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3.5 The output stream hierarchy

There are various classes of output streams, some of them with styling support. These “classes” are defined in an object-oriented programming style that resembles C++ or Java, but are actually implemented in C with a little bit of object orientation syntax. These definitions are preprocessed down to C. As a consequence, GNU libtextstyle is a C library and does not need to link with the C++ standard library.

All these classes are declared in <textstyle.h>.

The base output stream type is ‘ostream_t’. It is a pointer type to a (hidden) implementation type. Similarly for the subclasses.

When we say that ‘some_ostream_t’ is a subclass of ‘ostream_t’, what we mean is:

• Every ‘some_ostream_t’ object can be converted to an ‘ostream_t’, by virtue of a simple assignment. No cast is needed.
• The opposite conversion, from ‘ostream_t’ to ‘some_ostream_t’, can also be performed, provided that the object is actually an instance of ‘some_ostream_t’. You can test whether an object is an instance of ‘some_ostream_t’ by invoking the method ‘bool is_instance_of_some_ostream (ostream_t stream)’.
• Every method ‘ostream_foobar’ exists also as a method ‘some_ostream_foobar’ with compatible argument types and a compatible return type.

• The abstract ostream class
• The abstract styled_ostream class
• Concrete ostream subclasses without styling
• Concrete styled_ostream subclasses
• Accessor functions