different between New type[0] and New type[1]

This is a discussion on different between New type[0] and New type[1] within the C++ Programming forums, part of the General Programming Boards category; hi everyone: what the meaning of int*ptr=new int[0];? it's compiling, and there isn't a run time error, so what's the ...

  1. #1
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    different between New type[0] and New type[1]

    hi everyone:
    what the meaning of
    int*ptr=new int[0];?
    it's compiling, and there isn't a run time error,
    so what's the different between that and new int[1] or new int?
    thanks alot

  2. #2
    Guest Sebastiani's Avatar
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    Simply put, the first is semantically equivalent to setting the array to null, whereas the latter two allocate a single object.
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

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    i think you wrong, Sebastiani

    Quote Originally Posted by Sebastiani View Post
    the first is semantically equivalent to setting the array to null.
    thanks for the answer Sebastiani, but i don't think you right because the output of the next code:
    Code:
    #include<iostream>
    using namespace std;
    int main()
    {
    	int*ptr=new int[0];
    	cout<<ptr<<endl;
    	system("pause");
        return 0;
    }
    the output is an hex address (006148A8 in my case)
    and if were right than the output should be 0(NULL)

  4. #4
    and the hat of wrongness Salem's Avatar
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    Whether you get NULL or an address is implementation-specific, in response to allocating a zero-sized object.

    What you cannot do (in either case) is something like
    *ptr = value;
    ptr[0] = value;

    Both of these would be wrong, even if your pointer wasn't NULL.

    Likewise, if you did new[0], you must use delete [] to dispose of the object.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  5. #5
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    Quote Originally Posted by Salem View Post
    What you cannot do (in either case) is something like
    *ptr = value;
    ptr[0] = value;
    so why the next code compiling?
    Code:
    #include<iostream>
    using namespace std;
    int main()
    {
    	int*ptr=new int[0];
    	*ptr=976;
    	cout<<*ptr<<endl;
    	system("pause");
        return 0;
    }
    there isn't a run time error and the output is:976

  6. #6
    C++まいる!Cをこわせ!
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    Like Salem said, it's implementation defined.
    Run it on other compilers and see if it crashes and burns.
    It also leaks memory.

    Furthermore, trying this on Visual C++...
    Code:
    int main()
    {
    	int* p = new int[0];
    	*p = 10;
    	delete [] p;
    }
    ...fails on the delete statement because we allocated 0 bytes, yet tried to assign 4 bytes we didn't own!
    Last edited by Elysia; 03-03-2011 at 10:31 AM.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  7. #7
    and the hat of wrongness Salem's Avatar
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    > there isn't a run time error and the output is:976
    Good for you - now you understand that "anything" can happen when you make a mess of things.

    The safety net underneath the "C++ trapeze artist" is full of holes of varying sizes. Sometimes you get lucky and find a bit of net to save you from your mistakes. Other times, you find a hole and you plummet right into the ground.

    Because of this, you need to know the RULES of the language, rather than assuming that every single mistake you can make will be caught by the language. It won't. Sooner or later, a mistake like this would catch up with you.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  8. #8
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    all right guys i got it, thank you very much for your time and a special thanks to Salem, i'll follow the RULES from now on :-)

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