| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- Copyright (C) 1988-2015 Free Software Foundation, Inc. |
| |
| 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 the |
| Invariant Sections being "Free Software" and "Free Software Needs |
| Free Documentation", with the Front-Cover Texts being "A GNU Manual," |
| and with the Back-Cover Texts as in (a) below. |
| |
| (a) The FSF's Back-Cover Text is: "You are free to copy and modify |
| this GNU Manual. Buying copies from GNU Press supports the FSF in |
| developing GNU and promoting software freedom." --> |
| <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ --> |
| <head> |
| <title>Debugging with GDB: Xmethods In Python</title> |
| |
| <meta name="description" content="Debugging with GDB: Xmethods In Python"> |
| <meta name="keywords" content="Debugging with GDB: Xmethods In Python"> |
| <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="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> |
| <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> |
| <link href="Python-API.html#Python-API" rel="up" title="Python API"> |
| <link href="Xmethod-API.html#Xmethod-API" rel="next" title="Xmethod API"> |
| <link href="Unwinding-Frames-in-Python.html#Unwinding-Frames-in-Python" rel="prev" title="Unwinding Frames in Python"> |
| <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="Xmethods-In-Python"></a> |
| <div class="header"> |
| <p> |
| Next: <a href="Xmethod-API.html#Xmethod-API" accesskey="n" rel="next">Xmethod API</a>, Previous: <a href="Unwinding-Frames-in-Python.html#Unwinding-Frames-in-Python" accesskey="p" rel="prev">Unwinding Frames in Python</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> |
| </div> |
| <hr> |
| <a name="Xmethods-In-Python-1"></a> |
| <h4 class="subsubsection">23.2.2.13 Xmethods In Python</h4> |
| <a name="index-xmethods-in-Python"></a> |
| |
| <p><em>Xmethods</em> are additional methods or replacements for existing |
| methods of a C<tt>++</tt> class. This feature is useful for those cases |
| where a method defined in C<tt>++</tt> source code could be inlined or |
| optimized out by the compiler, making it unavailable to <small>GDB</small>. |
| For such cases, one can define an xmethod to serve as a replacement |
| for the method defined in the C<tt>++</tt> source code. <small>GDB</small> will |
| then invoke the xmethod, instead of the C<tt>++</tt> method, to |
| evaluate expressions. One can also use xmethods when debugging |
| with core files. Moreover, when debugging live programs, invoking an |
| xmethod need not involve running the inferior (which can potentially |
| perturb its state). Hence, even if the C<tt>++</tt> method is available, it |
| is better to use its replacement xmethod if one is defined. |
| </p> |
| <p>The xmethods feature in Python is available via the concepts of an |
| <em>xmethod matcher</em> and an <em>xmethod worker</em>. To |
| implement an xmethod, one has to implement a matcher and a |
| corresponding worker for it (more than one worker can be |
| implemented, each catering to a different overloaded instance of the |
| method). Internally, <small>GDB</small> invokes the <code>match</code> method of a |
| matcher to match the class type and method name. On a match, the |
| <code>match</code> method returns a list of matching <em>worker</em> objects. |
| Each worker object typically corresponds to an overloaded instance of |
| the xmethod. They implement a <code>get_arg_types</code> method which |
| returns a sequence of types corresponding to the arguments the xmethod |
| requires. <small>GDB</small> uses this sequence of types to perform |
| overload resolution and picks a winning xmethod worker. A winner |
| is also selected from among the methods <small>GDB</small> finds in the |
| C<tt>++</tt> source code. Next, the winning xmethod worker and the |
| winning C<tt>++</tt> method are compared to select an overall winner. In |
| case of a tie between a xmethod worker and a C<tt>++</tt> method, the |
| xmethod worker is selected as the winner. That is, if a winning |
| xmethod worker is found to be equivalent to the winning C<tt>++</tt> |
| method, then the xmethod worker is treated as a replacement for |
| the C<tt>++</tt> method. <small>GDB</small> uses the overall winner to invoke the |
| method. If the winning xmethod worker is the overall winner, then |
| the corresponding xmethod is invoked via the <code>__call__</code> method |
| of the worker object. |
| </p> |
| <p>If one wants to implement an xmethod as a replacement for an |
| existing C<tt>++</tt> method, then they have to implement an equivalent |
| xmethod which has exactly the same name and takes arguments of |
| exactly the same type as the C<tt>++</tt> method. If the user wants to |
| invoke the C<tt>++</tt> method even though a replacement xmethod is |
| available for that method, then they can disable the xmethod. |
| </p> |
| <p>See <a href="Xmethod-API.html#Xmethod-API">Xmethod API</a>, for API to implement xmethods in Python. |
| See <a href="Writing-an-Xmethod.html#Writing-an-Xmethod">Writing an Xmethod</a>, for implementing xmethods in Python. |
| </p> |
| <hr> |
| <div class="header"> |
| <p> |
| Next: <a href="Xmethod-API.html#Xmethod-API" accesskey="n" rel="next">Xmethod API</a>, Previous: <a href="Unwinding-Frames-in-Python.html#Unwinding-Frames-in-Python" accesskey="p" rel="prev">Unwinding Frames in Python</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> |
| </div> |
| |
| |
| |
| </body> |
| </html> |
