Accessing member functions of a class instance: garbage value and seg fault seen

This is a discussion on Accessing member functions of a class instance: garbage value and seg fault seen within the C++ Programming forums, part of the General Programming Boards category; Hello all, I'm trying to work through two separate but likely related problems: 1) When I try to use the ...

  1. #1
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    Accessing member functions of a class instance: garbage value and seg fault seen

    Hello all,

    I'm trying to work through two separate but likely related problems:

    1) When I try to use the class constructor below by setting the value of 'size'; the 'size' variable value is not maintained within the int location stored in the class. This is shown in the output I've copied below.

    2) When I try to access the member function getVal(...) I get a segmentation fault.

    I've boiled down my code to a subset that still shows this behavior.

    It has been quite some time since I've programmed in c/c++ and I've been trying to find where I could be going wrong here; for quite a while. Any advice to solve the issues above, or to explain why I'm getting this type of behavior, would be extremely appreciated.

    My system is:
    > uname -a
    Linux debian-richard 2.6.22.6 #1 SMP Thu Sep 6 21:18:12 EDT 2007 i686 GNU/Linux

    My compiler is:
    > g++ -v
    Using built-in specs.
    Target: i486-linux-gnu
    Configured with: ../src/configure -v --enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.1.3 --program-suffix=-4.1 --enable-__cxa_atexit --enable-clocale=gnu --enable-libstdcxx-debug --enable-mpfr --enable-checking=release i486-linux-gnu
    Thread model: posix
    gcc version 4.1.3 20070812 (prerelease) (Debian 4.1.2-15)

    thanks,
    Richard
    -----------------------------------
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <time.h>
    
    class data_matrix {
      private:
        int size;
        int *a;  
        data_matrix(int size, int val, bool random);
    
      public:
        data_matrix(int size);
        data_matrix(int size, int val);
        ~data_matrix();
        int getVal(int row, int col);
        int setVal(int row, int col, int val);
        int getSize();
    };
    
    data_matrix::data_matrix(int inpSize)
      {data_matrix(inpSize, 0, true);}
    
    data_matrix::data_matrix(int inpSize, int val) 
      {data_matrix(inpSize, val, false);}
    
    data_matrix::data_matrix(int inpSize, int val, bool random) {
      size = inpSize;
      printf("the input size=%d\nafter the update the size is now = %d\n",inpSize,size);
      a = new int[size*size];
    
      if (random) {
        srand(time(NULL));
        for (int i=0; i<size; i++)
          a[i] = rand();
    
      } else {
        for (int i=0; i<size; i++)
          a[i] = val;
      }
    }
    
    data_matrix::~data_matrix() {
      delete[] a;
      a = NULL;
    }
    
    int data_matrix::getVal(int row, int col) {
      return a[row*size + col];
    }
    
    int data_matrix::setVal(int row, int col, int val) {
      a[row*size + col] = val;
      return 0;
    }
    
    int data_matrix::getSize() {
       printf("the size variable is: %d\n",size);
       return size;
    }
    
    int main() {
      data_matrix b(100), c(200,2);
      printf("Starting tests\n");
      printf("b(100) size=%d\n",b.getSize());
      printf("c(200,2) size=%d\nc[1,1]=%d",c.getSize(),c.getVal(1,1));
    }
    -----------------------
    Has this output:
    -----------------------
    ./tester.exe
    the input size=100
    after the update the size is now = 100
    the input size=200
    after the update the size is now = 200
    Starting tests
    the size variable is: -1080196504
    b(100) size=-1080196504
    Segmentation fault

  2. #2
    Captain Crash brewbuck's Avatar
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    Portland, OR
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    7,230
    You can't call one constructor from inside another that way. It just ends up constructing then throwing away a temporary object.

  3. #3
    and the hat of wrongness Salem's Avatar
    Join Date
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    Location
    The edge of the known universe
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    To illustrate what brewbuck just said
    Code:
    data_matrix::data_matrix(int inpSize, int val, bool random) {
      size = inpSize;
      printf("ctor called - this = %p\n", (void*)this );
      printf("the input size=%d\nafter the update the size is now = %d\n",inpSize,size);
      a = new int[size*size];
    
      if (random) {
        srand(time(NULL));
        for (int i=0; i<size; i++)
          a[i] = rand();
    
      } else {
        for (int i=0; i<size; i++)
          a[i] = val;
      }
    }
    
    data_matrix::~data_matrix() {
      printf("dtor called - this = %p\n", (void*)this );
      delete[] a;
      a = NULL;
    }
    
    int main() {
        data_matrix b(100);
    }
    
    Result
    $ ./a.exe
    ctor called - this = 0x22cc80
    the input size=100
    after the update the size is now = 100
    dtor called - this = 0x22cc80
    dtor called - this = 0x22ccd0
    Even in this simple case, two things were created (as evidenced by two things being destroyed), but you only constructed one of them.


    You might need a default constructor and/or a copy constructor.
    http://www.parashift.com/c++-faq-lite/ctors.html
    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.
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