Re: Basic, but what the hell am I doing wrong here?

From: Kevin Mark (kmark+debian-user_at_pipeline.com)
Date: 06/11/04

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    Date: Thu, 10 Jun 2004 22:49:42 -0400
To: debian-user@lists.debian.org

    On Thu, Jun 10, 2004 at 09:36:04PM -0400, Stephen Touset wrote:
    > I've opened and read files hundreds of times in my life. What gives now?
    >
    > --
    > Stephen Touset <stephen@touset.org>

    > #include <fstream>
    >
    > using namespace std;
    >
    > int main(void)
    > {
    >
    > ifstream fin("test.cc");
    > char* str;
    >
    > if (!fin.is_open())
    > {
    > exit(1);
    > }
    >
    > while (!fin.eof())
    > {
    > fin.getline(str, 80);
    > }
    >
    > fin.close();
    >
    > return 0;
    >
    > }
    Hi Stephen,
    I can understand your confusion. you have a perception issue that is
    clouding your understanding. either that or you have been looking at
    this for umteen hours and too much caffine x-). The issue is the
    understading of memory allocation versus pointer definition.

    'char * str' defines a 'char *' called str. it allocates memory for this
    pointer. so, str has an address of 0xabcd (for example). It has not been
    initialized with anything, so it contains undeterminate data (eg.
    garbage). lets say it the contents of address 0xabcd is 'Z'. so, *str =
    'Z'. What does *(str + 1) contain? undeterminate data. As does *(str +
    2) ... *(str + 79). So you define str, don't initialize it and then read
    data into *(str) ... *(str + 79). Well not really, becase you have no
    right to write to those areas. Why? because you have not allocate memory
    at those addresses. How do you do this? well in c you'd 'malloc. But
    this is c++, so, you'd use 'new', I think.

    This should shead some light on you problem.
    -Kev 

    


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