Abstract base class for the paragraph and character format layers.

A format layer is a set of character or paragraph format attributes which may own a pointer to another format layer. This pointer is called a based-on link. The effective formatting of a text object may be built up from a chain of format layers - the final layer in the chain has a NULL based-on link. In case of conflict, attribute values set in upper layers (layers furthest from the layer with the NULL based-on link) override those set in lower layers. In rich text, additional formatting may be applied on top of these format layers.

The system of based-on links is implemented by the CFormatLayer class. It also implements persistence for chains of format layers.

When setting or sensing the attributes of a CParaFormatLayer or CCharFormatLayer, a format mask and container are specified as parameters. The container has data members for every format attribute, which may be set independently. When setting the layer, the mask indicates the attributes which will be taken from the container. Any attributes not specified in the mask will be taken from the system-provided default values.

When sensing a layer, on return, the mask indicates which attributes have been explicitly set in the layer, (i.e. not taken from the default values). In addition, a layer's effective format may be sensed. In this case, no mask is used because the format container will, on return, contain a value for every attribute.