Can the C++ `new` operator ever throw an exception

Question

Can the new operator throw an exception 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?

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See also:

• How often do you check for an exception in a C++ new instruction?
• Is it useful to test the return of “new” in C++?
• Will new return NULL in any case?

Answer 1

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.

Answer 2

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.

Answer 3

If you are running on a typical embedded processor running Linux without virtual memory it is quite likely your process will be terminated by the operating system before new fails if you allocate too much 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 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.

Answer 4

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.

Answer 5

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.

Answer 6

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.
}