C++ string question

If the constructor cannot allocate memory it should throw an exception.
An exception is a method commonly used in C++ to handle critic errors.
Another possibility, less common, is to use your own memory allocators (see custom std allocators).

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Original post by abeylin In C++ I invision doing this: <code> string s("hello world!"); </code> How do I know that there was enough memory to store the "hello world!" string?

Because the program keeps running. See below...

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It's possible the constructor failed to allocate memory, what would happen in that case?

Assuming a halfway decent compiler, and normal usage, an exception will be thrown to indicate the out-of-memory condition. This is what happens any time the C++ (default) operator new[] (or plain operator new, for that matter) is invoked and the needed memory is not available. It does not return null.

If the exception is not "caught", the program will terminate immediately, with very nicely defined behaviour (and depending on your compiler, possibly something like a stack trace or short debug dump). This is much, much nicer than the undefined behaviour (normally segfaults) that you get if you forget to check for a null return from malloc() and the memory isn't available. Also, you have much more freedom WRT how you catch the exception than you do with checking a return code. (Although in C++, you will need to be careful about "exception safety" if you deliberately design for exceptions to be thrown across function boundaries.)

[google] 'exception handling' for more info.

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How do I check the memory allocation that's happening behind the scenes in the string class?

You don't; the whole point of the class is to save you the effort of caring about all that stuff - because it's much more difficult than what you *really* want to get done, and not relevant to your high-level conception of the task.

wow, so "new()" throws an exception. It makes sense.
With that information I went to google to double check, and found that it doesn't always throw an exception: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vclang/html/_pluslang_the_new_and_delete_operators.asp
But you can force an exception to always be thrown.

Thanks for the info.

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Original post by abeylin wow, so "new()" throws an exception. It makes sense. With that information I went to google to double check, and found that it doesn't always throw an exception: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vclang/html/_pluslang_the_new_and_delete_operators.asp But you can force an exception to always be thrown. Thanks for the info.

No, new does not throw usually an exception. I write usually because in C++ you can also override the global operators (new and delete in particular).
So you could force new to throw an exception if the system cannot allocate memory ( simply redefine the new 'function' )
Commonly an exception is thrown inside a constructor if new returns NULL as failure response.

The c++ standard specifies that (excepting the nothrow versions) the new operator and new[] operator throw exceptions if they fail to allocate storage:

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C++ Standard, 3.7.3.1.3: An allocation function that fails to allocate storage can invoke the currently installed new_handler (18.4.2.2). [Note: A program supplied allocation function can obtain the address of the currently installed new_handler using the set_new_handler function (18.4.2.3). ] If a nothrow allocation function (18.4) fails to allocate storage, it shall return a null pointer. Any other allocation function that fails to allocate storage shall only indicate failure by throwing an exception of class std::bad_alloc (18.4.2.1) or a class derived from std::bad_alloc.

Microsoft Visual C++ 6.0 was written pre standardisation, and it's new and new[] operators return null on allocation failure. In order to retain backwards-compatability newer versions of Visual C++ all provide some method to have the new and new[] operators return null instead of throwing. This is only for backwards compatability and should never be used otherwise. If you require a non throwing new or new[] operation then use the standard-defined nothrow forms of the new and new[] operators.

Enigma

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Original post by Enigma Quote: C++ Standard, 3.7.3.1.3: An allocation function that fails to allocate storage can invoke the currently installed new_handler (18.4.2.2). [Note: A program supplied allocation function can obtain the address of the currently installed new_handler using the set_new_handler function (18.4.2.3). ] If a nothrow allocation function (18.4) fails to allocate storage, it shall return a null pointer. Any other allocation function that fails to allocate storage shall only indicate failure by throwing an exception of class std::bad_alloc (18.4.2.1) or a class derived from std::bad_alloc. Enigma

Interesting.