| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- Copyright (C) 1992-2015 Free Software Foundation, Inc. |
| Contributed by Cygnus Support. Written by Julia Menapace, Jim Kingdon, |
| and David MacKenzie. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation; with no |
| Invariant Sections, with no Front-Cover Texts, and with no Back-Cover |
| Texts. A copy of the license is included in the section entitled "GNU |
| Free Documentation License". --> |
| <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ --> |
| <head> |
| <title>STABS: Virtual Methods</title> |
| |
| <meta name="description" content="STABS: Virtual Methods"> |
| <meta name="keywords" content="STABS: Virtual Methods"> |
| <meta name="resource-type" content="document"> |
| <meta name="distribution" content="global"> |
| <meta name="Generator" content="makeinfo"> |
| <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> |
| <link href="index.html#Top" rel="start" title="Top"> |
| <link href="Symbol-Types-Index.html#Symbol-Types-Index" rel="index" title="Symbol Types Index"> |
| <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> |
| <link href="Cplusplus.html#Cplusplus" rel="up" title="Cplusplus"> |
| <link href="Inheritance.html#Inheritance" rel="next" title="Inheritance"> |
| <link href="Method-Modifiers.html#Method-Modifiers" rel="prev" title="Method Modifiers"> |
| <style type="text/css"> |
| <!-- |
| a.summary-letter {text-decoration: none} |
| blockquote.smallquotation {font-size: smaller} |
| div.display {margin-left: 3.2em} |
| div.example {margin-left: 3.2em} |
| div.indentedblock {margin-left: 3.2em} |
| div.lisp {margin-left: 3.2em} |
| div.smalldisplay {margin-left: 3.2em} |
| div.smallexample {margin-left: 3.2em} |
| div.smallindentedblock {margin-left: 3.2em; font-size: smaller} |
| div.smalllisp {margin-left: 3.2em} |
| kbd {font-style:oblique} |
| pre.display {font-family: inherit} |
| pre.format {font-family: inherit} |
| pre.menu-comment {font-family: serif} |
| pre.menu-preformatted {font-family: serif} |
| pre.smalldisplay {font-family: inherit; font-size: smaller} |
| pre.smallexample {font-size: smaller} |
| pre.smallformat {font-family: inherit; font-size: smaller} |
| pre.smalllisp {font-size: smaller} |
| span.nocodebreak {white-space:nowrap} |
| span.nolinebreak {white-space:nowrap} |
| span.roman {font-family:serif; font-weight:normal} |
| span.sansserif {font-family:sans-serif; font-weight:normal} |
| ul.no-bullet {list-style: none} |
| --> |
| </style> |
| |
| |
| </head> |
| |
| <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> |
| <a name="Virtual-Methods"></a> |
| <div class="header"> |
| <p> |
| Next: <a href="Inheritance.html#Inheritance" accesskey="n" rel="next">Inheritance</a>, Previous: <a href="Method-Modifiers.html#Method-Modifiers" accesskey="p" rel="prev">Method Modifiers</a>, Up: <a href="Cplusplus.html#Cplusplus" accesskey="u" rel="up">Cplusplus</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Types-Index.html#Symbol-Types-Index" title="Index" rel="index">Index</a>]</p> |
| </div> |
| <hr> |
| <a name="Virtual-Methods-1"></a> |
| <h3 class="section">8.11 Virtual Methods</h3> |
| |
| <p><< The following examples are based on a4.C >> |
| </p> |
| <p>The presence of virtual methods in a class definition adds additional |
| data to the class description. The extra data is appended to the |
| description of the virtual method and to the end of the class |
| description. Consider the class definition below: |
| </p> |
| <div class="example"> |
| <pre class="example">class A { |
| public: |
| int Adat; |
| virtual int A_virt (int arg) { return arg; }; |
| }; |
| </pre></div> |
| |
| <p>This results in the stab below describing class A. It defines a new |
| type (20) which is an 8 byte structure. The first field of the class |
| struct is ‘<samp>Adat</samp>’, an integer, starting at structure offset 0 and |
| occupying 32 bits. |
| </p> |
| <p>The second field in the class struct is not explicitly defined by the |
| C<tt>++</tt> class definition but is implied by the fact that the class |
| contains a virtual method. This field is the vtable pointer. The |
| name of the vtable pointer field starts with ‘<samp>$vf</samp>’ and continues with a |
| type reference to the class it is part of. In this example the type |
| reference for class A is 20 so the name of its vtable pointer field is |
| ‘<samp>$vf20</samp>’, followed by the usual colon. |
| </p> |
| <p>Next there is a type definition for the vtable pointer type (21). |
| This is in turn defined as a pointer to another new type (22). |
| </p> |
| <p>Type 22 is the vtable itself, which is defined as an array, indexed by |
| a range of integers between 0 and 1, and whose elements are of type |
| 17. Type 17 was the vtable record type defined by the boilerplate C<tt>++</tt> |
| type definitions, as shown earlier. |
| </p> |
| <p>The bit offset of the vtable pointer field is 32. The number of bits |
| in the field are not specified when the field is a vtable pointer. |
| </p> |
| <p>Next is the method definition for the virtual member function <code>A_virt</code>. |
| Its description starts out using the same format as the non-virtual |
| member functions described above, except instead of a dot after the |
| ‘<samp>A</samp>’ there is an asterisk, indicating that the function is virtual. |
| Since is is virtual some addition information is appended to the end |
| of the method description. |
| </p> |
| <p>The first number represents the vtable index of the method. This is a |
| 32 bit unsigned number with the high bit set, followed by a |
| semi-colon. |
| </p> |
| <p>The second number is a type reference to the first base class in the |
| inheritance hierarchy defining the virtual member function. In this |
| case the class stab describes a base class so the virtual function is |
| not overriding any other definition of the method. Therefore the |
| reference is to the type number of the class that the stab is |
| describing (20). |
| </p> |
| <p>This is followed by three semi-colons. One marks the end of the |
| current sub-section, one marks the end of the method field, and the |
| third marks the end of the struct definition. |
| </p> |
| <p>For classes containing virtual functions the very last section of the |
| string part of the stab holds a type reference to the first base |
| class. This is preceded by ‘<samp>~%</samp>’ and followed by a final semi-colon. |
| </p> |
| <div class="display"> |
| <pre class="display">.stabs "class_name(A):type_def(20)=sym_desc(struct)struct_bytes(8) |
| field_name(Adat):type_ref(int),bit_offset(0),field_bits(32); |
| field_name(A virt func ptr):type_def(21)=type_desc(ptr to)type_def(22)= |
| sym_desc(array)index_type_ref(range of int from 0 to 1); |
| elem_type_ref(vtbl elem type), |
| bit_offset(32); |
| meth_name(A_virt)::typedef(23)=sym_desc(method)returning(int); |
| :arg_type(int),protection(public)normal(yes)virtual(yes) |
| vtable_index(1);class_first_defining(A);;;~%first_base(A);", |
| N_LSYM,NIL,NIL,NIL |
| </pre></div> |
| |
| <div class="example"> |
| <pre class="example">.stabs "A:t20=s8Adat:1,0,32;$vf20:21=*22=ar1;0;1;17,32; |
| A_virt::23=##1;:i;2A*-2147483647;20;;;~%20;",128,0,0,0 |
| </pre></div> |
| |
| <hr> |
| <div class="header"> |
| <p> |
| Next: <a href="Inheritance.html#Inheritance" accesskey="n" rel="next">Inheritance</a>, Previous: <a href="Method-Modifiers.html#Method-Modifiers" accesskey="p" rel="prev">Method Modifiers</a>, Up: <a href="Cplusplus.html#Cplusplus" accesskey="u" rel="up">Cplusplus</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Types-Index.html#Symbol-Types-Index" title="Index" rel="index">Index</a>]</p> |
| </div> |
| |
| |
| |
| </body> |
| </html> |
