The Plotter class

The C++ binding for libplot is provided by a class library, named libplotter. This library implements a Plotter class, of which all Plotters are instances. Actually, a Plotter would normally be an instance of an appropriate derived class, determined by the Plotter's output format. Derived classes currently include XPlotter, XDrawablePlotter, PNMPlotter, GIFPlotter, AIPlotter, PSPlotter, FigPlotter, PCLPlotter, HPGLPlotter, TekPlotter, and MetaPlotter. The names should be self-explanatory. The operations that may be applied to any Plotter (e.g., the openpl operation, which begins a page of graphics) are implemented as public function members of the Plotter class.

At the time a Plotter is created, its input, output, and error streams must be specified. (The first is ignored, since at present, all Plotters are write-only.) The streams may be specified either as iostreams or as FILE pointers. That is, the two constructors

  Plotter(istream& instream, ostream& outstream, ostream& errstream);
  Plotter(FILE *infile, FILE *outfile, FILE *errfile);

are provided for the base Plotter class, and similarly for each of its derived classes. So, for example, both

PSPlotter plotter(cin, cout, cerr);

and

PSPlotter plotter(stdin, stdout, stderr);

are possible declarations of a Postscript Plotter that writes to standard output. In the iostream case, an ostream with a null stream buffer may be specified as the output stream and/or the error stream, to request that no output take place. In the FILE pointer case, specifying a null FILE pointer would accomplish the same thing. Instances of the XPlotter and XDrawablePlotter classes always ignore the output stream argument, since they write graphics to an X Display rather than to a stream.

The parameter-setting function parampl, which plays an important role in all bindings of libplot, is implemented in libplotter as a static function member of the Plotter class. The following is a formal description.

int Plotter::parampl (const char *parameter, void *value);

Plotter::parampl sets the value of the device driver parameter parameter to value. Device driver parameters specify Plotter options. The parameter values in effect at the time any Plotter is created are copied into it. For most parameters, value should be a char *, i.e., a string. Unrecognized parameters are ignored. For a list of the recognized parameters and their meaning, see section Device driver parameters.

Just as in the C binding, Plotter::parampl gives the programmer fine control over the parameters of subsequently created Plotters. The parameter values used by any Plotter are constant over the lifetime of the Plotter, and are those that were set when the Plotter was created. If at Plotter creation time a parameter is not set, its default value will be used, unless the parameter is string-valued and there is an environment variable of the same name, in which case the value of that environment variable will be used.

Once set, a parameter may be unset by the programmer by calling Plotter::parampl with a value argument of NULL. This further increases flexibility.

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