copy() segfaults

This is a discussion on copy() segfaults within the C++ Programming forums, part of the General Programming Boards category; Does anyone know why the following code segfaults? Valgrind gives me no hints. I compile with 'g++ knas.cpp -pedantic -Wall ...

  1. #1
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    copy() segfaults

    Does anyone know why the following code segfaults? Valgrind gives me no hints.
    I compile with 'g++ knas.cpp -pedantic -Wall -ansi -g -o knas' and get no errors/warnings.

    Code:
    #include <algorithm>
    #include <vector>
    #include <string>
    
    int main() {
      std::string str = "hello";
      std::vector<char> vec;
      copy(str.begin(), str.end(), vec.begin());
      return 0;
    }

  2. #2
    ZuK
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    Code:
    #include <algorithm>
    #include <vector>
    #include <string>
    
    int main() {
      std::string str = "hello";
      std::vector<char> vec;
      vec.reserve(str.length());
      copy(str.begin(), str.end(), vec.begin());
      return 0;
    }
    Kurt

  3. #3
    Registered User hk_mp5kpdw's Avatar
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    Or:
    Code:
    #include <algorithm>
    #include <vector>
    #include <string>
    #include <iterator>
    
    int main()
    {
      std::string str = "hello";
      std::vector<char> vec;
      std::copy(str.begin(), str.end(), std::back_inserter(vec) );
      return 0;
    }

    vec starts out without any memory allocated for storage of anything so trying to push data into it the wrong way is bad.


    Quote Originally Posted by ZuK View Post
    Code:
    #include <algorithm>
    #include <vector>
    #include <string>
    
    int main() {
      std::string str = "hello";
      std::vector<char> vec;
      vec.reserve(str.length());
      copy(str.begin(), str.end(), vec.begin());
      return 0;
    }
    Kurt
    Shouldn't that be resize?
    Last edited by hk_mp5kpdw; 09-29-2007 at 02:23 PM.
    "Owners of dogs will have noticed that, if you provide them with food and water and shelter and affection, they will think you are god. Whereas owners of cats are compelled to realize that, if you provide them with food and water and shelter and affection, they draw the conclusion that they are gods."
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  4. #4
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    Ah, thanks for the quick reply. Seems like that was it.

  5. #5
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    Code:
    #include <vector>
    #include <string>
    
    int main() {
      std::string str = "hello";
      std::vector<char> vec(str.begin(), str.end());
    }

  6. #6
    ZuK
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    Quote Originally Posted by hk_mp5kpdw View Post
    Shouldn't that be resize?
    Yes. Should have been.
    Reserve just sets the capacity of the vector that's why it wouldn't segfault.
    But the vector would still be empty after the call to copy.
    Kurt
    Last edited by ZuK; 09-29-2007 at 02:49 PM.

  7. #7
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    Quote Originally Posted by ZuK View Post
    Yes. Should have been.
    Reserve just sets the capacity of the vector that's why it wouldn't segfault.
    But the vector would still be empty after the call to copy.
    Kurt
    The vector would still have size 0, but if you accessed its elements by indexing as in vec[i], it would probably appear to work normally (this happened to me when I was first learning how to use vectors). However, vec.at(i) which does bounds checking would catch the problem. I still prefer the regular indexing since it's easier to read, but at() is good for debugging.

  8. #8
    Cat without Hat CornedBee's Avatar
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    Actually, any halfway decent standard library ought to assert if you use [] with an out-of-bound index in debug mode.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
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  9. #9
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    Quote Originally Posted by CornedBee View Post
    Actually, any halfway decent standard library ought to assert if you use [] with an out-of-bound index in debug mode.
    Thank you - I was completely unaware of this. Using -D_GLIBCXX_DEBUG with gcc generates the desired warnings, which saves me the trouble of making a second copy of the source with [] replaced by at().
    Code:
    #include <vector>
    
    int main() {
      std::vector<int> v;
      v.reserve(1);
      v[0] = 1;
    }
    Output when running executable compiled with -D_GLIBCXX_DEBUG:

    /usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c++/4.1.2/debug/vector:192:
    error: attempt to subscript container with out-of-bounds index 0, but
    container only holds 0 elements.

    Objects involved in the operation:
    sequence "this" @ 0x0xbfa91890 {
    type = N15__gnu_debug_def6vectorIiSaIiEEE;
    }
    Aborted

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