X-Git-Url: https://git.distorted.org.uk/~mdw/sod/blobdiff_plain/bf2e745293194a9fc27a86496623ccc515b7aa52..d9db9c73add5814057a80e9969c74419ce30e664:/doc/concepts.tex

diff --git a/doc/concepts.tex b/doc/concepts.tex
index d9239d0..eae67aa 100644
--- a/doc/concepts.tex
+++ b/doc/concepts.tex
@@ -63,22 +63,33 @@ having to implement additional syntax.
 For the most part, Sod takes a fairly traditional view of what it means to be
 an object system.
 
-An \emph{object} maintains \emph{state} and exhibits \emph{behaviour}.  An
-object's state is maintained in named \emph{slots}, each of which can store a
-C value of an appropriate (scalar or aggregate) type.  An object's behaviour
-is stimulated by sending it \emph{messages}.  A message has a name, and may
-carry a number of arguments, which are C values; sending a message may result
-in the state of receiving object (or other objects) being changed, and a C
-value being returned to the sender.
-
-Every object is a (direct) instance of some \emph{class}.  The class
-determines which slots its instances have, which messages its instances can
-be sent, and which methods are invoked when those messages are received.  The
-Sod translator's main job is to read class definitions and convert them into
-appropriate C declarations, tables, and functions.  An object cannot
+An \emph{object} maintains \emph{state} and exhibits \emph{behaviour}.
+(Here, we're using the term `object' in the usual sense of `object-oriented
+programming', rather than that of the ISO~C standard.  Once we have defined
+an `instance' below, we shall generally prefer that term, so as to prevent
+further confusion between these two uses of the word.)
+
+An object's state is maintained in named \emph{slots}, each of which can
+store a C value of an appropriate (scalar or aggregate) type.  An object's
+behaviour is stimulated by sending it \emph{messages}.  A message has a name,
+and may carry a number of arguments, which are C values; sending a message
+may result in the state of receiving object (or other objects) being changed,
+and a C value being returned to the sender.
+
+Every object is a \emph{direct instance} of exactly one \emph{class}.  The
+class determines which slots its instances have, which messages its instances
+can be sent, and which methods are invoked when those messages are received.
+The Sod translator's main job is to read class definitions and convert them
+into appropriate C declarations, tables, and functions.  An object cannot
 (usually) change its direct class, and the direct class of an object is not
 affected by, for example, the static type of a pointer to it.
 
+If an object~$x$ is a direct instance of some class~$C$, then we say that $C$
+is \emph{the class of}~$x$.  Note that the class of an object is a property
+of the object's value at runtime, and not of C's compile-time type system.
+We shall be careful in distinguishing C's compile-time notion of \emph{type}
+from Sod's run-time notion of \emph{class}.
+
 
 \subsection{Superclasses and inheritance}
 \label{sec:concepts.classes.inherit}
@@ -120,7 +131,7 @@ class then the only superclass of $C$ is $C$ itself, and $C$ has no proper
 superclasses.
 
 If an object is a direct instance of class~$C$ then the object is also an
-(indirect) instance of every superclass of $C$.
+(indirect) \emph{instance} of every superclass of $C$.
 
 If $C$ has a proper superclass $B$, then $B$ must not have $C$ as a direct
 superclass.  In different terms, if we construct a directed graph, whose
@@ -302,13 +313,14 @@ qualified by the defining class's nickname.
 \subsubsection{Slot initializers}
 As well as defining slot names and types, a class can also associate an
 \emph{initial value} with each slot defined by itself or one of its
-subclasses.  A class $C$ provides an \emph{initialization message} (see
-\xref{sec:concepts.lifecycle.birth}, and \xref{sec:structures.root.sodclass})
-whose methods set the slots of a \emph{direct} instance of the class to the
-correct initial values.  If several of $C$'s superclasses define initializers
-for the same slot then the initializer from the most specific such class is
-used.  If none of $C$'s superclasses define an initializer for some slot then
-that slot will be left uninitialized.
+superclasses.  A class $C$ provides an \emph{initialization message} (see
+\xref[\instead{sections}]{sec:concepts.lifecycle.birth}, and
+\ref{sec:structures.root.sodobject}) whose methods set the slots of a
+\emph{direct} instance of the class to the correct initial values.  If
+several of $C$'s superclasses define initializers for the same slot then the
+initializer from the most specific such class is used.  If none of $C$'s
+superclasses define an initializer for some slot then that slot will be left
+uninitialized.
 
 The initializer for a slot with scalar type may be any C expression.  The
 initializer for a slot with aggregate type must contain only constant
@@ -334,6 +346,9 @@ details.
 
 \subsection{C language integration} \label{sec:concepts.classes.c}
 
+It is very important to distinguish compile-time C \emph{types} from Sod's
+run-time \emph{classes}: see \xref{sec:concepts.classes}.
+
 For each class~$C$, the Sod translator defines a C type, the \emph{class
 type}, with the same name.  This is the usual type used when considering an
 object as an instance of class~$C$.  No entire object will normally have a
@@ -343,6 +358,10 @@ class type,\footnote{%
   chains.  See \xref{sec:structures.layout} for the full details.} %
 so access to instances is almost always via pointers.
 
+Usually, a value of type pointer-to-class-type of class~$C$ will point into
+an instance of class $C$.  However, clever (or foolish) use of pointer
+conversions can invalidate this relationship.
+
 \subsubsection{Access to slots}
 The class type for a class~$C$ is actually a structure.  It contains one
 member for each class in $C$'s superclass chain, named with that class's
@@ -516,6 +535,13 @@ Keyword arguments can be provided in three ways.
   call, which is useful when writing wrapper functions.
 \end{enumerate}
 
+Perhaps surprisingly, keyword arguments have a relatively small performance
+impact.  On the author's aging laptop, a call to a simple function, passing
+two out of three keyword arguments, takes about 30 cycles longer than calling
+a standard function which just takes integer arguments.  On the other hand,
+quite a lot of code is involved in decoding keyword arguments, so code size
+will naturally suffer.
+
 Keyword arguments are provided as a general feature for C functions.
 However, Sod has special support for messages which accept keyword arguments
 (\xref{sec:concepts.methods.keywords}); and they play an essential rÃ´le in
