Streams and Files

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

Table 10-1, Recipe 10.2

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