The declarations used in encoding definition files define
encoding-specific commands but they do not allow those commands to be
used without explicitly changing the encoding. For some commands,
such as symbols, this is not enough. For example, the OMS encoding
contains the symbol `§', but we need to be able to use the
command \S whatever the current encoding may be, without explicitly
selecting the encoding OMS.
To allow this, LaTeX has commands that declare default definitions
for commands; these defaults are used when the command is not defined
in the current encoding. For example, the default encoding for \S
is OMS, and so in an encoding (such as OT1) which does not
contain \S, the OMS encoding is selected in order to access this
glyph. But in an encoding (such as T1) which does contain \S, the
glyph in that encoding is used. The standard LaTeX2e format sets
up several such defaults using the following encodings: OT1, OMS
and OML.
Warning: These commands should not occur in encoding definition files, since those files should declare only commands for use when that encoding has been selected. They should instead be placed in packages; they must, of course, always refer to encodings that are known to be available.
\DeclareTextCommandDefault {<cmd>} {<definition>}
This command allows an encoding-specific command to be given a default
definition. For example, the default definition for \copyright is
defined be be a circled `c' with:
\DeclareTextCommandDefault{\copyright}{\textcircled{c}}
\DeclareTextAccentDefault {<cmd>} {<encoding>}
\DeclareTextSymbolDefault {<cmd>} {<encoding>}
These commands allow an encoding-specific command to be given a
default encoding. For example, the default encoding for \" and
\ae is set to be OT1 by:
\DeclareTextAccentDefault{\"}{OT1}
\DeclareTextSymbolDefault{\ae}{OT1}
Note that \DeclareTextAccentDefault can be used on any one-argument
encoding-specific command, not just those defined with
\DeclareTextAccent. Similarly, \DeclareTextSymbolDefault can be
used on any encoding-specific command with no arguments, not just
those defined with \DeclareTextSymbol.
For more examples of these definitions, see ltoutenc.dtx.
\ProvideTextCommandDefault {<cmd>} {<definition>}
This command is the same as \DeclareTextCommandDefault, except that
if the command already has a default definition, then the definition
is ignored. This is useful to give `faked' definitions of symbols
which may be given `real' definitions by other packages. For example,
a package might give a fake definition of \textonequarter by saying:
\ProvideTextCommandDefault{\textonequarter}{$\m@th\frac14$}