Name Encoding in GNU C++

In order to support its strong typing rules and the ability to provide function overloading, the C++ programming language encodes information about functions and objects, so that conflicts across object files can be detected during linking. (3) These rules tend to be unique to each individual implementation of C++.

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

• In addition to the basic types void, char, short, int, long, float, double, and long double, GNU C++ supports two additional types: wchar_t, the wide character type, and long long (if the host supports it). The encodings for these are `w' and `x' respectively.
• According to the ARM, qualified names (e.g., `foo::bar::baz') are encoded with a leading `Q'. Followed by the number of qualifications (in this case, three) and the respective names, this might be encoded as `Q33foo3bar3baz'. GNU C++ adds a leading underscore to the list, producing `_Q33foo3bar3baz'.
• The operator `*=' is encoded as `__aml', not `__amu', to match the normal `*' operator, which is encoded as `__ml'.
• In addition to the normal operators, GNU C++ also offers the minimum and maximum operators `>?' and `<?', encoded as `__mx' and `__mn', and the conditional operator `?:', encoded as `__cn'.
• Constructors are encoded as simply `__name', where name is the encoded name (e.g., 3foo for the 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 `foo::~foo' is encoded as `_$_3foo'.
• Virtual tables are encoded with a prefix of `_vt', rather than `__vtbl'. The names of their classes are separated by dollar signs (or periods), and not encoded as normal: the virtual table for foo is `__vt$foo', and the table for foo::bar is named `__vt$foo$bar'.
• 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 foo contains a static member `bar', its encoding would be `_3foo$bar'.
• GNU C++ is not as aggressive as other compilers when it comes to always generating `Fv' for functions with no arguments. In particular, the compiler does not add the sequence to conversion operators. The function `foo::bar()' is encoded as `bar__3foo', not `bar__3fooFv'.
• The argument list for methods is not prefixed by a leading `F'; it is considered implied.
• GNU C++ approaches the task of saving space in encodings differently from that noted in the ARM. It does use the `Tn' and `Nxy' codes to signify copying the nth argument's type, and making the next x arguments be the type of the yth argument, respectively. However, the values for n and y begin at zero with GNU C++, whereas the ARM describes them as starting at one. For the function `foo (bartype, bartype)', GNU C++ uses `foo__7bartypeT0', while compilers following the ARM example generate `foo__7bartypeT1'.
• GNU C++ does not bother using the space-saving methods for types whose encoding is a single character (like an integer, encoded as `i'). This is useful in the most common cases (two ints would result in using three letters, instead of just `ii').