allocation and freeing of an array of ints

This is a discussion on allocation and freeing of an array of ints within the C++ Programming forums, part of the General Programming Boards category; first of all, how come you can do this in C++ and not in C int a[n]; secondly, why is ...

  1. #1
    Registered User
    Join Date
    Mar 2010
    Posts
    44

    allocation and freeing of an array of ints

    first of all, how come you can do this in C++ and not in C

    int a[n];

    secondly, why is the array "a" being printed after i freed it in the following piece of code:

    int *a = new int[10];
    for (int i = 0; i < 10; ++i)
    a[i] = i + 1;

    delete [] a;

    for (int i = 0; i < 10; ++i)
    cout << a[i] << " ";

    kind regards

  2. #2
    C++ Witch laserlight's Avatar
    Join Date
    Oct 2003
    Location
    Singapore
    Posts
    23,773
    Quote Originally Posted by boxden
    first of all, how come you can do this in C++ and not in C

    int a[n];
    Unless n is a constant positive integer, you cannot do that in C++, but under certain circumstances you can do that in C even if n is not a constant, with respect to the 1999 edition of the C standard (this is the variable length array feature).

    If you are able to use variable length arrays in C++, then you are relying on your compiler's language extension.

    Quote Originally Posted by boxden
    secondly, why is the array "a" being printed after i freed it in the following piece of code:
    Undefined behaviour. You should not be accessing that destroyed array.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  3. #3
    Registered User
    Join Date
    Mar 2009
    Posts
    399
    >first of all, how come you can do this in C++ and not in C
    >
    >int a[n];

    The feature you're using there is actually a C99 feature that's not available in C++ unless you use non-standard compiler extensions.

  4. #4
    Registered User
    Join Date
    Mar 2010
    Posts
    44
    thank you for the reply guys, i was compiling it like this
    g++ *.cpp

    what flags do i need to set to have the same behavior as C's gcc -ansi -Wall -pedantic?

    also laserlight, could you tell me more about C's possibility of using int n[var]? I thought ansi C prohibited it?

    thanks in advance

  5. #5
    C++ Witch laserlight's Avatar
    Join Date
    Oct 2003
    Location
    Singapore
    Posts
    23,773
    Quote Originally Posted by boxden
    thank you for the reply guys, i was compiling it like this
    g++ *.cpp

    what flags do i need to set to have the same behavior as C's gcc -ansi -Wall -pedantic?
    Use -Wall and -pedantic too

    Quote Originally Posted by boxden
    also laserlight, could you tell me more about C's possibility of using int n[var]? I thought ansi C prohibited it?
    You might want to search the Web, e.g., The New C: Why Variable Length Arrays?
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

Popular pages Recent additions subscribe to a feed

Similar Threads

  1. Back To The Basics: Freeing An Array on the Heap?
    By Deo in forum C++ Programming
    Replies: 12
    Last Post: 04-07-2007, 04:42 AM
  2. Understanding Memory Allocation
    By Ragsdale85 in forum C Programming
    Replies: 7
    Last Post: 10-31-2005, 07:36 AM
  3. 2D arrays:dynamic allocation and freeing
    By bravetanveer in forum C Programming
    Replies: 5
    Last Post: 02-05-2005, 06:24 AM
  4. memory allocation and freeing
    By Jase in forum Linux Programming
    Replies: 1
    Last Post: 05-25-2003, 06:26 AM
  5. dynamic allocation question
    By vale in forum C++ Programming
    Replies: 1
    Last Post: 08-26-2001, 04:23 PM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21