40 Using the Standard

Streams to Format

Technique Data

Save Time By

Understanding stream classes

Formatting data with stream classes

Understanding your output

If you’ve been programming in C++ for a long time, you’re probably used to outputting data with the printf, fprintf, and sprintf functions that date back to the C programming days. It is now time to take

the plunge into using the stream components of the standard C++ library, because these components will save you lots of time and heartache. The stream components support input, output, and formatting for data in C++ applications. Like the printf, fprintf, and sprintf functions, streams exist to write to the console, to files, and to format data into strings. Unlike the aforementioned functions, streams are type-safe and extensible, which saves you time by reducing the amount of code you need to write and the amount of debugging you need to do to find problems in output.

The stream components save time by being type-safe, well written, and comprehensive. If you use streams instead of more specific output functions, you will find that your code is smaller, easier to understand, and more portable.

Although most programmers are aware that you can input and output data through the stream classes, most are unaware that the stream classes have a wealth of formatting functionality built into them.

In this technique, I show you how to work with the formatting functionality of the stream classes, how to extract data from a stream, and how to output columns and change floating point precision for data.

Working with Streams

In order to understand just how a stream component can be used in your application to save time and effort, let’s take a look at a simple example of a stream being used in an application. In this case, we will create some data in our program, and then output that data to the user. In addition, we will examine how to extend the stream class by creating our own output control.

226 Technique 40: Using the Standard Streams to Format Data

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 ch40.cpp,
	although you can use whatever you choose.
2.	Type the code from Listing 40-1 into your file.
	Better yet, copy the code from the source file on	}
	this book’s companion Web site.

vector<A>::iterator iter;

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

{

out.width(8); out << (*iter);

}

out << endl; return out;

LISTING 40-1: USING STREAMS

#include <stdio.h> #include <iostream> #include <sstream> #include <vector>

using namespace std;

void PrintDoubleRow( int numElements, double *dArray, ostream& out )

{

int main(int argc, char **argv)

{

double	dArray[20];
int	nCount = 0;

//See whether they gave us any on the command line.

if ( argc > 2 )

{

for ( int i=1; i<argc; ++i )

{

stringstream str; double number;

//First, set up some elements of the

ostream. str.setf(ios::fixed,

//Set the output floating point precision to 2 decimal places.

out.precision(4);

//Only show the decimal point if it is not a whole number.

out << showpoint;

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

{

//Set each column to be 8 spaces. out.width(8);

//Output the float.

out << dArray[i];

}

out << endl;

}

template	< class A >			4
ostream&	operator<<(	ostream& out,

vector< A >& dVector )

{

std::ios_base::floatfield); str.width(0); str.precision(4);

str << argv[i]; str >> number;

dArray[nCount] = number; nCount++;

}

}

else

{

// Prompt the user for input. bool bDone = false;

while ( !bDone )

{

char szBuffer[80];

cout << “Enter a number (or a dash (*) to quit): “;

memset( szBuffer, 0, 80 ); cin >> szBuffer;

if ( szBuffer[0] == ‘*’ ) bDone = true;

else

{

Working with Streams 227

stringstream str; double number;

	str.setf(ios::fixed,
		std::ios_base::float-
		field);
	str.width(0);
	str.precision(4);
	str << szBuffer;
	str >> number;
	dArray[nCount] = number;
	nCount++;
	}
	}
	}
	PrintDoubleRow( nCount, dArray,					1
	cout );
	// Now display it as a vector.						2
	vector< double > dVector;
	for ( int i=0; i<nCount; ++i )
	dVector.insert( dVector.end(),
	dArray[i] );
	cout << “Vector: “ << endl;					3
}	cout << dVector << endl;
	Let’s take a look at what is going on here. First,
	we create a standard array of double values and
	put data received from the user into the array.
	That array is then printed out using the standard
	stream class (see	1). Next, we are creating an
	“array” using the Standard			Template Library
	vector class (see	2). We then print that vec-
	tor out using a stream		shown at		3. But wait,
	how does this work? Vectors are not among the
	standard supported types for streams. If you
	look at the templated function marked with					4,

you will see that we have created an overloaded operator that takes a vector object and outputs it to a stream. The compiler will match up our overloaded operator along with the streaming of the vector, and make sure that it all works properly. Note also the use of the width and precision methods of the stream class to set the output width of each column in the vector properly, and only output the right number of decimal points.

3. Save the source-code file in the code editor and close the editor application.

4. Compile the application in your favorite compiler, on your favorite operating system.

If you have done everything right, when you run the application with the following command-line input:

1 2 3 4

you should see the following output from the application on the console window:

$ ./a.exe 1 2 3 4

1.000 2.000 3.000 4.000 Vector:

1.000 2.000 3.000 4.000

As you can see, the output is the same for both the array and vector classes. We can also see that the width of the columns is fixed at eight characters, as we specified in the width method of the stream. Finally, note that the number of decimal points is fixed at three for each entry, once again as specified in the precision method.

Alternatively, you can enter the data at the prompt. To do so, run the program with no input arguments, and then enter the values when prompted from the user. In this case, you should see the following output from the program in the console window:

$ ./a.exe
Enter a number (or		a dash (*) to quit): 1
Enter a number (or		a dash (*) to quit): 2
Enter a number (or		a dash (*) to quit): 3
Enter a number (or		a dash (*) to quit): 4
Enter a number (or		a dash (*) to quit): *
1.000	2.000	3.000	4.000
Vector:
1.000	2.000	3.000	4.000

The output from the program is the same, the only difference is how the data got into the system. Note again that the width of the columns is still fixed and the number of decimal points is still what we specified.

41 Reading In and

Processing Files

Technique

Save Time By

Reading in files with stream classes

Processing files with stream classes

Creating a test file

Interpreting your output

Processing files in C++ is really the same as processing any other sort of input or output. Unlike similar functions in C, however, the file-processing functions in C++ allow you to use the same code for

processing data — from either the keyboard or a file. This generality makes it considerably easier to write code that is easy to test, run, and maintain. And, of course, when code is faster to write and easier to test, it saves you time in the project.

If you use stream classes instead of C-style file functions to access data, you will find the code quicker to write and test — and errors easier to trap. See Technique 40 for more on using stream classes.

This technique shows you how to use the file-stream classes to read in — and process — a simple preferences file. I also tell you how this method compares to the old style of doing things, so that you can easily drop in this code wherever you are using the older C-style functions.

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 ch41.cpp, although you can use whatever you choose.

2. Type the code from Listing 41-1 into your file.

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

LISTING 41-1: THE FILE-READING CLASS

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

#include <fstream> #include <ios> #include <iostream> #include <string> #include <vector>

Reading In and Processing Files 229

using namespace std;

// The old fashioned way

class Entry

{

private:

char *strName; char *strValue;

void Init()

{

strName = NULL; strValue = NULL;

}

public:

Entry()

{

Init();

}

Entry( const char *name, const char *value )

{

Init();

setName( name ); setValue ( value );

}

Entry( const Entry& aCopy )

{

Init();

setName( aCopy.strName ); setValue( aCopy.strValue );

}

~Entry()

{

if	( strName )
	delete []	strName;
if	( strValue	)
	delete []	strValue;

}

Entry operator=( const Entry& aCopy )

{

setName( aCopy.strName ); setValue( aCopy.strValue ); return *this;

}

void setName(const char *name)

{

if ( strName )

delete [] strName;

strName = new char[strlen(name)+1 ]; strcpy( strName, name );

}

void setValue( const char *value )

(continued)

230 Technique 41: Reading In and Processing Files

LISTING 41-1 (continued)

{

if ( strValue )

delete [] strValue; strValue = new char[strlen

(value)+1 ];

strcpy( strValue, value );

}

const char *getName( void )

{

return strName;

}

const char *getValue( void )

{

return strValue;

}

};

bool OpenFileAndReadOld( const char *strFileName, Entry* array, int nMaxEntries, int *numFound )

{

FILE *fp	= fopen ( strFileName, “r” );
if ( fp == NULL )
return false;
int nPos	= 0;
while ( !feof(fp) )
{			1
char	szBuffer[ 257 ];
memset( szBuffer, 0, 256 );
if (	fgets( szBuffer, 256, fp ) ==
NULL )
	break;

//Look for the position of the ‘=’ sign.

char *str = strstr(szBuffer, “=”); if ( str )

{

//First, get the name. char szName[256]; memset( szName, 0, 256 ); strncpy(szName, szBuffer,

strlen(szBuffer)-strlen(str) );

//Now, get the value.

char szValue[256];

memset( szValue, 0, 256 ); strncpy(szValue, str+1,

strlen(str)-1 );

if ( szValue[strlen(szValue)-1] == ‘\n’ )

szValue[strlen(szValue)-1] = 0;

Entry e( szName, szValue ); if ( nPos < nMaxEntries )

{

array[ nPos ] = e; nPos ++;

}

}

}

*numFound = nPos;

fclose(fp);

return true;

}

The code in Listing 41-1 does things the oldfashioned way (C-style), using file-based functions. Trying it with streams creates reusable operators along the way. That’s next.

3. Now, append the code in Listing 41-2 to the source file using your favorite source-code editor.

LISTING 41-2: USING STREAMS FOR FILE READING

//Various operators used by the application.

ifstream& operator<<(		string& sIn, ifstream&
{	in )	3
	while ( !in.eof()	)
	{

char c; in.get(c);

if ( in.fail() ) return in;

sIn += c;

if ( c == ‘\n’ ) return in;

}

return in;

}

				Reading In and Processing Files		231
string operator-( string& sIn,			4	Entry e(name.c_str(),
	char cIn )			value.c_str());
{				entries.insert( entries.end(),
	string sOut = “”;			e );
	for ( int i=0; i<sIn.length(); ++i )			}
	if ( sIn[i] != cIn )			}
	sOut += sIn[i];
}	return sOut;			in.close( );
bool OpenFileAndReadNew( const char				return true;
				}
	*szFileName, std::vector< Entry >&
{	entries )
	ifstream in;
				As you can see, the code is much easier to
	in.open( szFileName );
				understand and maintain in the stream version.
	if ( in.fail() )			Readability is important in coding because it
	{			takes less time for the maintenance programmer
	printf(“Unable to open file %s\n”,			to read and understand your objective. The
	szFileName );			stream versions of the code form their own
	return false;
				description of what we are trying to do, improv-
	}
				ing on the confusing C-style functions. More
	// Process the file			importantly, if we want to test the functions from
				the keyboard, it is trivial to pass in the standard
	while ( !in.eof() )
	{			input object instead of a file. The code can cope
	// Get an input line			with both types of input.
	string sLine = “”;		2	To understand just how simple the stream ver-
	sLine << in;
				sion is compared to the older version, take a
	// Skip comments			look at two similar segments of the code. The
	if ( sLine.length() && sLine[0]			line marked	1 in the original listing shows
	== ‘#’ ) continue;			how we read a line in from the input source. The
				corresponding line in the updated stream ver-
				sion is marked	2. Note that the stream version
	// Remove all carriage returns and			is not only smaller and easier to read, but also it
				handles problems the original code did not. For
	line feeds
				example, the string class can handle an almost
	sLine = sLine - ‘\n’;
				infinite number of characters, whereas the buffer
	sLine = sLine - ‘\r’;
				used in the original code is fixed in size.
	// Now, extract the pieces			Likewise, the stream version automatically
	int ePos = sLine.find_first_of			enters the number of characters into the string
	(‘=’, 0);			class, which makes checking for blank lines sim-
	if ( ePos != string::npos )			ple. Finally, checking for substrings in stream
	{			classes is considerably easier than using the
	// Copy the name			clunky old strstr function that required you

	string name =	to check for NULL	returns and end of string
	sLine.substr(0,ePos);

string value =	comparisons.
sLine.substr(ePos+1);	4. Save the source code as a file in the code

editor.