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

You can't call one constructor from inside another that way. It just ends up constructing then throwing away a temporary object.

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