How to Draw a Diagram

Follow the rules below when you are asked to draw a diagram.

Rules 1-3

For each stack frame of a function, draw a scratch pad. Label the pad with the function name.
All local variables should reside inside the corresponding pad. Label them.
Draw a box only for an object that is occupied in the memory.
- A variable should have a box, since its value is stored in memory.
- A constant value should not have a box.

Example 1


void f(char );

int main()
{
  int n = 4;
  char c = 'h';

  f(c);
  return 0;
}

void f(char c)
{
  double x = 4.7;
  cout << c << endl;

  //*** point1 ***
}

Rule 1 is satisfied. There are two pads labeled with main and f.
Rule 2 is satisfied. You can see all the local variables properly labeled.
Rule 3 is satisfied. There are four boxes.

Example 2


void f(char );

int main()
{
  int n = 4;

  f('h');
  return 0;
}

void f(char c)
{
  double x = 4.7;
  cout << c << endl;

  //*** point1 ***
}

Observe how rule 3-2 is applied in the slighted changed code.

Rules 4-5

If an array is located in the heap area, the array should be drawn outside the stack frame (scratch pad) of any function.
Inside each box, specify the value that it stores.
- For example, the box contains an int, give an actual integer value.
- If the box contains an address (i.e., if it's a pointer variable), give an arrow that starts from inside the box.

Example 3


void f(char );

int main()
{
  int n = 4;
  int* A;
  A = new int[n]{1, 3, 5, 7};
  f('h');
  return 0;
}

void f(char c)
{
  double x = 4.7;
  cout << c << endl;

  //*** point1 ***
}

Rule 4 is satisfied, since the array is outside of the pads.
Rule 5 is satisfied. Note how the arrow is drawn for variable A.
- The starting point is inside the box.
- The ending point is the first element of the array.

Rules 6

When two pointers are equal, they point to the same thing.

Example 4


void f(int* B);

int main()
{
  int n = 4;
  int* A;
  A = new int[n]{1, 3, 5, 7};
  f(A);
  return 0;
}

void f(int* B)
{
  B[1] = 0;
  //*** point1 ***
}

Example 5


void f(int* q);

int main()
{
  int n = 4;
  int* A;
  A = new int[n]{1, 3, 5, 7};
  int* p = &A[2];
  f(p);
  return 0;
}

void f(int* q)
{
  *q = 0;
  //*** point1 ***
}

Rules 7

When the function is finished, the scratch pad should be discarded.

Example 6


int* f();

int main()
{
  int* A;
  A = f();
  //*** control point 2 ***
  return 0;
}

int* f()
{
  int* B = new int[4] = {1, 3, 5, 7};
  //*** control point 1 *** 
  return B;
}

control point 1

between control points 1 and 2

control point 2