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GNU Info File | 1995-11-26 | 38KB | 681 lines

This is Info file gcc.info, produced by Makeinfo-1.55 from the input
file gcc.texi.
   This file documents the use and the internals of the GNU compiler.
   Published by the Free Software Foundation 59 Temple Place - Suite 330
Boston, MA 02111-1307 USA
   Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995 Free Software
Foundation, Inc.
   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the sections entitled "GNU General Public License," "Funding for
Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are
included exactly as in the original, and provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.
   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the sections entitled "GNU General Public
License," "Funding for Free Software," and "Protect Your Freedom--Fight
`Look And Feel'", and this permission notice, may be included in
translations approved by the Free Software Foundation instead of in the
original English.
File: gcc.info,  Node: Asm Labels,  Next: Explicit Reg Vars,  Prev: Extended Asm,  Up: C Extensions
Controlling Names Used in Assembler Code
========================================
   You can specify the name to be used in the assembler code for a C
function or variable by writing the `asm' (or `__asm__') keyword after
the declarator as follows:
     int foo asm ("myfoo") = 2;
This specifies that the name to be used for the variable `foo' in the
assembler code should be `myfoo' rather than the usual `_foo'.
   On systems where an underscore is normally prepended to the name of
a C function or variable, this feature allows you to define names for
the linker that do not start with an underscore.
   You cannot use `asm' in this way in a function *definition*; but you
can get the same effect by writing a declaration for the function
before its definition and putting `asm' there, like this:
     extern func () asm ("FUNC");

     func (x, y)
          int x, y;
     ...
   It is up to you to make sure that the assembler names you choose do
not conflict with any other assembler symbols.  Also, you must not use a
register name; that would produce completely invalid assembler code.
GNU CC does not as yet have the ability to store static variables in
registers.  Perhaps that will be added.
File: gcc.info,  Node: Explicit Reg Vars,  Next: Alternate Keywords,  Prev: Asm Labels,  Up: C Extensions
Variables in Specified Registers
================================
   GNU C allows you to put a few global variables into specified
hardware registers.  You can also specify the register in which an
ordinary register variable should be allocated.
   * Global register variables reserve registers throughout the program.
     This may be useful in programs such as programming language
     interpreters which have a couple of global variables that are
     accessed very often.
   * Local register variables in specific registers do not reserve the
     registers.  The compiler's data flow analysis is capable of
     determining where the specified registers contain live values, and
     where they are available for other uses.
     These local variables are sometimes convenient for use with the
     extended `asm' feature (*note Extended Asm::.), if you want to
     write one output of the assembler instruction directly into a
     particular register.  (This will work provided the register you
     specify fits the constraints specified for that operand in the
     `asm'.)
* Menu:
* Global Reg Vars::
* Local Reg Vars::
File: gcc.info,  Node: Global Reg Vars,  Next: Local Reg Vars,  Up: Explicit Reg Vars
Defining Global Register Variables
----------------------------------
   You can define a global register variable in GNU C like this:
     register int *foo asm ("a5");
Here `a5' is the name of the register which should be used.  Choose a
register which is normally saved and restored by function calls on your
machine, so that library routines will not clobber it.
   Naturally the register name is cpu-dependent, so you would need to
conditionalize your program according to cpu type.  The register `a5'
would be a good choice on a 68000 for a variable of pointer type.  On
machines with register windows, be sure to choose a "global" register
that is not affected magically by the function call mechanism.
   In addition, operating systems on one type of cpu may differ in how
they name the registers; then you would need additional conditionals.
For example, some 68000 operating systems call this register `%a5'.
   Eventually there may be a way of asking the compiler to choose a
register automatically, but first we need to figure out how it should
choose and how to enable you to guide the choice.  No solution is
evident.
   Defining a global register variable in a certain register reserves
that register entirely for this use, at least within the current
compilation.  The register will not be allocated for any other purpose
in the functions in the current compilation.  The register will not be
saved and restored by these functions.  Stores into this register are
never deleted even if they would appear to be dead, but references may
be deleted or moved or simplified.
   It is not safe to access the global register variables from signal
handlers, or from more than one thread of control, because the system
library routines may temporarily use the register for other things
(unless you recompile them specially for the task at hand).
   It is not safe for one function that uses a global register variable
to call another such function `foo' by way of a third function `lose'
that was compiled without knowledge of this variable (i.e. in a
different source file in which the variable wasn't declared).  This is
because `lose' might save the register and put some other value there.
For example, you can't expect a global register variable to be
available in the comparison-function that you pass to `qsort', since
`qsort' might have put something else in that register.  (If you are
prepared to recompile `qsort' with the same global register variable,
you can solve this problem.)
   If you want to recompile `qsort' or other source files which do not
actually use your global register variable, so that they will not use
that register for any other purpose, then it suffices to specify the
compiler option `-ffixed-REG'.  You need not actually add a global
register declaration to their source code.
   A function which can alter the value of a global register variable
cannot safely be called from a function compiled without this variable,
because it could clobber the value the caller expects to find there on
return.  Therefore, the function which is the entry point into the part
of the program that uses the global register variable must explicitly
save and restore the value which belongs to its caller.
   On most machines, `longjmp' will restore to each global register
variable the value it had at the time of the `setjmp'.  On some
machines, however, `longjmp' will not change the value of global
register variables.  To be portable, the function that called `setjmp'
should make other arrangements to save the values of the global register
variables, and to restore them in a `longjmp'.  This way, the same
thing will happen regardless of what `longjmp' does.
   All global register variable declarations must precede all function
definitions.  If such a declaration could appear after function
definitions, the declaration would be too late to prevent the register
from being used for other purposes in the preceding functions.
   Global register variables may not have initial values, because an
executable file has no means to supply initial contents for a register.
   On the Sparc, there are reports that g3 ... g7 are suitable
registers, but certain library functions, such as `getwd', as well as
the subroutines for division and remainder, modify g3 and g4.  g1 and
g2 are local temporaries.
   On the 68000, a2 ... a5 should be suitable, as should d2 ... d7.  Of
course, it will not do to use more than a few of those.
File: gcc.info,  Node: Local Reg Vars,  Prev: Global Reg Vars,  Up: Explicit Reg Vars
Specifying Registers for Local Variables
----------------------------------------
   You can define a local register variable with a specified register
like this:
     register int *foo asm ("a5");
Here `a5' is the name of the register which should be used.  Note that
this is the same syntax used for defining global register variables,
but for a local variable it would appear within a function.
   Naturally the register name is cpu-dependent, but this is not a
problem, since specific registers are most often useful with explicit
assembler instructions (*note Extended Asm::.).  Both of these things
generally require that you conditionalize your program according to cpu
type.
   In addition, operating systems on one type of cpu may differ in how
they name the registers; then you would need additional conditionals.
For example, some 68000 operating systems call this register `%a5'.
   Eventually there may be a way of asking the compiler to choose a
register automatically, but first we need to figure out how it should
choose and how to enable you to guide the choice.  No solution is
evident.
   Defining such a register variable does not reserve the register; it
remains available for other uses in places where flow control determines
the variable's value is not live.  However, these registers are made
unavailable for use in the reload pass.  I would not be surprised if
excessive use of this feature leaves the compiler too few available
registers to compile certain functions.
File: gcc.info,  Node: Alternate Keywords,  Next: Incomplete Enums,  Prev: Explicit Reg Vars,  Up: C Extensions
Alternate Keywords
==================
   The option `-traditional' disables certain keywords; `-ansi'
disables certain others.  This causes trouble when you want to use GNU C
extensions, or ANSI C features, in a general-purpose header file that
should be usable by all programs, including ANSI C programs and
traditional ones.  The keywords `asm', `typeof' and `inline' cannot be
used since they won't work in a program compiled with `-ansi', while
the keywords `const', `volatile', `signed', `typeof' and `inline' won't
work in a program compiled with `-traditional'.
   The way to solve these problems is to put `__' at the beginning and
end of each problematical keyword.  For example, use `__asm__' instead
of `asm', `__const__' instead of `const', and `__inline__' instead of
`inline'.
   Other C compilers won't accept these alternative keywords; if you
want to compile with another compiler, you can define the alternate
keywords as macros to replace them with the customary keywords.  It
looks like this:
     #ifndef __GNUC__
     #define __asm__ asm
     #endif
   `-pedantic' causes warnings for many GNU C extensions.  You can
prevent such warnings within one expression by writing `__extension__'
before the expression.  `__extension__' has no effect aside from this.
File: gcc.info,  Node: Incomplete Enums,  Next: Function Names,  Prev: Alternate Keywords,  Up: C Extensions
Incomplete `enum' Types
=======================
   You can define an `enum' tag without specifying its possible values.
This results in an incomplete type, much like what you get if you write
`struct foo' without describing the elements.  A later declaration
which does specify the possible values completes the type.
   You can't allocate variables or storage using the type while it is
incomplete.  However, you can work with pointers to that type.
   This extension may not be very useful, but it makes the handling of
`enum' more consistent with the way `struct' and `union' are handled.
   This extension is not supported by GNU C++.
File: gcc.info,  Node: Function Names,  Prev: Incomplete Enums,  Up: C Extensions
Function Names as Strings
=========================
   GNU CC predefines two string variables to be the name of the current
function.  The variable `__FUNCTION__' is the name of the function as
it appears in the source.  The variable `__PRETTY_FUNCTION__' is the
name of the function pretty printed in a language specific fashion.
   These names are always the same in a C function, but in a C++
function they may be different.  For example, this program:
     extern "C" {
     extern int printf (char *, ...);
     }

     class a {
      public:
       sub (int i)
         {
           printf ("__FUNCTION__ = %s\n", __FUNCTION__);
           printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__);
         }
     };

     int
     main (void)
     {
       a ax;
       ax.sub (0);
       return 0;
     }
gives this output:
     __FUNCTION__ = sub
     __PRETTY_FUNCTION__ = int  a::sub (int)
   These names are not macros: they are predefined string variables.
For example, `#ifdef __FUNCTION__' does not have any special meaning
inside a function, since the preprocessor does not do anything special
with the identifier `__FUNCTION__'.
File: gcc.info,  Node: C++ Extensions,  Next: Trouble,  Prev: C Extensions,  Up: Top
Extensions to the C++ Language
******************************
   The GNU compiler provides these extensions to the C++ language (and
you can also use most of the C language extensions in your C++
programs).  If you want to write code that checks whether these
features are available, you can test for the GNU compiler the same way
as for C programs: check for a predefined macro `__GNUC__'.  You can
also use `__GNUG__' to test specifically for GNU C++ (*note Standard
Predefined Macros: (cpp.info)Standard Predefined.).
* Menu:
* Naming Results::      Giving a name to C++ function return values.
* Min and Max::        C++ Minimum and maximum operators.
* Destructors and Goto:: Goto is safe to use in C++ even when destructors
                           are needed.
* C++ Interface::       You can use a single C++ header file for both
                         declarations and definitions.
* Template Instantiation:: Methods for ensuring that exactly one copy of
                         each needed template instantiation is emitted.
* C++ Signatures::    You can specify abstract types to get subtype
             polymorphism independent from inheritance.
File: gcc.info,  Node: Naming Results,  Next: Min and Max,  Up: C++ Extensions
Named Return Values in C++
==========================
   GNU C++ extends the function-definition syntax to allow you to
specify a name for the result of a function outside the body of the
definition, in C++ programs:
     TYPE
     FUNCTIONNAME (ARGS) return RESULTNAME;
     {
       ...
       BODY
       ...
     }
   You can use this feature to avoid an extra constructor call when a
function result has a class type.  For example, consider a function
`m', declared as `X v = m ();', whose result is of class `X':
     X
     m ()
     {
       X b;
       b.a = 23;
       return b;
     }
   Although `m' appears to have no arguments, in fact it has one
implicit argument: the address of the return value.  At invocation, the
address of enough space to hold `v' is sent in as the implicit argument.
Then `b' is constructed and its `a' field is set to the value 23.
Finally, a copy constructor (a constructor of the form `X(X&)') is
applied to `b', with the (implicit) return value location as the
target, so that `v' is now bound to the return value.
   But this is wasteful.  The local `b' is declared just to hold
something that will be copied right out.  While a compiler that
combined an "elision" algorithm with interprocedural data flow analysis
could conceivably eliminate all of this, it is much more practical to
allow you to assist the compiler in generating efficient code by
manipulating the return value explicitly, thus avoiding the local
variable and copy constructor altogether.
   Using the extended GNU C++ function-definition syntax, you can avoid
the temporary allocation and copying by naming `r' as your return value
at the outset, and assigning to its `a' field directly:
     X
     m () return r;
     {
       r.a = 23;
     }
The declaration of `r' is a standard, proper declaration, whose effects
are executed *before* any of the body of `m'.
   Functions of this type impose no additional restrictions; in
particular, you can execute `return' statements, or return implicitly by
reaching the end of the function body ("falling off the edge").  Cases
     X
     m () return r (23);
     {
       return;
     }
(or even `X m () return r (23); { }') are unambiguous, since the return
value `r' has been initialized in either case.  The following code may
be hard to read, but also works predictably:
     X
     m () return r;
     {
       X b;
       return b;
     }
   The return value slot denoted by `r' is initialized at the outset,
but the statement `return b;' overrides this value.  The compiler deals
with this by destroying `r' (calling the destructor if there is one, or
doing nothing if there is not), and then reinitializing `r' with `b'.
   This extension is provided primarily to help people who use
overloaded operators, where there is a great need to control not just
the arguments, but the return values of functions.  For classes where
the copy constructor incurs a heavy performance penalty (especially in
the common case where there is a quick default constructor), this is a
major savings.  The disadvantage of this extension is that you do not
control when the default constructor for the return value is called: it
is always called at the beginning.
File: gcc.info,  Node: Min and Max,  Next: Destructors and Goto,  Prev: Naming Results,  Up: C++ Extensions
Minimum and Maximum Operators in C++
====================================
   It is very convenient to have operators which return the "minimum"
or the "maximum" of two arguments.  In GNU C++ (but not in GNU C),
`A <? B'
     is the "minimum", returning the smaller of the numeric values A
     and B;
`A >? B'
     is the "maximum", returning the larger of the numeric values A and
     B.
   These operations are not primitive in ordinary C++, since you can
use a macro to return the minimum of two things in C++, as in the
following example.
     #define MIN(X,Y) ((X) < (Y) ? : (X) : (Y))
You might then use `int min = MIN (i, j);' to set MIN to the minimum
value of variables I and J.
   However, side effects in `X' or `Y' may cause unintended behavior.
For example, `MIN (i++, j++)' will fail, incrementing the smaller
counter twice.  A GNU C extension allows you to write safe macros that
avoid this kind of problem (*note Naming an Expression's Type: Naming
Types.).  However, writing `MIN' and `MAX' as macros also forces you to
use function-call notation notation for a fundamental arithmetic
operation.  Using GNU C++ extensions, you can write `int min = i <? j;'
instead.
   Since `<?' and `>?' are built into the compiler, they properly
handle expressions with side-effects;  `int min = i++ <? j++;' works
correctly.
File: gcc.info,  Node: Destructors and Goto,  Next: C++ Interface,  Prev: Min and Max,  Up: C++ Extensions
`goto' and Destructors in GNU C++
=================================
   In C++ programs, you can safely use the `goto' statement.  When you
use it to exit a block which contains aggregates requiring destructors,
the destructors will run before the `goto' transfers control.  (In ANSI
C++, `goto' is restricted to targets within the current block.)
   The compiler still forbids using `goto' to *enter* a scope that
requires constructors.
File: gcc.info,  Node: C++ Interface,  Next: Template Instantiation,  Prev: Destructors and Goto,  Up: C++ Extensions
Declarations and Definitions in One Header
==========================================
   C++ object definitions can be quite complex.  In principle, your
source code will need two kinds of things for each object that you use
across more than one source file.  First, you need an "interface"
specification, describing its structure with type declarations and
function prototypes.  Second, you need the "implementation" itself.  It
can be tedious to maintain a separate interface description in a header
file, in parallel to the actual implementation.  It is also dangerous,
since separate interface and implementation definitions may not remain
parallel.
   With GNU C++, you can use a single header file for both purposes.
     *Warning:* The mechanism to specify this is in transition.  For the
     nonce, you must use one of two `#pragma' commands; in a future
     release of GNU C++, an alternative mechanism will make these
     `#pragma' commands unnecessary.
   The header file contains the full definitions, but is marked with
`#pragma interface' in the source code.  This allows the compiler to
use the header file only as an interface specification when ordinary
source files incorporate it with `#include'.  In the single source file
where the full implementation belongs, you can use either a naming
convention or `#pragma implementation' to indicate this alternate use
of the header file.
`#pragma interface'
`#pragma interface "SUBDIR/OBJECTS.h"'
     Use this directive in *header files* that define object classes,
     to save space in most of the object files that use those classes.
     Normally, local copies of certain information (backup copies of
     inline member functions, debugging information, and the internal
     tables that implement virtual functions) must be kept in each
     object file that includes class definitions.  You can use this
     pragma to avoid such duplication.  When a header file containing
     `#pragma interface' is included in a compilation, this auxiliary
     information will not be generated (unless the main input source
     file itself uses `#pragma implementation').  Instead, the object
     files will contain references to be resolved at link time.
     The second form of this directive is useful for the case where you
     have multiple headers with the same name in different directories.
     If you use this form, you must specify the same string to `#pragma
     implementation'.
`#pragma implementation'
`#pragma implementation "OBJECTS.h"'
     Use this pragma in a *main input file*, when you want full output
     from included header files to be generated (and made globally
     visible).  The included header file, in turn, should use `#pragma
     interface'.  Backup copies of inline member functions, debugging
     information, and the internal tables used to implement virtual
     functions are all generated in implementation files.
     If you use `#pragma implementation' with no argument, it applies to
     an include file with the same basename(1) as your source file.
     For example, in `allclass.cc', `#pragma implementation' by itself
     is equivalent to `#pragma implementation "allclass.h"'.
     In versions of GNU C++ prior to 2.6.0 `allclass.h' was treated as
     an implementation file whenever you would include it from
     `allclass.cc' even if you never specified `#pragma
     implementation'.  This was deemed to be more trouble than it was
     worth, however, and disabled.
     If you use an explicit `#pragma implementation', it must appear in
     your source file *before* you include the affected header files.
     Use the string argument if you want a single implementation file to
     include code from multiple header files.  (You must also use
     `#include' to include the header file; `#pragma implementation'
     only specifies how to use the file--it doesn't actually include
     it.)
     There is no way to split up the contents of a single header file
     into multiple implementation files.
   `#pragma implementation' and `#pragma interface' also have an effect
on function inlining.
   If you define a class in a header file marked with `#pragma
interface', the effect on a function defined in that class is similar to
an explicit `extern' declaration--the compiler emits no code at all to
define an independent version of the function.  Its definition is used
only for inlining with its callers.
   Conversely, when you include the same header file in a main source
file that declares it as `#pragma implementation', the compiler emits
code for the function itself; this defines a version of the function
that can be found via pointers (or by callers compiled without
inlining).  If all calls to the function can be inlined, you can avoid
emitting the function by compiling with `-fno-implement-inlines'.  If
any calls were not inlined, you will get linker errors.
   ---------- Footnotes ----------
   (1)  A file's "basename" was the name stripped of all leading path
information and of trailing suffixes, such as `.h' or `.C' or `.cc'.
File: gcc.info,  Node: Template Instantiation,  Next: C++ Signatures,  Prev: C++ Interface,  Up: C++ Extensions
Where's the Template?
=====================
   C++ templates are the first language feature to require more
intelligence from the environment than one usually finds on a UNIX
system.  Somehow the compiler and linker have to make sure that each
template instance occurs exactly once in the executable if it is needed,
and not at all otherwise.  There are two basic approaches to this
problem, which I will refer to as the Borland model and the Cfront
model.
Borland model
     Borland C++ solved the template instantiation problem by adding
     the code equivalent of common blocks to their linker; template
     instances are emitted in each translation unit that uses them, and
     they are collapsed together at run time.  The advantage of this
     model is that the linker only has to consider the object files
     themselves; there is no external complexity to worry about.  This
     disadvantage is that compilation time is increased because the
     template code is being compiled repeatedly.  Code written for this
     model tends to include definitions of all member templates in the
     header file, since they must be seen to be compiled.
Cfront model
     The AT&T C++ translator, Cfront, solved the template instantiation
     problem by creating the notion of a template repository, an
     automatically maintained place where template instances are
     stored.  As individual object files are built, notes are placed in
     the repository to record where templates and potential type
     arguments were seen so that the subsequent instantiation step
     knows where to find them.  At link time, any needed instances are
     generated and linked in.  The advantages of this model are more
     optimal compilation speed and the ability to use the system
     linker; to implement the Borland model a compiler vendor also
     needs to replace the linker.  The disadvantages are vastly
     increased complexity, and thus potential for error; theoretically,
     this should be just as transparent, but in practice it has been
     very difficult to build multiple programs in one directory and one
     program in multiple directories using Cfront.  Code written for
     this model tends to separate definitions of non-inline member
     templates into a separate file, which is magically found by the
     link preprocessor when a template needs to be instantiated.
   Currently, g++ implements neither automatic model.  In the mean time,
you have three options for dealing with template instantiations:
  1. Do nothing.  Pretend g++ does implement automatic instantiation
     management.  Code written for the Borland model will work fine, but
     each translation unit will contain instances of each of the
     templates it uses.  In a large program, this can lead to an
     unacceptable amount of code duplication.
  2. Add `#pragma interface' to all files containing template
     definitions.  For each of these files, add `#pragma implementation
     "FILENAME"' to the top of some `.C' file which `#include's it.
     Then compile everything with -fexternal-templates.  The templates
     will then only be expanded in the translation unit which
     implements them (i.e. has a `#pragma implementation' line for the
     file where they live); all other files will use external
     references.  If you're lucky, everything should work properly.  If
     you get undefined symbol errors, you need to make sure that each
     template instance which is used in the program is used in the file
     which implements that template.  If you don't have any use for a
     particular instance in that file, you can just instantiate it
     explicitly, using the syntax from the latest C++ working paper:
          template class A<int>;
          template ostream& operator << (ostream&, const A<int>&);
     This strategy will work with code written for either model.  If
     you are using code written for the Cfront model, the file
     containing a class template and the file containing its member
     templates should be implemented in the same translation unit.
     A slight variation on this approach is to use the flag
     -falt-external-templates instead; this flag causes template
     instances to be emitted in the translation unit that implements
     the header where they are first instantiated, rather than the one
     which implements the file where the templates are defined.  This
     header must be the same in all translation units, or things are
     likely to break.
     *Note Declarations and Definitions in One Header: C++ Interface,
     for more discussion of these pragmas.
  3. Explicitly instantiate all the template instances you use, and
     compile with -fno-implicit-templates.  This is probably your best
     bet; it may require more knowledge of exactly which templates you
     are using, but it's less mysterious than the previous approach,
     and it doesn't require any `#pragma's or other g++-specific code.
     You can scatter the instantiations throughout your program, you
     can create one big file to do all the instantiations, or you can
     create tiny files like
          #include "Foo.h"
          #include "Foo.cc"

          template class Foo<int>;
     for each instance you need, and create a template instantiation
     library from those.  I'm partial to the last, but your mileage may
     vary.  If you are using Cfront-model code, you can probably get
     away with not using -fno-implicit-templates when compiling files
     that don't `#include' the member template definitions.
File: gcc.info,  Node: C++ Signatures,  Prev: Template Instantiation,  Up: C++ Extensions
Type Abstraction using Signatures
=================================
   In GNU C++, you can use the keyword `signature' to define a
completely abstract class interface as a datatype.  You can connect this
abstraction with actual classes using signature pointers.  If you want
to use signatures, run the GNU compiler with the `-fhandle-signatures'
command-line option.  (With this option, the compiler reserves a second
keyword `sigof' as well, for a future extension.)
   Roughly, signatures are type abstractions or interfaces of classes.
Some other languages have similar facilities.  C++ signatures are
related to ML's signatures, Haskell's type classes, definition modules
in Modula-2, interface modules in Modula-3, abstract types in Emerald,
type modules in Trellis/Owl, categories in Scratchpad II, and types in
POOL-I.  For a more detailed discussion of signatures, see `Signatures:
A Language Extension for Improving Type Abstraction and Subtype
Polymorphism in C++' by Gerald Baumgartner and Vincent F. Russo (Tech
report CSD-TR-95-051, Dept. of Computer Sciences, Purdue University,
August 1995, a slightly improved version appeared in
*Software--Practice & Experience*, 25(8), pp. 863-889, August 1995).
You can get the tech report by anonymous FTP from `ftp.cs.purdue.edu'
in `pub/gb/Signature-design.ps.gz'.
   Syntactically, a signature declaration is a collection of member
function declarations and nested type declarations.  For example, this
signature declaration defines a new abstract type `S' with member
functions `int foo ()' and `int bar (int)':
     signature S
     {
       int foo ();
       int bar (int);
     };
   Since signature types do not include implementation definitions, you
cannot write an instance of a signature directly.  Instead, you can
define a pointer to any class that contains the required interfaces as a
"signature pointer".  Such a class "implements" the signature type.
   To use a class as an implementation of `S', you must ensure that the
class has public member functions `int foo ()' and `int bar (int)'.
The class can have other member functions as well, public or not; as
long as it offers what's declared in the signature, it is suitable as
an implementation of that signature type.
   For example, suppose that `C' is a class that meets the requirements
of signature `S' (`C' "conforms to" `S').  Then
     C obj;
     S * p = &obj;
defines a signature pointer `p' and initializes it to point to an
object of type `C'.  The member function call `int i = p->foo ();'
executes `obj.foo ()'.
   Abstract virtual classes provide somewhat similar facilities in
standard C++.  There are two main advantages to using signatures
instead:
  1. Subtyping becomes independent from inheritance.  A class or
     signature type `T' is a subtype of a signature type `S'
     independent of any inheritance hierarchy as long as all the member
     functions declared in `S' are also found in `T'.  So you can
     define a subtype hierarchy that is completely independent from any
     inheritance (implementation) hierarchy, instead of being forced to
     use types that mirror the class inheritance hierarchy.
  2. Signatures allow you to work with existing class hierarchies as
     implementations of a signature type.  If those class hierarchies
     are only available in compiled form, you're out of luck with
     abstract virtual classes, since an abstract virtual class cannot
     be retrofitted on top of existing class hierarchies.  So you would
     be required to write interface classes as subtypes of the abstract
     virtual class.
   There is one more detail about signatures.  A signature declaration
can contain member function *definitions* as well as member function
declarations.  A signature member function with a full definition is
called a *default implementation*; classes need not contain that
particular interface in order to conform.  For example, a class `C' can
conform to the signature
     signature T
     {
       int f (int);
       int f0 () { return f (0); };
     };
whether or not `C' implements the member function `int f0 ()'.  If you
define `C::f0', that definition takes precedence; otherwise, the
default implementation `S::f0' applies.
File: gcc.info,  Node: Trouble,  Next: Bugs,  Prev: C++ Extensions,  Up: Top
Known Causes of Trouble with GNU CC
***********************************
   This section describes known problems that affect users of GNU CC.
Most of these are not GNU CC bugs per se--if they were, we would fix
them.  But the result for a user may be like the result of a bug.
   Some of these problems are due to bugs in other software, some are
missing features that are too much work to add, and some are places
where people's opinions differ as to what is best.
* Menu:
* Actual Bugs::              Bugs we will fix later.
* Installation Problems::     Problems that manifest when you install GNU CC.
* Cross-Compiler Problems::   Common problems of cross compiling with GNU CC.
* Interoperation::      Problems using GNU CC with other compilers,
               and with certain linkers, assemblers and debuggers.
* External Bugs::    Problems compiling certain programs.
* Incompatibilities::   GNU CC is incompatible with traditional C.
* Fixed Headers::       GNU C uses corrected versions of system header files.
                           This is necessary, but doesn't always work smoothly.
* Standard Libraries::  GNU C uses the system C library, which might not be
                           compliant with the ISO/ANSI C standard.
* Disappointments::     Regrettable things we can't change, but not quite bugs.
* C++ Misunderstandings::     Common misunderstandings with GNU C++.
* Protoize Caveats::    Things to watch out for when using `protoize'.
* Non-bugs::        Things we think are right, but some others disagree.
* Warnings and Errors:: Which problems in your code get warnings,
                         and which get errors.
File: gcc.info,  Node: Actual Bugs,  Next: Installation Problems,  Up: Trouble
Actual Bugs We Haven't Fixed Yet
================================
   * The `fixincludes' script interacts badly with automounters; if the
     directory of system header files is automounted, it tends to be
     unmounted while `fixincludes' is running.  This would seem to be a
     bug in the automounter.  We don't know any good way to work around
     it.
   * The `fixproto' script will sometimes add prototypes for the
     `sigsetjmp' and `siglongjmp' functions that reference the
     `jmp_buf' type before that type is defined.  To work around this,
     edit the offending file and place the typedef in front of the
     prototypes.
   * There are several obscure case of mis-using struct, union, and
     enum tags that are not detected as errors by the compiler.
   * When `-pedantic-errors' is specified, GNU C will incorrectly give
     an error message when a function name is specified in an expression
     involving the comma operator.
   * Loop unrolling doesn't work properly for certain C++ programs.
     This is a bug in the C++ front end.  It sometimes emits incorrect
     debug info, and the loop unrolling code is unable to recover from
     this error.