Chapter 9
The Spreadsheet Class
Chapter 8 introduced the SpreadsheetCell class. This chapter moves on to write the Spreadsheet class. As with the SpreadsheetCell class, the Spreadsheet class will evolve throughout this chapter. Thus, the various attempts do not always illustrate the best way to do every aspect of class writing. To start,
a Spreadsheet is simply a two-dimensional array of SpreadsheetCells, with methods to set and retrieve cells at specific locations in the Spreadsheet. Although most spreadsheet applications use letters in one direction and numbers in the other to refer to cells, this Spreadsheet uses numbers in both directions. Here is a first attempt at a class definition for a simple Spreadsheet class:
// Spreadsheet.h
#include “SpreadsheetCell.h”
class Spreadsheet
{
public:
Spreadsheet(int inWidth, int inHeight);
void setCellAt(int x, int y, const SpreadsheetCell& cell); SpreadsheetCell getCellAt(int x, int y);
protected:
bool inRange(int val, int upper);
int mWidth, mHeight; SpreadsheetCell** mCells;
};
Note that the Spreadsheet class does not contain a standard two-dimensional array of
SpreadsheetCells. Instead, it contains a SpreadsheetCell**. The reason is that each Spreadsheet object might have different dimensions, so the constructor of the class must dynamically allocate the two-dimensional array based on the client-specified height and width. In order to allocate dynamically a two-dimensional array you need to write the following code:
#include “Spreadsheet.h”
Spreadsheet::Spreadsheet(int inWidth, int inHeight) : mWidth(inWidth), mHeight(inHeight)
{
mCells = new SpreadsheetCell* [mWidth]; for (int i = 0; i < mWidth; i++) {
mCells[i] = new SpreadsheetCell[mHeight];
}
}
The resultant memory for a Spreadsheet called s1 on the stack with width four and height three is shown in Figure 9-1.
184
|
|
Mastering Classes and Objects |
|
stack |
heap |
4 |
3 |
Each element is an |
unnamed SpreadsheetCell* |
||
int mWidth |
int mHeight |
|
SpreadsheetCell**mCells |
|
|
Each element is an unnamed |
Spreadsheet s1 |
SpreadsheetCell. |
Figure 9-1
If this code confuses you, consult Chapter 13 for details on memory management.
The implementations of the set and retrieval methods are straightforward:
void Spreadsheet::setCellAt(int x, int y, const SpreadsheetCell& cell)
{
if (!inRange(x, mWidth) || !inRange(y, mHeight)) { return;
}
mCells[x][y] = cell;
}
SpreadsheetCell Spreadsheet::getCellAt(int x, int y)
{
SpreadsheetCell empty;
if (!inRange(x, mWidth) || !inRange(y, mHeight)) { return (empty);
}
return (mCells[x][y]);
}
Note that these two methods use a helper method inRange() to check that x and y represent valid coordinates in the spreadsheet. Attempting to access an invalid field in the array will cause the program to malfunction. A production application would probably use exceptions to report error conditions, as described in Chapter 15.
185
Chapter 9
Freeing Memory with Destructors
Whenever you are finished with dynamically allocated memory, you should free it. If you dynamically allocate memory in an object, the place to free that memory is in the destructor. The compiler guarantees that the destructor will be called when the object is destroyed. Here is the Spreadsheet class definition from earlier with a destructor:
class Spreadsheet
{
public:
Spreadsheet(int inWidth, int inHeight); ~Spreadsheet();
void setCellAt(int x, int y, const SpreadsheetCell& inCell); SpreadsheetCell getCellAt(int x, int y);
protected:
bool inRange(int val, int upper);
int mWidth, mHeight; SpreadsheetCell** mCells;
};
The destructor has the same name as the name of the class (and of the constructors), preceded by a tilde (~). The destructor takes no arguments, and there can only be one of them.
Here is the implementation of the Spreadsheet class destructor:
Spreadsheet::~Spreadsheet()
{
for (int i = 0; i < mWidth; i++) { delete[] mCells[i];
}
delete[] mCells;
}
This destructor frees the memory that was allocated in the constructor. However, no dictate requires you only to free memory in the destructor. You can write whatever code you want in the destructor, but it is a good idea to use it only for freeing memory or disposing of other resources.
Handling Copying and Assignment
Recall from Chapter 8 that, if you don’t write a copy constructor and an assignment operator yourself, C++ writes them for you. These compiler-generated methods recursively call the copy constructor or assignment operator, respectively, on object data members. However, for primitives, such as int, double, and pointers, they provide shallow or bitwise copying or assignment: they just copy or assign the data members from the source object directly to the destination object. That presents problems when you dynamically allocate memory in your object. For example, the following code copies the spreadsheet s1 to initialize s when s1 is passed to the printSpreadsheet() function.
186
Mastering Classes and Objects
#include “Spreadsheet.h”
void printSpreadsheet(Spreadsheet s)
{
// Code omitted for brevity.
}
int main(int argc, char** argv)
{
Spreadsheet s1(4, 3); printSpreadsheet(s1);
return (0);
}
The Spreadsheet contains one pointer variable: mCells. A shallow copy of a spreadsheet gives the destination object a copy of the mCells pointer, but not a copy of the underlying data. Thus, you end up with a situation where both s and s1 have a pointer to the same data, as shown in Figure 9-2.
|
stack |
heap |
4 |
3 |
Each element is an |
unnamed SpreadsheetCell* |
||
int mWidth |
int mHeight |
|
SpreadsheetCell**mCells |
|
|
Each element is an unnamed |
Spreadsheet s1 |
SpreadsheetCell. |
4 |
3 |
int mWidth |
int mHeight |
SpreadsheetCell**mCells
Spreadsheet s
Figure 9-2
187
Chapter 9
If s were to change something to which mCells points, that change would show up in s1 too. Even worse, when the printSpreadsheet() function exits, s’s destructor is called, which frees the memory pointed to by mCells. That leaves the situation shown in Figure 9-3.
|
stack |
heap |
4 |
3 |
Freed memory |
|
||
int mWidth |
int mHeight |
|
SpreadsheetCell**mCells
Spreadsheet s1
Figure 9-3
Now s1 has a dangling pointer!
Unbelievably, the problem is even worse with assignment. Suppose that you had the following code:
Spreadsheet s1(2, 2), s2(4, 3);
s1 = s2;
After both objects are constructed, you would have the memory layout shown in Figure 9-4.
188
Mastering Classes and Objects
|
stack |
heap |
4 |
3 |
|
int mWidth |
int mHeight |
|
SpreadsheetCell**mCells
Spreadsheet s2
2 |
2 |
int mWidth |
int mHeight |
SpreadsheetCell**mCells
Spreadsheet s1
Figure 9-4
After the assignment statement, you would have the layout shown in Figure 9-5.
189
Chapter 9
|
stack |
4 |
3 |
int mWidth |
int mHeight |
SpreadsheetCell**mCells
Spreadsheet s2
2 |
2 |
int mWidth |
int mHeight |
SpreadsheetCell**mCells
Spreadsheet s1
Figure 9-5
heap
Orphaned memory!
Now, not only do the mCells pointers in s1 and s2 point to the same memory, but you have orphaned the memory to which mCells in s1 previously pointed. That is why in assignment operators you must first free the old memory, and then do a deep copy.
As you can see, relying on C++’s default copy constructor or assignment operator is not always a good idea. Whenever you have dynamically allocated memory in a class, you should write your own copy constructor to provide a deep copy of the memory.
The Spreadsheet Copy Constructor
Here is a declaration for a copy constructor in the Spreadsheet class:
class Spreadsheet
{
public:
Spreadsheet(int inWidth, int inHeight); Spreadsheet(const Spreadsheet& src);
190
Mastering Classes and Objects
~Spreadsheet();
void setCellAt(int x, int y, const SpreadsheetCell& cell); SpreadsheetCell getCellAt(int x, int y);
protected:
bool inRange(int val, int upper);
int mWidth, mHeight; SpreadsheetCell** mCells;
};
Here is the definition of the copy constructor:
Spreadsheet::Spreadsheet(const Spreadsheet& src)
{
int i, j;
mWidth = src.mWidth; mHeight = src.mHeight;
mCells = new SpreadsheetCell* [mWidth]; for (i = 0; i < mWidth; i++) {
mCells[i] = new SpreadsheetCell[mHeight];
}
for (i = 0; i < mWidth; i++) {
for (j = 0; j < mHeight; j++) { mCells[i][j] = src.mCells[i][j];
}
}
}
Note that the copy constructor copies all data members, including mWidth and mHeight, not just the pointer data members. The rest of the code in the copy constructor provides a deep copy of the mCells dynamically allocated two-dimensional array.
Copy all data members in a copy constructor, not just pointer members.
The Spreadsheet Assignment Operator
Here is the definition for the Spreadsheet class with an assignment operator:
class Spreadsheet
{
public:
Spreadsheet(int inWidth, int inHeight); Spreadsheet(const Spreadsheet& src); ~Spreadsheet();
Spreadsheet& operator=(const Spreadsheet& rhs);
void setCellAt(int x, int y, const SpreadsheetCell& cell); SpreadsheetCell getCellAt(int x, int y);
191
Chapter 9
protected:
bool inRange(int val, int upper);
int mWidth, mHeight; SpreadsheetCell** mCells;
};
Here is the implementation of the assignment operator for the Spreadsheet class, with explanations interspersed. Note that when an object is assigned to, it already has been initialized. Thus, you must free any dynamically allocated memory before allocating new memory. You can think of an assignment operator as a combination of a destructor and a copy constructor. You are essentially “reincarnating” the object with new life (or data) when you assign to it.
Spreadsheet& Spreadsheet::operator=(const Spreadsheet& rhs)
{
int i, j;
// Check for self-assignment. if (this == &rhs) {
return (*this);
}
The above code checks for self-assignment.
// Free the old memory.
for (i = 0; i < mWidth; i++) { delete[] mCells[i];
}
delete[] mCells;
This chunk of code is identical to the destructor. You must free all the memory before reallocating it, or you will create a memory leak.
// Copy the new memory. mWidth = rhs.mWidth; mHeight = rhs.mHeight;
mCells = new SpreadsheetCell* [mWidth]; for (i = 0; i < mWidth; i++) {
mCells[i] = new SpreadsheetCell[mHeight];
}
for (i = 0; i < mWidth; i++) {
for (j = 0; j < mHeight; j++) { mCells[i][j] = rhs.mCells[i][j];
}
}
This chunk of code is identical to the copy constructor.
return (*this);
}
192
Mastering Classes and Objects
The assignment operator completes the “big 3” routines for managing dynamically allocated memory in an object: the destructor, the copy constructor, and the assignment operator. Whenever you find yourself writing one of those methods you should write all of them.
Whenever a class dynamically allocates memory, write a destructor, copy constructor, and assignment operator.
Common Helper Routines for Copy Constructor and Assignment Operator
The copy constructor and the assignment operator are quite similar. Thus, it’s usually convenient to factor the common tasks into a helper method. For example, you could add a copyFrom() method to the Spreadsheet class, and rewrite the copy constructor and assignment operator to use it like this:
void Spreadsheet::copyFrom(const Spreadsheet& src)
{
int i, j;
mWidth = src.mWidth; mHeight = src.mHeight;
mCells = new SpreadsheetCell* [mWidth]; for (i = 0; i < mWidth; i++) {
mCells[i] = new SpreadsheetCell[mHeight];
}
for (i = 0; i < mWidht; i++) {
for (j = 0; j < mHeight; j++) { mCells[i][j] = src.mCells[i][j];
}
}
}
Spreadsheet::Spreadsheet(const Spreadsheet &src)
{
copyFrom(src);
}
Spreadsheet& Spreadsheet::operator=(const Spreadsheet& rhs)
{
int i;
//Check for self-assignment. if (this == &rhs) {
return (*this);
}
//Free the old memory.
for (i = 0; i < mWidth; i++) { delete[] mCells[i];
}
193