source: trunk/third/gcc/cp/gpcompare.texi @ 8834

@node ANSI
@chapter @sc{gnu} C++ Conformance to @sc{ansi} C++

These changes in the @sc{gnu} C++ compiler were made to comply more
closely with the @sc{ansi} base document, @cite{The Annotated C++
Reference Manual} (the @sc{arm}).  Further reducing the divergences from
@sc{ansi} C++ is a continued goal of the @sc{gnu} C++ Renovation
Project.

@b{Section 3.4}, @i{Start and Termination}.  It is now invalid to take
the address of the function @samp{main()}.

@b{Section 4.8}, @i{Pointers to Members}.  The compiler produces
an error for trying to convert between a pointer to a member and the type
@samp{void *}.

@b{Section 5.2.5}, @i{Increment and Decrement}.  It is an error to use
the increment and decrement operators on an enumerated type.

@b{Section 5.3.2}, @i{Sizeof}.  Doing @code{sizeof} on a function is now
an error.

@b{Section 5.3.4}, @i{Delete}.  The syntax of a @i{cast-expression} is
now more strictly controlled.

@b{Section 7.1.1}, @i{Storage Class Specifiers}.  Using the
@code{static} and @code{extern} specifiers can now only be applied to
names of objects, functions, and anonymous unions.

@b{Section 7.1.1}, @i{Storage Class Specifiers}.  The compiler no longer complains
about taking the address of a variable which has been declared to have @code{register}
storage.

@b{Section 7.1.2}, @i{Function Specifiers}.  The compiler produces an
error when the @code{inline} or @code{virtual} specifiers are
used on anything other than a function.

@b{Section 8.3}, @i{Function Definitions}.  It is now an error to shadow
a parameter name with a local variable; in the past, the compiler only
gave a warning in such a situation.

@b{Section 8.4.1}, @i{Aggregates}.  The rules concerning declaration of
an aggregate are now all checked in the @sc{gnu} C++ compiler; they
include having no private or protected members and no base classes.

@b{Section 8.4.3}, @i{References}.  Declaring an array of references is
now forbidden.  Initializing a reference with an initializer list is
also considered an error.

@b{Section 9.5}, @i{Unions}.  Global anonymous unions must be declared
@code{static}.

@b{Section 11.4}, @i{Friends}.  Declaring a member to be a friend of a
type that has not yet been defined is an error.

@b{Section 12.1}, @i{Constructors}.  The compiler generates a
default copy constructor for a class if no constructor has been declared.

@ignore
@b{Section 12.4}, @i{Destructors}.  In accordance with the @sc{ansi} C++
draft standard working paper, a pure virtual destructor must now be
defined.
@end ignore

@b{Section 12.6.2}, @i{Special Member Functions}.  When using a
@i{mem-initializer} list, the compiler will now initialize class members
in declaration order, not in the order in which you specify them.
Also, the compiler enforces the rule that non-static @code{const}
and reference members must be initialized with a @i{mem-initializer}
list when their class does not have a constructor.

@b{Section 12.8}, @i{Copying Class Objects}.  The compiler generates
default copy constructors correctly, and supplies default assignment
operators compatible with user-defined ones.

@b{Section 13.4}, @i{Overloaded Operators}.  An overloaded operator may
no longer have default arguments.

@b{Section 13.4.4}, @i{Function Call}.  An overloaded @samp{operator ()}
must be a non-static member function.

@b{Section 13.4.5}, @i{Subscripting}.  An overloaded @samp{operator []}
must be a non-static member function.

@b{Section 13.4.6}, @i{Class Member Access}.  An overloaded @samp{operator ->}
must be a non-static member function.

@b{Section 13.4.7}, @i{Increment and Decrement}.  The compiler will now
make sure a postfix @samp{@w{operator ++}} or @samp{@w{operator --}} has an
@code{int} as its second argument.


@node Encoding
@chapter Name Encoding in @sc{gnu} C++

@c FIXME!! rewrite name encoding section
@c ...to give complete rules rather than diffs from ARM.
@c To avoid plagiarism, invent some different way of structuring the
@c description of the rules than what ARM uses.

@cindex mangling
@cindex name encoding
@cindex encoding information in names
In order to support its strong typing rules and the ability to provide
function overloading, the C++ programming language @dfn{encodes}
information about functions and objects, so that conflicts across object
files can be detected during linking. @footnote{This encoding is also
sometimes called, whimsically enough, @dfn{mangling}; the corresponding
decoding is sometimes called @dfn{demangling}.} These rules tend to be
unique to each individual implementation of C++.

The scheme detailed in the commentary for 7.2.1 of @cite{The Annotated
Reference Manual} offers a description of a possible implementation
which happens to closely resemble the @code{cfront} compiler.  The
design used in @sc{gnu} C++ differs from this model in a number of ways:

@itemize @bullet
@item
In addition to the basic types @code{void}, @code{char}, @code{short},
@code{int}, @code{long}, @code{float}, @code{double}, and @code{long
double}, @sc{gnu} C++ supports two additional types: @code{wchar_t}, the wide
character type, and @code{long long} (if the host supports it).  The
encodings for these are @samp{w} and @samp{x} respectively.

@item
According to the @sc{arm}, qualified names (e.g., @samp{foo::bar::baz}) are
encoded with a leading @samp{Q}.  Followed by the number of
qualifications (in this case, three) and the respective names, this
might be encoded as @samp{Q33foo3bar3baz}.  @sc{gnu} C++ adds a leading
underscore to the list, producing @samp{_Q33foo3bar3baz}.
 
@item
The operator @samp{*=} is encoded as @samp{__aml}, not @samp{__amu}, to
match the normal @samp{*} operator, which is encoded as @samp{__ml}.

@c XXX left out ->(), __wr
@item
In addition to the normal operators, @sc{gnu} C++ also offers the minimum and
maximum operators @samp{>?} and @samp{<?}, encoded as @samp{__mx} and
@samp{__mn}, and the conditional operator @samp{?:}, encoded as @samp{__cn}.

@cindex destructors, encoding of
@cindex constructors, encoding of
@item
Constructors are encoded as simply @samp{__@var{name}}, where @var{name}
is the encoded name (e.g., @code{3foo} for the @code{foo} class
constructor).  Destructors are encoded as two leading underscores
separated by either a period or a dollar sign, depending on the
capabilities of the local host, followed by the encoded name.  For
example, the destructor @samp{foo::~foo} is encoded as @samp{_$_3foo}.

@item
Virtual tables are encoded with a prefix of @samp{_vt}, rather than
@samp{__vtbl}.  The names of their classes are separated by dollar signs
(or periods), and not encoded as normal: the virtual table for
@code{foo} is @samp{__vt$foo}, and the table for @code{foo::bar} is
named @samp{__vt$foo$bar}.

@item
Static members are encoded as a leading underscore, followed by the
encoded name of the class in which they appear, a separating dollar sign
or period, and finally the unencoded name of the variable.  For example,
if the class @code{foo} contains a static member @samp{bar}, its
encoding would be @samp{_3foo$bar}.

@item
@sc{gnu} C++ is not as aggressive as other compilers when it comes to always
generating @samp{Fv} for functions with no arguments.  In particular,
the compiler does not add the sequence to conversion operators.  The
function @samp{foo::bar()} is encoded as @samp{bar__3foo}, not
@samp{bar__3fooFv}.

@item
The argument list for methods is not prefixed by a leading @samp{F}; it
is considered implied.

@item
@sc{gnu} C++ approaches the task of saving space in encodings
differently from that noted in the @sc{arm}.  It does use the
@samp{T@var{n}} and @samp{N@var{x}@var{y}} codes to signify copying the
@var{n}th argument's type, and making the next @var{x} arguments be the
type of the @var{y}th argument, respectively.  However, the values for
@var{n} and @var{y} begin at zero with @sc{gnu} C++, whereas the
@sc{arm} describes them as starting at one.  For the function @samp{foo
(bartype, bartype)}, @sc{gnu} C++ uses @samp{foo__7bartypeT0}, while
compilers following the @sc{arm} example generate @samp{foo__7bartypeT1}.

@c Note it loses on `foo (int, int, int, int, int)'.
@item
@sc{gnu} C++ does not bother using the space-saving methods for types whose
encoding is a single character (like an integer, encoded as @samp{i}).
This is useful in the most common cases (two @code{int}s would result in
using three letters, instead of just @samp{ii}).
@end itemize

@c @node Cfront
@c @chapter @code{cfront} Compared to @sc{gnu} C++
@c 
@c 
@c FIXME!! Fill in.  Consider points in the following:
@c 
@c @display
@c Date: Thu, 2 Jan 92 21:35:20 EST
@c From: raeburn@@cygnus.com
@c Message-Id: <9201030235.AA10999@@cambridge.cygnus.com>
@c To: mrs@@charlie.secs.csun.edu
@c Cc: g++@@cygnus.com
@c Subject: Re: ARM and GNU C++ incompatabilities
@c 
@c Along with that, we should probably describe how g++ differs from
@c cfront, in ways that the users will notice.  (E.g., cfront supposedly
@c allows "free (new char[10])"; does g++?  How do the template
@c implementations differ?  "New" placement syntax?)
@c @end display
@c
@c XXX For next revision.
@c
@c GNU C++:
@c * supports expanding inline functions in many situations,
@c   including those which have static objects, use `for' statements,
@c   and other situations.  Part of this versatility is due to is
@c   ability to not always generate temporaries for assignments.
@c * deliberately allows divide by 0 and mod 0, since [according
@c   to Wilson] there are actually situations where you'd like to allow
@c   such things.  Note on most systems it will cause some sort of trap
@c   or bus error.  Cfront considers it an error.
@c * does [appear to] support nested classes within templates.
@c * conversion functions among baseclasses are all usable by
@c   a class that's derived from all of those bases.
@c * sizeof works even when the class is defined within its ()'s
@c * conditional expressions work with member fns and pointers to
@c    members.
@c * can handle non-trivial declarations of variables within switch
@c   statements.
@c
@c Cfront: