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1 @node ANSI
2 @chapter @sc{gnu} C++ Conformance to @sc{ansi} C++
3
4 These changes in the @sc{gnu} C++ compiler were made to comply more
5 closely with the @sc{ansi} base document, @cite{The Annotated C++
6 Reference Manual} (the @sc{arm}). Further reducing the divergences from
7 @sc{ansi} C++ is a continued goal of the @sc{gnu} C++ Renovation
8 Project.
9
10 @b{Section 3.4}, @i{Start and Termination}. It is now invalid to take
11 the address of the function @samp{main()}.
12
13 @b{Section 4.8}, @i{Pointers to Members}. The compiler produces
14 an error for trying to convert between a pointer to a member and the type
15 @samp{void *}.
16
17 @b{Section 5.2.5}, @i{Increment and Decrement}. It is an error to use
18 the increment and decrement operators on an enumerated type.
19
20 @b{Section 5.3.2}, @i{Sizeof}. Doing @code{sizeof} on a function is now
21 an error.
22
23 @b{Section 5.3.4}, @i{Delete}. The syntax of a @i{cast-expression} is
24 now more strictly controlled.
25
26 @b{Section 7.1.1}, @i{Storage Class Specifiers}. Using the
27 @code{static} and @code{extern} specifiers can now only be applied to
28 names of objects, functions, and anonymous unions.
29
30 @b{Section 7.1.1}, @i{Storage Class Specifiers}. The compiler no longer complains
31 about taking the address of a variable which has been declared to have @code{register}
32 storage.
33
34 @b{Section 7.1.2}, @i{Function Specifiers}. The compiler produces an
35 error when the @code{inline} or @code{virtual} specifiers are
36 used on anything other than a function.
37
38 @b{Section 8.3}, @i{Function Definitions}. It is now an error to shadow
39 a parameter name with a local variable; in the past, the compiler only
40 gave a warning in such a situation.
41
42 @b{Section 8.4.1}, @i{Aggregates}. The rules concerning declaration of
43 an aggregate are now all checked in the @sc{gnu} C++ compiler; they
44 include having no private or protected members and no base classes.
45
46 @b{Section 8.4.3}, @i{References}. Declaring an array of references is
47 now forbidden. Initializing a reference with an initializer list is
48 also considered an error.
49
50 @b{Section 9.5}, @i{Unions}. Global anonymous unions must be declared
51 @code{static}.
52
53 @b{Section 11.4}, @i{Friends}. Declaring a member to be a friend of a
54 type that has not yet been defined is an error.
55
56 @b{Section 12.1}, @i{Constructors}. The compiler generates a
57 default copy constructor for a class if no constructor has been declared.
58
59 @ignore
60 @b{Section 12.4}, @i{Destructors}. In accordance with the @sc{ansi} C++
61 draft standard working paper, a pure virtual destructor must now be
62 defined.
63 @end ignore
64
65 @b{Section 12.6.2}, @i{Special Member Functions}. When using a
66 @i{mem-initializer} list, the compiler will now initialize class members
67 in declaration order, not in the order in which you specify them.
68 Also, the compiler enforces the rule that non-static @code{const}
69 and reference members must be initialized with a @i{mem-initializer}
70 list when their class does not have a constructor.
71
72 @b{Section 12.8}, @i{Copying Class Objects}. The compiler generates
73 default copy constructors correctly, and supplies default assignment
74 operators compatible with user-defined ones.
75
76 @b{Section 13.4}, @i{Overloaded Operators}. An overloaded operator may
77 no longer have default arguments.
78
79 @b{Section 13.4.4}, @i{Function Call}. An overloaded @samp{operator ()}
80 must be a non-static member function.
81
82 @b{Section 13.4.5}, @i{Subscripting}. An overloaded @samp{operator []}
83 must be a non-static member function.
84
85 @b{Section 13.4.6}, @i{Class Member Access}. An overloaded @samp{operator ->}
86 must be a non-static member function.
87
88 @b{Section 13.4.7}, @i{Increment and Decrement}. The compiler will now
89 make sure a postfix @samp{@w{operator ++}} or @samp{@w{operator --}} has an
90 @code{int} as its second argument.
91
92
93 @node Encoding
94 @chapter Name Encoding in @sc{gnu} C++
95
96 @c FIXME!! rewrite name encoding section
97 @c ...to give complete rules rather than diffs from ARM.
98 @c To avoid plagiarism, invent some different way of structuring the
99 @c description of the rules than what ARM uses.
100
101 @cindex mangling
102 @cindex name encoding
103 @cindex encoding information in names
104 In order to support its strong typing rules and the ability to provide
105 function overloading, the C++ programming language @dfn{encodes}
106 information about functions and objects, so that conflicts across object
107 files can be detected during linking. @footnote{This encoding is also
108 sometimes called, whimsically enough, @dfn{mangling}; the corresponding
109 decoding is sometimes called @dfn{demangling}.} These rules tend to be
110 unique to each individual implementation of C++.
111
112 The scheme detailed in the commentary for 7.2.1 of @cite{The Annotated
113 Reference Manual} offers a description of a possible implementation
114 which happens to closely resemble the @code{cfront} compiler. The
115 design used in @sc{gnu} C++ differs from this model in a number of ways:
116
117 @itemize @bullet
118 @item
119 In addition to the basic types @code{void}, @code{char}, @code{short},
120 @code{int}, @code{long}, @code{float}, @code{double}, and @code{long
121 double}, @sc{gnu} C++ supports two additional types: @code{wchar_t}, the wide
122 character type, and @code{long long} (if the host supports it). The
123 encodings for these are @samp{w} and @samp{x} respectively.
124
125 @item
126 According to the @sc{arm}, qualified names (e.g., @samp{foo::bar::baz}) are
127 encoded with a leading @samp{Q}. Followed by the number of
128 qualifications (in this case, three) and the respective names, this
129 might be encoded as @samp{Q33foo3bar3baz}. @sc{gnu} C++ adds a leading
130 underscore to the list, producing @samp{_Q33foo3bar3baz}.
131
132 @item
133 The operator @samp{*=} is encoded as @samp{__aml}, not @samp{__amu}, to
134 match the normal @samp{*} operator, which is encoded as @samp{__ml}.
135
136 @c XXX left out ->(), __wr
137 @item
138 In addition to the normal operators, @sc{gnu} C++ also offers the minimum and
139 maximum operators @samp{>?} and @samp{<?}, encoded as @samp{__mx} and
140 @samp{__mn}, and the conditional operator @samp{?:}, encoded as @samp{__cn}.
141
142 @cindex destructors, encoding of
143 @cindex constructors, encoding of
144 @item
145 Constructors are encoded as simply @samp{__@var{name}}, where @var{name}
146 is the encoded name (e.g., @code{3foo} for the @code{foo} class
147 constructor). Destructors are encoded as two leading underscores
148 separated by either a period or a dollar sign, depending on the
149 capabilities of the local host, followed by the encoded name. For
150 example, the destructor @samp{foo::~foo} is encoded as @samp{_$_3foo}.
151
152 @item
153 Virtual tables are encoded with a prefix of @samp{_vt}, rather than
154 @samp{__vtbl}. The names of their classes are separated by dollar signs
155 (or periods), and not encoded as normal: the virtual table for
156 @code{foo} is @samp{__vt$foo}, and the table for @code{foo::bar} is
157 named @samp{__vt$foo$bar}.
158
159 @item
160 Static members are encoded as a leading underscore, followed by the
161 encoded name of the class in which they appear, a separating dollar sign
162 or period, and finally the unencoded name of the variable. For example,
163 if the class @code{foo} contains a static member @samp{bar}, its
164 encoding would be @samp{_3foo$bar}.
165
166 @item
167 @sc{gnu} C++ is not as aggressive as other compilers when it comes to always
168 generating @samp{Fv} for functions with no arguments. In particular,
169 the compiler does not add the sequence to conversion operators. The
170 function @samp{foo::bar()} is encoded as @samp{bar__3foo}, not
171 @samp{bar__3fooFv}.
172
173 @item
174 The argument list for methods is not prefixed by a leading @samp{F}; it
175 is considered implied.
176
177 @item
178 @sc{gnu} C++ approaches the task of saving space in encodings
179 differently from that noted in the @sc{arm}. It does use the
180 @samp{T@var{n}} and @samp{N@var{x}@var{y}} codes to signify copying the
181 @var{n}th argument's type, and making the next @var{x} arguments be the
182 type of the @var{y}th argument, respectively. However, the values for
183 @var{n} and @var{y} begin at zero with @sc{gnu} C++, whereas the
184 @sc{arm} describes them as starting at one. For the function @samp{foo
185 (bartype, bartype)}, @sc{gnu} C++ uses @samp{foo__7bartypeT0}, while
186 compilers following the @sc{arm} example generate @samp{foo__7bartypeT1}.
187
188 @c Note it loses on `foo (int, int, int, int, int)'.
189 @item
190 @sc{gnu} C++ does not bother using the space-saving methods for types whose
191 encoding is a single character (like an integer, encoded as @samp{i}).
192 This is useful in the most common cases (two @code{int}s would result in
193 using three letters, instead of just @samp{ii}).
194 @end itemize
195
196 @c @node Cfront
197 @c @chapter @code{cfront} Compared to @sc{gnu} C++
198 @c
199 @c
200 @c FIXME!! Fill in. Consider points in the following:
201 @c
202 @c @display
203 @c Date: Thu, 2 Jan 92 21:35:20 EST
204 @c From: raeburn@@cygnus.com
205 @c Message-Id: <9201030235.AA10999@@cambridge.cygnus.com>
206 @c To: mrs@@charlie.secs.csun.edu
207 @c Cc: g++@@cygnus.com
208 @c Subject: Re: ARM and GNU C++ incompatabilities
209 @c
210 @c Along with that, we should probably describe how g++ differs from
211 @c cfront, in ways that the users will notice. (E.g., cfront supposedly
212 @c allows "free (new char[10])"; does g++? How do the template
213 @c implementations differ? "New" placement syntax?)
214 @c @end display
215 @c
216 @c XXX For next revision.
217 @c
218 @c GNU C++:
219 @c * supports expanding inline functions in many situations,
220 @c including those which have static objects, use `for' statements,
221 @c and other situations. Part of this versatility is due to is
222 @c ability to not always generate temporaries for assignments.
223 @c * deliberately allows divide by 0 and mod 0, since [according
224 @c to Wilson] there are actually situations where you'd like to allow
225 @c such things. Note on most systems it will cause some sort of trap
226 @c or bus error. Cfront considers it an error.
227 @c * does [appear to] support nested classes within templates.
228 @c * conversion functions among baseclasses are all usable by
229 @c a class that's derived from all of those bases.
230 @c * sizeof works even when the class is defined within its ()'s
231 @c * conditional expressions work with member fns and pointers to
232 @c members.
233 @c * can handle non-trivial declarations of variables within switch
234 @c statements.
235 @c
236 @c Cfront: