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CGAL 4.8 - IO Streams

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User Manual

Authors
  Andreas Fabri, Geert-Jan Giezeman, and Lutz Kettner

All classes in the CGAL kernel provide input and output operators for IO streams. Classes external to CGAL are also supported, by means of `oformat()` (Section IO for Non CGAL Types). The basic task of such an operator is to produce a representation of an object that can be written as a sequence of characters on devices as a console, a file, or a pipe. In CGAL we distinguish between a raw ascii, a raw binary and a pretty printing format.

enum Mode {ASCII = 0, BINARY, PRETTY};

In `ASCII` mode, objects are written as a set of numbers, e.g. the coordinates of a point or the coefficients of a line, in a machine independent format. In `BINARY` mode, data are written in a binary format, e.g. a double is represented as a sequence of four byte. The format depends on the machine. The mode `PRETTY` serves mainly for debugging as the type of the geometric object is written, as well as the data defining the object. For example for a point at the origin with Cartesian double coordinates, the output would be `PointC2(0.0, 0.0)`. At the moment CGAL does not provide input operations for pretty printed data. By default a stream is in Ascii mode.

CGAL provides the following functions to modify the mode of an IO stream.

IO::Mode set_mode(std::ios& s, IO::Mode m);
IO::Mode set_ascii_mode(std::ios& s);
IO::Mode set_binary_mode(std::ios& s);
IO::Mode set_pretty_mode(std::ios& s);

The following functions allow to test whether a stream is in a certain mode.

IO::Mode get_mode(std::ios& s);
bool is_ascii(std::ios& s);
bool is_binary(std::ios& s);
bool is_pretty(std::ios& s);

# Output Operator

CGAL defines output operators for classes that are derived from the class `ostream`. This allows to write to ostreams as `std::cout` or `std::cerr`, as well as to `std::ostringstream` and `std::ofstream`. The output operator is defined for all classes in the CGAL `Kernel` and for the class `Color` as well. Let `os` be an output stream.

// Inserts object `c` in the stream `os`. Returns `os`.
ostream& operator<<(ostream& os, Class c);

## Example

#include <iostream>
#include <fstream>
#include <CGAL/Cartesian.h>
#include <CGAL/Segment_2.h>
typedef CGAL::Point_2< CGAL::Cartesian<double> > Point;
typedef CGAL::Segment_2< CGAL::Cartesian<double> > Segment;
int main()
{
Point p(0,1), q(2,2);
Segment s(p,q);
CGAL::set_pretty_mode(std::cout);
std::cout << p << std::endl << q << std::endl;
std::ofstream f("data.txt");
CGAL::set_binary_mode(f);
f << s << p ;
return 1;
}

# Input Operator

CGAL defines input operators for classes that are derived from the class `istream`. This allows to read from istreams as `std::cin`, as well as from `std::istringstream` and `std::ifstream`. The input operator is defined for all classes in the CGAL `Kernel`. Let `is` be an input stream.

// Extracts object `c` from the stream `is`. Returns `is`.
istream& operator>>(istream& is, Class c);

## Example

#include <iostream>
#include <fstream>
#include <CGAL/Cartesian.h>
#include <CGAL/Segment_2.h>
typedef CGAL::Point_2< CGAL::Cartesian<double> > Point;
typedef CGAL::Segment_2< CGAL::Cartesian<double> > Segment;
int
main()
{
Point p, q;
Segment s;
CGAL::set_ascii_mode(std::cin);
std::cin >> p >> q;
std::ifstream f("data.txt");
CGAL::set_binary_mode(f);
f >> s >> p;
return 1;
}

# IO for Non-CGAL Types

## Using Output Formatting

To ensure that non-CGAL types are formatted correctly (i.e., respecting `IO::Mode`), `oformat()` can be used. For types with a `Output_rep` specialization, the respective output routine of `Output_rep` will be called by `oformat()`. Otherwise, the stream output operator will be called.

## Example

std::cout << CGAL::oformat( myobject );

Optional, you can provide a second template parameter `F` as a formatting tag:

## Example

std::cout << CGAL::oformat( myobject, My_formatting_tag() );

For a list of formatting tags supported by the type `T`, please refer to the documentation of the respective type.

## Customizing Output Formatting

In some situations, you want to control the output formatting for a type `T`. For external types (third party libraries etc.), there might be problems if their stream output operator does not respect `IO::Mode`. The purpose of `Output_rep` is to provide a way to control output formatting that works independently of the object's stream output operator.

Instead of putting `T` directly into an output stream, `T` is wrapped into an output representation `Output_rep`. For convenience, a function `oformat()` exists which constructs an instance of `Output_rep`.

If you do not specialize `Output_rep` for `T`, `T`'s stream output operator is called from within `Output_rep`, by default. If you want another behaviour for your type `T`, you have to provide a specialization for that type. Furthermore, you can provide specializations with a second template parameter (a formatting tag). The second template parameter defaults to `Null_tag` and means default behaviour.

For example, specializing `Output_rep` for `CORE::BigRat` (without a formatting tag parameter) could look like this:

## Example

template <class F>
class Output_rep< ::CORE::BigRat, F> {
const ::CORE::BigRat& t;
public:
Output_rep( const ::CORE::BigRat& tt) : t(tt) {}
std::ostream& operator()( std::ostream& out) const {
switch (get_mode(out)) {
case IO::PRETTY:{
if(CGAL_CORE_DENOMINATOR(t) == ::CORE::BigRat(1))
return out <<CGAL_CORE_NUMERATOR(t);
else
return out << CGAL_CORE_NUMERATOR(t)
<< "/"
<< CGAL_CORE_DENOMINATOR(t);
break;
}
default:
return out << CGAL_CORE_NUMERATOR(t)
<< "/"
<< CGAL_CORE_DENOMINATOR(t);
}
}
};

# Colors

An object of the class `Color` is a color available for drawing operations in many CGAL output streams.

Each color is defined by a triple of integers `(r,g,b)` with 0 \( \le \) r,g,b \( \le \) 255, the so-called rgb-value of the color. There are a 11 predefined `Color` constants available: `BLACK`, `WHITE`, `GRAY`, `RED`, `GREEN`, `DEEPBLUE`, `BLUE`, `PURPLE`, `VIOLET`, `ORANGE`, and `YELLOW`.

# Stream Support

Three classes are provided by CGAL as adaptors to input and output stream iterators. The class `Istream_iterator` is an input iterator adaptor and is particularly useful for classes that are similar but not compatible to `std::istream`. Similarly, the class `Ostream_iterator` is an output iterator adaptor. The class `Verbose_ostream` can be used as an output stream. The stream output operator `<<` is defined for any type. The class stores in an internal state a stream and whether the output is active or not. If the state is active, the stream output operator `<<` uses the internal stream to output its argument. If the state is inactive, nothing happens.