page is tested by IsBadReadPtr, further accesses to that page will not raise exceptions!
There is no, and that's why you should never do things like this.
Perhaps try using segvcatch, which can convert segfaults into C++ exceptions.
One is to use C++ exceptions: try catch blocks. But freeing dynamic memory will be an issue when an exception is raised.
Second is to use C style: errno variable
Third is just to return -1 on error and 0 on success :)
Which way should be chosen for a mid-size project and why? Any other better approach..?
But freeing dynamic memory will be an issue when an exception is raised.
No it's not. std::vector<int> v(100); Done.
The concept here is called Scope-Bound Resource Management (SBRM), also known by the much more common (and awkward) name Resource Acquisition Is Initialization (RAII). Basically, all resources are contained in some object which will clean up the resource in the destructor (which is always guaranteed to be run for an automatically allocated object). So whether or not the function exists normally or via exception, the destructor is run and your resource is cleaned up.
Never do an allocation where you need to free it explicitly, use containers and smart pointers.
Second is to use C style: errno variable
Third is just to return -1 on error and 0 on success :)
And how do they help solving your problem of freeing dynamic memory? They also use an early-exit strategy, same as throw.
So in summary, they don’t have an advantage over C++ exceptions (according to you).
In the first place, you should strive for a program with minimum error cases. (Because errors are not cool.)
Exceptions are a nice tool but should be used conservatively: reserve them for "exceptional cases", do not use them to control the flow of your program.
For example, do not use exceptions to test whether a user input is correct or not. (For such a case, return an error code.)
One is to use C++ exceptions: try
catch blocks. But freeing dynamic
memory will be an issue when an
exception is raised.
#see RAII.
Exceptions should be your preferred method of dealing with exceptional runtime situations like running out of memory. Note that something like std::map::find doesn't throw (and it shouldn't) because it's not necessarily an error or particularly exceptional case to search for a key that doesn't exist: the function can inform the client whether or not the key exists. It's not like a violation of a pre-condition or post-condition like requiring a file to exist for a program to operate correctly and finding that the file isn't there.
The beauty of exception-handling, if you do it correctly (again, #see RAII), is that it avoids the need to litter error-handling code throughout your system.
Let's consider a case where function A calls function B which calls C then D and so on, all the way up to 'Z'. Z is the only function that can throw, and A is the only one interested in recovering from an error (A is the entry point for a high-level operation, e.g., like loading an image). If you stick to RAII which will be helpful for more than exception-handling, then you only need to put a line of code in Z to throw an exception and a little try/catch block in A to catch the exception and, say, display an error message to the user.
Unfortunately a lot of people don't adhere to RAII as strictly as they should in practice, so a lot of real world code has more try/catch blocks than should be necessary to deal with manual resource cleanup (which shouldn't have to be manual). Nevertheless, this is the ideal you should strive to achieve in your code, and it's more practical if it's a mid-sized project. Likewise, in real world scenarios, people often ignore error codes returned by functions. if you're going to put the extra mile in favor of robustness, you might as well start with RAII because that will help your application regardless of whether you use exception handling or error code handling.
There is a caveat: you should not throw exceptions across module boundaries. If you do, you should consider a hybrid between error codes (as in returning error codes, not using a global error status like errno) and exceptions.
It is worth noting that if you use operator new in your code without specifying nothrow everywhere, ex:
int* p = new int(123); // can throw std::bad_alloc
int* p = new(std::nothrow) int(123); // returns a null pointer on failure
... then you already need to catch and handle bad_alloc exceptions in your code for it to be robust against out of memory exceptions.
Have a look at this comment by Herb Sutter on try catch for C++ GOTW. And do go through his whole set of articles. He does have a lot to say on when and how to check and save yourself from error conditions and how to handle them in the best ways possible.
Throw an exception. Destructors of variables are always called when an exception is thrown, and if your stack-based variables don't clean up after themselves (if for example you used a raw pointer when you need to delete the result), then you get what you deserve. Use smart pointers, no memory leaks.
But freeing dynamic memory will be an issue when an exception is raised.
Freeing memory (or any other resource for that matter) doesn't suddenly become a non-issue because you don't use exceptions. The techniques that make dealing with these problems while exceptions can be thrown easy, also make it easier when there can be "error conditions".
Exceptions are good for passing control from one context to another.
You let the compiler do the work of unrolling the stack between the contexts then in the new context compensate for the exception (and then hopefully continue).
If your error happens and can be corrected in the same context then error codes are a good method to do error handling and clean up (Don't take this to mean you should not be using RAII you still need that). But for example within a class an error occurs in a function and the calling function can correct for that type of error (then it probably is not an exceptional circumstance so no exceptions) then error code are useful.
You should not use error codes when you have to pass information out of a library or sub system as you are then relying on the developer using the code to actually check and handle the code to make sure it works correctly and more often than not they will ignore error codes.
Been doing Java for number of years so haven't been tracking C++. Has finally clause been added to C++ exception handling in the language definition?
Is there a favored idiom that mimics Java's try/finally?
Am also bothered that C++ doesn't have an ultimate super type for all possible exceptions that could be thrown - like Java's Throwable class.
I can write:
try {
// do something
} catch(...) {
// alas, can't examine the exception
// can only do cleanup code and perhaps rethrow, ala:
throw;
}
ADDENDUM EDIT:
I ended up accepting the answer that
had the most up votes, i.e., use
destructors to do cleanup. Of course,
from my own comments, it is clear I
don't entirely agree with that.
However, C++ is what it is and so in
the application endeavor I have in
mind, I'm going to more or less strive
to adhere to common community
practice. I'll use template classes to
wrap resources that don't already have
a class destructor (i.e., C library
resources), thus bestowing on them
destructor semantics.
NEW ADDENDUM EDIT:
Hmm, instead of finally then a closure
feature perhaps? A closure combined with
ScopeGuard approach (see one of the
answers below) would be a way to
accomplish cleanup with arbitrary
actions and access to the cleanup
code's outer scope context. Cleanup could be done in the idiom fashion that is seen in Ruby programming where they supply cleanup blocks when a resource is being opened. Isn't a
closure feature being considered for
C++?
By making effective use of destructors. When an exception is thrown in a try block, any object created within it will be destroyed immediately (and hence its destructor called).
This is different from Java where you have no idea when an object's finalizer will be called.
UPDATE: Straight from the horse's mouth: Why doesn't C++ provide a "finally" construct?
My $.02. I've been programming in managed languages like C# and Java for years, but was forced to make the switch to C++ for the purposes of speed. At first I couldn't believe how I had to write out the method signature twice in the header file and then the cpp file, and I didn't like how there was no finally block, and no garbage collection meant tracking memory leaks everywhere - gosh I didn't like it at all!
However, as I said I was forced to use C++. So I was forced to seriously learn it, and now I've finally understood all the programming idioms like RAII and I get all the subtleties of the language and such. It took me a while but now I see just how different of a language it is compared to C# or Java.
These days I think C++ is the best language there is! Yes, I can understand that there is a little more what I call 'chaff' sometimes (seemingly unnecessary stuff to write), but after actually using the language seriously, I've changed my mind about it completely.
I used to have memory leaks all the time. I used to write all my code into the .h file because I hated the separation of code, I couldn't understand why they would do that! And I used to always end up with stupid cyclic include dependencies, and heaps more. I was really hung up on C# or Java, to me C++ was a huge step down. These days I get it. I almost never have memory leaks, I enjoy the separation of interface and implementation, and I don't have problems with cycle dependencies anymore.
And I don't miss the finally block either. To be honest, my opinion is that these C++ programmers that you talk about writing repeated cleanup actions in catch blocks just sound to me like they're just bad C++ programmers. I mean, it doesn't look like any of the other C++ programmers in this thread are having any of the problems you mention. RAII really does make finally redundant, and if anything, it's less work. You write one destructor and then you never have to write another finally ever! Well at least for that type.
With respect, what I think is going on is you're just used to Java now, just like I had been.
C++'s answer is RAII: The object's destructor will be executed when they go out of scope. Whether by a return, by an exception or whatever. If you handle the exception somewhere else, you can be sure all objects from the called function down to your handler will be properly destructed by having their destructor called. They will clean up for you.
Read http://en.wikipedia.org/wiki/Resource_acquisition_is_initialization
No finally has not been added to C++, nor is it likely to ever be added.
The way C++ uses constructor/destructor makes the need for finally unnecessary.
If you are using catch(...) to cleanup then you are not using C++ properly. The cleanup code should all be in the destructor.
Though it is not a requirement to use it C++ does have a std::exception.
Forcing developers to derive from a specific class to use exception goes against the keep it simple philosophy of C++. Its also why we don't require all classes to derive from Object.
Read: Does C++ support 'finally' blocks? (And what's this 'RAII' I keep hearing about?)
The use of finally is more error prone than destructors to do clean up.
This is because you are forcing the user of the object to do clean up rather than the designer/implementer of the class.
Ok, I have to add in an answer to the points you made in a separate answer post:
(It would be a lot more convenient if you'd edited this into the original question, so it doesn't end up at the bottom below the answers to it.
If all cleanup always gets done in
destructors then there wouldn't need
to be any cleanup code in a catch
block - yet C++ has catch blocks where
cleanup actions get done. Indeed it
has a block for catch(...) where it is
only possible to do cleanup actions
(well, certainly can't get at any
exception information to do any
logging).
catch has a completely separate purpose, and as a Java programmer you should be aware of that. The finally clause is for "unconditional" cleanup actions. No matter how the block is exited, this must be done. Catch is for conditional cleanup. If this type of exception is thrown, we need to perform a few extra actions.
The cleanup in a finally block will
get done whether there was an
exception thrown or not - which is
what one always wants to happen when
cleanup code does exist.
Really? If we want it to always happen for this type (say, we always want to close a database connection when we're done with it), then why don't we define it once? In the type itself? Make the database connection close itself, rather than having to put a try/finally around every single use of it?
That's the point in destructors. They guarantee that each type is able to take care of its own cleanup, every time it's used, without the caller having to think of it.
C++ developers from day one have been
plagued with having to repeat cleanup
actions that appear in catch blocks in
the code flow that occurs upon
successful exit from the try block.
Java and C# programmers just do it
once in the finally block.
No. C++ programmers have never been plagued by that. C programmers have. And C programmers who realized that c++ had classes, and then called themselves C++ programmers have.
I program in C++ and C# daily, and I feel I'm plagued by C#'s ridiculous insistence that I must supply a finally clause (or a using block) EVERY SINGLE TIME I use a database connection or something else that must be cleaned up.
C++ lets me specify once and for all that "whenever we're done with this type, it should perform these actions". I don't risk forgetting to release memory. I don't risk forgetting to close file handles, sockets or database connections. Because my memory, my handles, sockets and db connections do it themselves.
How can it ever be preferable to have to write duplicate cleanup code every time you use a type? If you need to wrap the type because it doesn't have a destructor itself, you have two easy options:
Look for a proper C++ library which provides this destructor (hint: Boost)
Use boost::shared_ptr to wrap it, and supply it with a custom functor at runtime, specifying the cleanup to be done.
When you write application server
software like Java EE app servers
Glassfish, JBoss, etc., you want to be
able to catch and log exception
information - as opposed to let it
fall on the floor. Or worse fall into
the runtime and cause a ungraceful
abrupt exit of the application server.
That's why it's very desirable to have
an overarching base class for any
possible exception.
And C++ has just such a class. std::exception.
Have done C++ since the CFront days
and Java/C# most of this decade. Is
clear to see there's just an enormous
culture gap in how fundamentally
similar things are approached.
No, you've never done C++. You've done CFront, or C with classes. Not C++. There's a huge difference. Quit calling the answers lame, and you might learn something about the language you thought you knew. ;)
Cleanup functions, themselves, are thoroughly lame. They have low cohesion, in that they are expected to perform a series of activities only related in when they happen. They have high coupling, in that they need to have their internals modified when the functions that actually do something are changed. Because of this, they're error-prone.
The try...finally construct is a framework for cleanup functions. It is a language-encouraged way to write lousy code. Moreover, since it encourages writing the same cleanup code over and over, it undermines the DRY principle.
The C++ way is far preferable for these purposes. The cleanup code for a resource is written precisely once, in the destructor. It's in the same place as the rest of the code for that resource, and therefore has good cohesiveness. The cleanup code doesn't have to be put into unrelated modules, and therefore this cuts down on coupling. It is written precisely once, when well designed.
Moreover, the C++ way is much more uniform. C++, with the smart pointer additions, handles all sorts of resources in the same way, while Java handles memory well and provides inadequate constructs to release other resources.
There are plenty of problems with C++, but this isn't one of them. There are ways in which Java is better than C++, but this isn't one of them.
Java would be much better off with a way to implement RAII instead of try...finally.
To avoid having to define a wrapper class for every releasable resource, you may be interested in ScopeGuard (http://www.ddj.com/cpp/184403758) which allows one to create "cleaners" on the fly.
For example:
FILE* fp = SomeExternalFunction();
// Will automatically call fclose(fp) when going out of scope
ScopeGuard file_guard = MakeGuard(fclose, fp);
An Example of how difficult it is to use finally correctly.
Open and closing two files.
Where you want to guarantee that the file is closed correctly.
Waiting for the GC is not an option as the files may be re-used.
In C++
void foo()
{
std::ifstream data("plop");
std::ofstream output("plep");
// DO STUFF
// Files closed auto-magically
}
In a language with no destructors but has a finally clause.
void foo()
{
File data("plop");
File output("plep");
try
{
// DO STUFF
}
finally
{
// Must guarantee that both files are closed.
try {data.close();} catch(Throwable e){/*Ignore*/}
try {output.close();}catch(Throwable e){/*Ignore*/}
}
}
This is a simple example and already the code is getting convoluted. Here we are only trying to marshal 2 simple resources. But as the number of resources that need to be managed increases and/or their complexity increases the use of a finally block becomes harder and harder to use correctly in the presence of exceptions.
The use of finally moves responsibility for correct usage onto the user of an object. By using constructor/destructor mechanism provided by C++ you move the responsibility of correct usage to the designer/implementer of the class. This is inheritanly safer as the designer only needs to do it correctly once at the class level (rather than have different users try and do it correctly in different ways).
Using C++11 with its lambda expressions, I've recently started using the following code to mimic finally:
class FinallyGuard {
private:
std::function<void()> f_;
public:
FinallyGuard(std::function<void()> f) : f_(f) { }
~FinallyGuard() { f_(); }
};
void foo() {
// Code before the try/finally goes here
{ // Open a new scope for the try/finally
FinallyGuard signalEndGuard([&]{
// Code for the finally block goes here
});
// Code for the try block goes here
} // End scope, will call destructor of FinallyGuard
// Code after the try/finally goes here
}
The FinallyGuard is an object which is constructed with a callable function-like argument, preferrably a lambda expression. It will simply remember that function until its destructor is called, which is the case when the object goes out of scope, either due to normal control flow or due to stack unwinding during exception handling. In both cases, the destructor will call the function, thus executing the code in question.
It is a bit strange that you have to write the code for the finally before the code for the try block, but apart from that it actually feels a lot like a genuine try/finally from Java. I guess one should not abuse this for situations where an object with its own proper destructor would be more appropriate, but there are cases where I consider this approach above more suitable. I discussed one such scenario in this question.
As far as I understand things, std::function<void()> will use some pointer indirection and at least one virtual function call to perform its type erasure, so there will be a performance overhead. Don't use this technique in a tight loop where performance is critical. In those cases, a specialized object whose destructor does one thing only would be more appropriate.
C++ destructors make finally redundant. You can get the same effect by moving the cleanup code from finally to corresponding destructors.
I think that you are missing the point of what catch (...) can do.
You say in your example "alas, can't examine the exception". Well, you have no information about the type of the exception. You don't even know if it's a polymorphic type so even if you had some sort of an untyped reference to it, you couldn't even safely attempt a dynamic_cast.
If you know about certain exceptions or exception hierarchies that you can do something with then this is the place for catch blocks with explicity named types.
catch (...) is not often useful in C++. It can be used in places which have to guarantee that they don't throw, or only throw certain contracted exceptions. If you are using catch (...) for cleanup then there is a very good chance that your code is not robustly exception safe in any case.
As mentioned in other answers, if you are using local objects to manage resources (RAII) then it can be surprising and enlightening how few catch blocks you need, often - if you don't need to do anything locally with an exception - even the try block can be redundant as you let the exceptions flow out to the client code that can respond to them while still guaranteeing no resource issues.
To answer your original question, if you need some piece of code to run at the end of a block, exception or no exception, then a recipe would be.
class LocalFinallyReplacement {
~LocalFinallyReplacement() { /* Finally code goes here */ }
};
// ...
{ // some function...
LocalFinallyReplacement lfr; // must be a named object
// do something
}
Note how we can completely do away with try, catch and throw.
If you had data in the function that was originally declared outside the try block that you needed access to in the "finally" block, then you may need to add that to the constructor of the helper class and store it until the destructor. However, at this point I would seriously reconsider whether the problem could be resolved by altering the design of the local resource handling objects as it would imply something awry in the design.
Not completetely offtopic.
Boiler Plating DB Resource Cleanup in Java
sarcasm mode: Isn't the Java idiom wonderful?
I've done plenty of class design and template wrapper design in C++ over those 15 years and done it all the C++ way in terms of destructors cleaning up. Every project, though, also invariably involved the use of C libraries that provided resources with the open it, use it, close it usage model. A try/finally would mean such a resource can just be consumed where it needs to be - in a completely robust manner - and be done with it. The least tedium approach to programming that situation. Could deal with all the other state going on during the logic of that cleanup without having to be scoped away in some wrapper destructor.
I did most of my C++ coding on Windows so could always resort to using Microsoft's __try/__finally for such situations. (Their structured exception handling has some powerful abilities for interacting with exceptions.) Alas, doesn't look like C language has ever ratified any portable exception handling constructs.
That wasn't ideal solution, though, because it was not straightforward to blend C and C++ code in a try block where either style of exception might get thrown. A finally block added to C++ would have been helpful for those situations and would enable portability.
Regarding your addendum-edit, yes closures are being considered for C++0x. They can be used with RAII scoped guards to provide an easy to use solution, check Pizer's weblog. They can also be used to mimic try-finally, see this answer ; but is this really a good idea ? .
Thought I'd add my own solution to this - a kind of smart pointer wrapper for when you have to deal with non-RAII types.
Used like this:
Finaliser< IMAPITable, Releaser > contentsTable;
// now contentsTable can be used as if it were of type IMAPITable*,
// but will be automatically released when it goes out of scope.
So here's the implementation of Finaliser:
/* Finaliser
Wrap an object and run some action on it when it runs out of scope.
(A kind of 'finally.')
* T: type of wrapped object.
* R: type of a 'releaser' (class providing static void release( T* object )). */
template< class T, class R >
class Finaliser
{
private:
T* object_;
public:
explicit Finaliser( T* object = NULL )
{
object_ = object;
}
~Finaliser() throw()
{
release();
}
Finaliser< T, R >& operator=( T* object )
{
if (object_ != object && object_ != NULL)
{
release();
}
object_ = object;
return *this;
}
T* operator->() const
{
return object_;
}
T** operator&()
{
return &object_;
}
operator T*()
{
return object_;
}
private:
void release() throw()
{
R::release< T >( object_ );
}
};
... and here's Releaser:
/* Releaser
Calls Release() on the object (for use with Finaliser). */
class Releaser
{
public:
template< class T > static void release( T* object )
{
if (object != NULL)
{
object->Release();
}
}
};
I have a few different kinds of releaser like this, including one for free() and one for CloseHandle().