Changeset 43c6dc82
 Timestamp:
 Feb 9, 2018, 5:30:40 PM (5 years ago)
 Branches:
 aaronthesis, armeh, cleanupdtors, deferred_resn, demangler, enum, forallpointerdecay, jacob/cs343translation, jenkinssandbox, master, newast, newastuniqueexpr, newenv, no_list, persistentindexer, pthreademulation, qualifiedEnum, resolvnew, with_gc
 Children:
 48786bc8, cfe2f0a
 Parents:
 298ed08
 File:

 1 edited
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doc/papers/general/Paper.tex
r298ed08 r43c6dc82 1583 1583 The operator overloading feature of \CFA provides a natural means to implement this truth value comparison for arbitrary types, but the C type system is not precise enough to distinguish an equality comparison with @0@ from an equality comparison with an arbitrary integer or pointer. 1584 1584 To provide this precision, \CFA introduces a new type @zero_t@ as type type of literal @0@ (somewhat analagous to @nullptr_t@ and @nullptr@ in \CCeleven); @zero_t@ can only take the value @0@, but has implicit conversions to the integer and pointer types so that standard C code involving @0@ continues to work properly. 1585 With this addition, the \CFA compiler rewrites @if (x)@ and similar expressions to @if (x != 0)@ or the appropriate analogue, and any type @T@ can be made ``truthy'' by defining a single function @int ?!=?(T, zero_t)@. 1586 1587 \TODO{Clean up and integrate this paragraph} As well, restricted constant overloading is allowed for the values @0@ and @1@, which have special status in C, \eg the value @0@ is both an integer and a pointer literal, so its meaning depends on context. 1588 In addition, several operations are defined in terms values @0@ and @1@, \eg: 1589 \begin{lstlisting} 1590 int x; 1591 if (x) x++ $\C{// if (x != 0) x += 1;}$ 1592 \end{lstlisting} 1593 Every @if@ and iteration statement in C compares the condition with @0@, and every increment and decrement operator is semantically equivalent to adding or subtracting the value @1@ and storing the result. 1594 Due to these rewrite rules, the values @0@ and @1@ have the types @zero_t@ and @one_t@ in \CFA, which allows overloading various operations for new types that seamlessly connect to all special @0@ and @1@ contexts. 1595 The types @zero_t@ and @one_t@ have special built in implicit conversions to the various integral types, and a conversion to pointer types for @0@, which allows standard C code involving @0@ and @1@ to work as normal. 1596 1585 With this addition, the \CFA compiler rewrites @if (x)@ and similar expressions to @if ((x) != 0)@ or the appropriate analogue, and any type @T@ can be made ``truthy'' by defining an operator overload @int ?!=?(T, zero_t)@. 1586 \CC makes types truthy by adding a conversion to @bool@; prior to the addition of explicit cast operators in \CCeleven this approach had the pitfall of making truthy types transitively convertable to any numeric type; our design for \CFA avoids this issue. 1587 1588 \CFA also includes a special type for @1@, @one_t@; like @zero_t@, @one_t@ has builtin implicit conversions to the various integral types so that @1@ maintains its expected semantics in legacy code. 1589 The addition of @one_t@ allows generic algorithms to handle the unit value uniformly for types where that is meaningful. 1590 \TODO{Make this sentence true} In particular, polymorphic functions in the \CFA prelude define @++x@ and @x++@ in terms of @x += 1@, allowing users to idiomatically define all forms of increment for a type @T@ by defining the single function @T& ?+=(T&, one_t)@; analogous overloads for the decrement operators are present as well. 1597 1591 1598 1592 \subsection{Units}
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