Can the C++ `new` operator ever throw an exception in real life?_问答_开发者

Can the new operator throw an exc开发者_运维技巧eption in real life?

And if so, do I have any options for handling such an exception apart from killing my application?

Update:

Do any real-world, new-heavy applications check for failure and recover when there is no memory?

If you are running your program on a machine with less physical memory than the maximum of virtual memory (2 GB on standard Windows) you will find that once you have allocated an amount of memory approximately equal to the available physical memory, further allocations will succeed but will cause paging to disk. This will bog your program down and you might not actually be able to get to the point of exhausting virtual memory. So you might not get an exception thrown.

If you have more physical memory than the virtual memory, and you simply keep allocating memory, you will get an exception when you have exhausted virtual memory to the point where you can not allocate the block size you are requesting.

If you have a long-running program that allocates and frees in many different block sizes, including small blocks, with a wide variety of lifetimes, the virtual memory may become fragmented to the point where new will be unable to find a large enough block to satisfy a request. Then new will throw an exception. If you happen to have a memory leak that leaks the occasional small block in a random location that will eventually fragment memory to the point where an arbitrarily small block allocation will fail, and an exception will be thrown.

If you have a program error that accidentally passes a huge array size to new[], new will fail and throw an exception. This can happen for example if the array size is actually some sort of random byte pattern, perhaps derived from uninitialized memory or a corrupted communication stream.

All the above is for the default global new. However, you can replace global new and you can provide class-specific new. These too can throw, and the meaning of that situation depends on how you programmed it. it is usual for new to include a loop that attempts all possible avenues for getting the requested memory. It throws when all those are exhausted. What you do then is up to you.

You can catch an exception from new and use the opportunity it provides to document the program state around the time of the exception. You can "dump core". If you have a circular instrumentation buffer allocated at program startup, you can dump it to disk before you terminate the program. The program termination can be graceful, which is an advantage over simply not handling the exception.

I have not personally seen an example where additional memory could be obtained after the exception. One possibility however, is the following: Suppose you have a memory allocator that is highly efficient but not good at reclaiming free space. For example, it might be prone to free space fragmentation, in which free blocks are adjacent but not coalesced. You could use an exception from new, caught in a new_handler, to run a compaction procedure for free space before retrying.

Serious programs should treat memory as a potentially scarce resource, control its allocation as much as possible, monitor its availability and react appropriately if something seems to have gone dramatically wrong. For example, you could make a case that in any real program there is quite a small upper bound on the size parameter passed to the memory allocator, and anything larger than this should cause some kind of error handling, whether or not the request can be satisfied. You could argue that the rate of memory increase of a long-running program should be monitored, and if it can be reasonably predicted that the program will exhaust available memory in the near future, an orderly restart of the process should be begun.

In Unix systems, it's customary to run long-running processes with memory limits (using ulimit) so that it doesn't eat up all of a system's memory. If your program hits that limit, you will get std::bad_alloc.

Update for OP's edit: the most typical case of programs recovering from an out-of-memory condition is in garbage-collected systems, which then performs a GC and continues. Though, this sort of on-demand GC is really for last-ditch efforts only; usually, good programs try to GC periodically to reduce stress on the collector.

It's less usual for non-GC programs to recover from out-of-memory issues, but for Internet-facing servers, one way to recover is to simply reject the request that's causing the memory to run out with a "temporary" error. ("First in, first served" strategy.)

osgx said:

Does any real-world applications checks a lot number of news and can recover when there is no memory?

I have answered this previously in my answer to this question, which is quoted below:

It is very difficult to handle this sort of situation. You may want to return a meaningful error to the user of your application, but if it's a problem caused by lack of memory, you may not even be able to afford the memory to allocate the error message. It's a bit of a catch-22 situation really.

There is a defensive programming technique (sometimes called a memory parachute or rainy day fund) where you allocate a chunk of memory when your application starts. When you then handle the bad_alloc exception, you free this memory up, and use the available memory to close down the application gracefully, including displaying a meaningful error to the user. This is much better than crashing :)

You don't need to handle the exception in every single new :) Exceptions can propagate. Design your code so that there are certain points in each "module" where that error is handled.

It depends on the compiler/runtime and on the operator new that you are using (e.g. certain versions of Visual Studio will not throw out of the box, but would rather return a NULL pointer a la malloc instead.)

You can always catch a std::bad_alloc exception, or explicitly use nothrow new to return NULL instead of throwing. (Also see past StackOverflow posts revolving around the subject.)

Note that operator new, like malloc, will fail when you have run out of memory, out of address space (e.g. 2-3GB in a 32-bit process depending on the OS), out of quota (ulimit was already mentioned) or out of contiguous address space (e.g. fragmented heap.)

Yes, new can throw std::bad_alloc (a subclass of std::exception), which you may catch.

If you absolutely want to avoid this exception, and instead are ready to test the result of new for a null pointer, you may add a nothrow argument:

T* p = new (nothrow) T(...);
if (p == 0)
{
    // Do something about the bad allocation!
}
else
{
    // Here you may use p.
}

Yes new will throw an exception if there is no more memory available, but that doesn't mean you should wrap every new in a try ... catch. Only catch the exception if your program can actually do something about it.

If the program cannot do anything to handle that exceptional situation, what is often the case if you run out of memory, there is no use in catching the exception. If the only thing you could reasonably do is to abort the program you can as well just let the exception bubble up to top level, where it will terminate the program as well.

In many cases there's no reasonable recovery for an out of memory situation, in which case it's probably perfectly reasonable to let the application terminate. You might want to catch the exception at a high level to display a nicer error message than the compiler might give by default, but you might have to play some tricks to get even that to work (since the process is likely to be very low on resources at that point).

Unless you have a special situation that can be handled and recovered, there's probably no reason to spend a lot of effort trying to handle the exception.

Note that in Windows, very large new/mallocs will just allocate from virtual memory. In practice, your machine will crash before you see that exception.

char *pCrashMyMachine = new char[TWO_GIGABYTES];

Try it if you dare!

I use Mac OS X, and I've never seen malloc return NULL (which would imply an exception from new in C++). The machine bogs down, does its best to allocate dwindling memory to processes, and finally sends SIGSTOP and invites the user to kill processes rather than have them deal with allocation failure.

However, that's just one platform. CERTAINLY there are platforms where the default allocator does throw. And, as Chris says, ulimit may introduce an artificial constraint so that an exception would be the expected behavior.

Also, there are allocators besides the default one/malloc. If a class overrides operator new, you use custom arguments to new(…), or you pass an allocator object into a container, it probably defines its own conditions to throw bad_alloc.

new operator will throw std::bad_alloc exception when there are not enough available memory in the pool to fulfill runtime request.

This can happen on bad design or when memory allocated are not freed correctly.

Handling of such exception is based on your design, one way will be pause and retry some time later, hoping more memory returned to the pool and the request may succeed.

Most realistically new will throw due to a decision to limit a resource. Say this class (which may be memory intensive) takes memory out of the physicals pool and if to many objects take from it (we need memory for other things like sound, textures etc) it may throw instead of crashing later on when something that should be able to allocate memory takes it. (looks like a weird side effect).

Overloading new can be useful in devices with restricted memory. Such as handhelds or on consoles when its too easy to go overboard with cool effects.

Yes, new can and will throw.

Since you are asking about 'real' programs: I've worked on various shrink-wrapped commercial software applications for over 20 years. 'Real' programs with millions of users. That you can go and buy off the shelf today. Yes, new can throw.

There are various ways to handle this.

First, write your own new_handler (this is called before new gives up and throws - see set_new_handler() function). When your new_handler is called, see if you can free some things you don't really need. Also warn the user that they are running low on memory. (yes, it can be hard to warn the user about anything if you are really low).

One thing is to have pre-allocated, at the start of your program some 'extra' memory. When you run out of memory, use this extra memory to help save a copy of the user's document to disk. Then warn, and maybe exit gracefully.

Etc. This is just a overview, obviously there is more to it.

Handling low memory is not easy.

The new-handler function is the function called by allocation functions whenever new attempt to allocate the memory fails. We can have our own logging or some special action, eg,g arranging for more memory etc. Its intended purpose is one of three things: 1) make more memory available 2) terminate the program (e.g. by calling std::terminate) 3) throw exception of type std::bad_alloc or derived from std::bad_alloc. The default implementation throws std::bad_alloc. The user can have his own new-handler, which may offer behavior different than the default one. THis should be use only when you really need. See the example for more clarification and default behaviour,

#include <iostream>
#include <new>

void handler()
{
    std::cout << "Memory allocation failed, terminating\n";
    std::set_new_handler(nullptr);
}

int main()
{
    std::set_new_handler(handler);
    try {
        while (true) {
            new int[100000000ul];
        }
    } catch (const std::bad_alloc& e) {
        std::cout << e.what() << '\n';
    }
}

It's good to check/catch this exception when you are allocating memory based from something given from outside (from user space, network e.g.), because it could mean an attempt to compromise your application/service/system and you shouldn't allow this to happen.

new operator will throw std::bad_alloc exception when you run out of the memory ( virtual memory to be precise).

If new throws an exception then it is a serious error:

More than available VM is getting allocated ( it fails eventually). You can try reducing the amount of memory than exiting the program by catching std::bad_alloc exception.