Creating the Column Class 217

When you’re implementing a complex system, you can save immense time by breaking it down into the most discrete simple components you can make. This way, when change is needed later on, the amount of required effort is smaller and the ripple effect throughout the system is minimal.

Creating the Column Class

The first element of the spreadsheet is the column. Let’s build a simple class that maintains information about the column, and contains methods to work with that information. Here’s how:

1. In the code editor of your choice, create a new file to hold the code for the implementation of the source file.

In this example, the file is named ch39.cpp, although you can use whatever you choose.

2. Type the code from Listing 39-1 into your file.

Better yet, copy the code from the source file on this book’s companion Web site.

LISTING 39-1: THE COLUMN CLASS

#include <stdio.h> #include <string> #include <vector>

class Column

{

std::string _format; std::string _value;

public:

Column(void)

{

_format = “%s”; _value = “”;

}

Column( const char *format, const char *value )

{

_format = format; _value = value;

}

Column( const Column& aCopy )

{

_format = aCopy._format; _value = aCopy._value;

}

virtual ~Column()

{

}

Column operator=( const Column& aCopy )

{

_format = aCopy._format; _value = aCopy._value; return *this;

}

Column operator=(const char *value)

{

_value = value; return *this;

}

void setValue( const char *value )

{

_value = value;

}

std::string getValue( void )

{

return _value;

}

void setFormat( const char *format )

{

_format = format;
}
std::string getFormat( void )
{
return _format;
}
virtual std::string getFormattedString		1
( void ) const
{

char szBuffer[ 100 ]; sprintf(szBuffer, _format.c_str(), _value.c_str());

std::string sRet = szBuffer; return sRet;

}

};

218 Technique 39: Implementing a Spreadsheet

This code implements the most basic element of the spreadsheet system, the column. As you can see, we implement a complete class by adding methods for the constructors, destructors, assignment operators, and accessor methods. It also implements a virtual method (shown at

1) for returning the contents of the column in a formatted manner. Doing so allows other column types to be defined later on down the line, if you so desire.

3. Save your file in your code editor.

The next step is to implement the Row class that will hold an array of columns.

Creating the Row Class

After we have created a Column class, the next thing to do is to create a Row class that contains the columns we wish to store in the spreadsheet. You can think of the Column class as the data for a single cell, and the Row class as a list of cells for a given row.

1. Append the code from Listing 39-2 to the end of your file.

LISTING 39-2: THE ROW CLASS

class Row

{

std::vector< Column > _columns;

void Copy( const Row& aCopy )

{

std::vector< Column >::const_ iterator iter;

for ( iter = aCopy._columns.begin(); iter != aCopy._columns.end(); ++iter )

_columns.insert( _columns.end(), (*iter) );

}

public: Row(void)

{

}

Row( unsigned int numColumns )

{

for ( int i=0; i<numColumns; ++i )

{

Column c;

_columns.insert( _columns.end(),

c );
}
}
Row( const Row& aCopy )
{
Copy( aCopy );
}
Row operator=( const Row& aCopy )
{
Copy( aCopy );
}		3
Column& operator[]( int idx )
{
if ( idx < 0 || idx > _columns.
size()-1 )
throw “Row: Index out		2
of range”;
return _columns[ idx ];

}

int NumColumns( void )

{

return _columns.size();

}

void Clear()

{

std::vector< Column >::iterator iter;

for ( iter = _columns.begin(); iter != _columns.end(); ++iter )

(*iter).setValue( “” );

}

void Print() const

{

std::vector< Column >::const_ iterator iter;

for ( iter = _columns.begin(); iter != _columns.end(); ++iter )

printf(“%s “, (*iter).getFormattedString().c_str() );

printf(“\n”);

}

};

Creating the Spreadsheet Class 219

Note that the Row class does not do anything with the columns, except to store them and give the end-user access to the ones they want. Note also that we use exception handling (shown at

2) to deal with the exceptional cases of arrayindices out of bounds. There are no good defaults possible here, so we just assume that it is a fatal error to ask for an invalid column number.

One thing that could be changed here is that the Row class does not handle resizing. Instead, the Row class simply assumes that the array of columns is always being instantiated from scratch. To properly resize a row, you would need to create a new array of columns of the right size, and then copy the existing columns into that row.

2. Save your file.

The final step of the process of implementing the class is to put together the actual Spreadsheet class. The next section shows how.

Creating the Spreadsheet Class

Finally, we come to the important part for the enduser: the Spreadsheet class itself. A spreadsheet, of course, is simply a list of the rows that make up the sheet, which in turn is a list of the columns that make up each row. Our spreadsheet will always be “square” — that is, it will contain an equal number of columns in each row.

1. Append the code from Listing 39-3 to the end of your file.

LISTING 39-3: THE SPREADSHEET CLASS

class Spreadsheet
{
int	_cols;
std::vector< Row > _rows;
std::string	_name;

void _BuildSheet( int nRows, int nCols )

{

//If there is anything already here, remove it.

_rows.erase(_rows.begin(), _rows.end());

// Now, add in the rows.

for ( int i=0; i<nRows; ++i )

{

Row row(nCols); _rows.insert( _rows.end(),

row );

}

}

void _InternalSetRows( const unsigned int nRows )

{

_BuildSheet( nRows, _cols );

}

void _InternalSetCols( const unsigned int nCols )

{

//Save the number of rows, so we can rebuild it.

int nRowCount = _rows.size();

//Set the number of columns. _cols = nCols;

//Now rebuild the rows.

_BuildSheet( nRowCount, nCols );

}

void Copy( const Spreadsheet& aCopy )

{

_InternalSetCols( aCopy. NumColumns() );

std::vector< Row >::const_iterator iter;

for ( iter = aCopy._rows.begin(); iter != aCopy._rows.end(); ++iter )

_rows.insert( _rows.end(), (*iter) );

_name = aCopy._name;

}

public:

Spreadsheet(void)

{

}

Spreadsheet( const char *name )

{

_name = name;

}

Spreadsheet( const char *name, unsigned int nRows, unsigned int nCols )

{

(continued)

220 Technique 39: Implementing a Spreadsheet

LISTING 39-3 (continued)

_name = name; _InternalSetCols( nCols ); _InternalSetRows( nRows );

}

Spreadsheet( const Spreadsheet& aCopy )

{

Copy( aCopy );
}
Spreadsheet operator=( const
Spreadsheet& aCopy )
{
Copy( aCopy );
return *this;
}		4
Row& operator[]( int idx )
{

if ( idx < 0 || idx > _rows. size()-1 )

throw “Spreadsheet: Index out of range”;

return _rows[idx];

}

Spreadsheet operator()(int r1, int c1, int r2, int c2)

{

Spreadsheet ret;

//Assign the pieces. ret.setNumColumns( c2-c1+1 ); ret.setNumRows( r2-r1+1 );

//Now copy over the chunk they want. try

{

for ( int r = r1; r <= r2; ++r ) for ( int c = c1; c <= c2;

++c ) ret[r-r1][c-c1] =

(*this)[r][c];

}

catch ( ... )

{

throw “Spreadsheet: Index out of range”;

}

return ret;

}

void setNumColumns( int nCols )

{

_InternalSetCols( nCols );

}

void setNumRows( int nRows )

{

_InternalSetRows( nRows );

}

int NumColumns() const

{

return _cols;

}

int NumRows() const

{

return _rows.size();

}

void setName( const char *name )

{

_name = name;

}

std::string getName( void ) const

{

return _name;

}

void Print() const

{

std::vector< Row >::const_iterator iter;

printf(“Sheet: %s\n”, _name.c_str() );

for ( iter = _rows.begin(); iter != _rows.end(); ++iter )

{

(*iter).Print();

}

}

void Clear()

{

std::vector< Row >::iterator iter; for ( iter = _rows.begin(); iter !=

_rows.end(); ++iter ) (*iter).Clear();

}

};

As I mentioned earlier in this technique, the spreadsheet is really just a holder of rows, which in turn are a holder of columns. The Column class is the only one that “understands” what the data being stored looks like, or how it is formatted, or how it will be displayed. The Spreadsheet class