@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: