[RESOLVED] new operator
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Thread: [RESOLVED] new operator

  1. #1
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    [RESOLVED] new operator

    Hi all,

    When operator new fails bad::alloc exception is thrown.
    Does the c++ standard guarantee that the pointer will be set to NULL ?

    Many thanks

  2. #2
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    Re: new operator

    No. Anyway, because of the exception, the pointer would be left unchanged.
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  3. #3
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    Re: new operator

    AFAIK, when exception was thrown, stack unwinding is happen and all the local copy get destroyed. Why you said the pointer was unchanged ?

    Does it related to new allocate memory on heap ?

    Thanks.
    Thanks for your help.

  4. #4
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    Re: new operator

    Quote Originally Posted by Peter_APIIT
    AFAIK, when exception was thrown, stack unwinding is happen and all the local copy get destroyed. Why you said the pointer was unchanged ?
    If the exception causes control to reach a point where the pointer goes out of scope, then yes, the pointer would be destroyed. However, it is not necessarily the case that the pointer will go out of scope since it could be assigned to in a try block where the exception is caught in the catch block.
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  5. #5
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    Re: new operator

    Sorry for my stupidity.

    Do you have any example code that explain your idea ?

    Thanks.
    Thanks for your help.

  6. #6
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    Re: new operator

    Quote Originally Posted by Peter_APIIT
    Do you have any example code that explain your idea ?
    Well, a simple but rather silly example for some type T:
    Code:
    T* p = 0;
    try
    {
        p = new T;
    }
    catch (const std::exception&)
    {
        // do nothing
    }
    delete p;
    My main point is that because the exception is thrown, the assignment does not take place, so the pointer retains its value. If the pointer happens to be destroyed immediately after that, so be it.
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  7. #7
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    Re: new operator

    Thanks you!


    EDIT: How do I mark the message as a resolved?? Thank you

  8. #8
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    Re: new operator

    You're welcome
    You can use the thread tools to mark the thread as resolved.
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  9. #9
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    Re: [RESOLVED] new operator

    Just a complement...

    Still, it's possible to use new in the "old" form. This is done with the nothrow version of new and is guaranteed by the Standard to return 0 in case of failure.

    Code:
    SomeType p = new (std::nothrow) SomeType;
    if (!p)
    {
      //Failure...
    }

  10. #10
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    Re: [RESOLVED] new operator

    Quote Originally Posted by ltcmelo View Post
    Just a complement...

    Still, it's possible to use new in the "old" form. This is done with the nothrow version of new and is guaranteed by the Standard to return 0 in case of failure.

    Code:
    SomeType p = new (std::nothrow) SomeType;
    if (!p)
    {
      //Failure...
    }
    Watch out from std::nothrow() when there is a chance that your code may be used in environments that define thier own version of new operayor (MFC for example).
    http://www.codeguru.com/forum/showthread.php?t=470168

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