info ginac

6.3.3 Adding new output formats

Creating a new output format involves subclassing print_context, which is somewhat similar to adding a new algebraic class (see section Adding classes). There is a macro GINAC_DECLARE_PRINT_CONTEXT that needs to go into the class definition, and a corresponding macro GINAC_IMPLEMENT_PRINT_CONTEXT that has to appear at global scope. Every print_context class needs to provide a default constructor and a constructor from an std::ostream and an unsigned options value.

Here is an example for a user-defined print_context class:

class print_myformat : public print_dflt
{
    GINAC_DECLARE_PRINT_CONTEXT(print_myformat, print_dflt)
public:
    print_myformat(std::ostream & os, unsigned opt = 0)
     : print_dflt(os, opt) {}
};

print_myformat::print_myformat() : print_dflt(std::cout) {}

GINAC_IMPLEMENT_PRINT_CONTEXT(print_myformat, print_dflt)

That's all there is to it. None of the actual expression output logic is implemented in this class. It merely serves as a selector for choosing a particular format. The algorithms for printing expressions in the new format are implemented as print methods, as described above.

print_myformat is a subclass of print_dflt, so it behaves exactly like GiNaC's default output format:

{
    symbol x("x");
    ex e = pow(x, 2) + 1;

    // this prints "1+x^2"
    cout << e << endl;
    
    // this also prints "1+x^2"
    e.print(print_myformat()); cout << endl;

    ...
}

To fill print_myformat with life, we need to supply appropriate print methods with set_print_func(), like this:

// This prints powers with '**' instead of '^'. See the LaTeX output
// example above for explanations.
void print_power_as_myformat(const power & p,
                             const print_myformat & c,
                             unsigned level)
{
    unsigned power_prec = p.precedence();
    if (level >= power_prec)
        c.s << '(';
    p.op(0).print(c, power_prec);
    c.s << "**";
    p.op(1).print(c, power_prec);
    if (level >= power_prec)
        c.s << ')';
}

{
    ...
    // install a new print method for power objects
    set_print_func<power, print_myformat>(print_power_as_myformat);

    // now this prints "1+x**2"
    e.print(print_myformat()); cout << endl;

    // but the default format is still "1+x^2"
    cout << e << endl;
}