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Streams and Files


Over the course of this five-part series, you will learn how to handle stream output in C++, how to operate on files, and more. This article is excerpted from chapter 10 of the C++ Cookbook, written by Ryan Stephens, Christopher Diggins, Jonathan Turkanis and Jeff Cogswell (O'Reilly; ISBN: 0596007612). Copyright 2007 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.

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By: O'Reilly Media
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October 16, 2008
TABLE OF CONTENTS:
  1. · Streams and Files
  2. · 10.1 Lining Up Text Output Problem
  3. · Tables for Text Manipulation
  4. · TableFormatter Template

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Streams and Files - TableFormatter Template
(Page 4 of 4 )

As with all other character-based classes in the standard library, manipulators work on streams that use narrow or wide characters. Therefore, you can use them with templates to write formatting utilities that operate on streams of any kind of character. Example 10-2 presents the class template TableFormatter, which formats data into equal-width columns and writes it to a stream.

Example 10-2. A generic class for tabular data

#include <iostream>
#include <iomanip>
#include <string>
#include <vector>

using namespace std;

// TableFormatter formats data for output to a stream of characters
// of type T.
template<typename T>
class TableFormatter {

public:
   TableFormatter(basic_ostream<T>& os) : out_(os) {}
  ~TableFormatter() {out_ << flush;}

   template<typename valT>
   void writeTableRow(const vector<valT>& v, int width);
   //...

private:
  basic_ostream<T>& out_;
};

template<typename T>     // refers to class template param list
template<typename valT>  // refers to mem fn template param list
void TableFormatter<T>::writeTableRow(const std::vector<valT>& v,
                                     
int width) {

  ios_base::fmtflags flags = out_.flags();

  out_.flush();
  out_ << setprecision(2) << fixed;  // Set the precision, in case
                                     // this is floating-point data
  for (vector<valT>::const_iterator p = v.begin();
       p != v.end(); ++p)
    out_ << setw(width) << left << *p;  // Set the width, justify,
                                      
// and write the element
  out_ << endl;     // Flush
  out_.setf(flags); // Set the flags back to normal
}

int main() {

   TableFormatter<char> fmt(cout);

   vector<string> vs;

   vs.push_back( "Sunday" );
   vs.push_back( "Monday" );
   vs.push_back( "Tuesday" );

   fmt.writeTableRow(vs, 12);
   fmt.writeTableRow(vs, 12);
   fmt.writeTableRow(vs, 12);

   vector<double> vd;

   vd.push_back(4.0);
   vd.push_back(3.0);
   vd.push_back(2.0);
   vd.push_back(1.0);

   fmt.writeTableRow(vd, 5);
}

The output from Example 10-2 looks like this:

  Sunday     Monday     Tuesday
  4.00 3.00 2.00 1.00

See Also

Table 10-1, Recipe 10.2

Please check back next week for the continuation of this article.


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