Adding new Python example

gdb-best-friend.tex

@@ -376,12 +376,66 @@ CallerIs()
 (gdb) source caller_is.py
 (gdb) break foo if $caller_is ("bar")
 			\end{lstlisting}
-			\end{block}
+		\end{block}
 		\item{Internal variables and functions on \gdb{} start with \verb|$|.}
 		\item{\texttt{caller\_is.py} is part of upstream \gdb{}.}
 	\end{itemize}
 \end{frame}
 
+\begin{frame}[fragile]
+	\frametitle{\python{}$^5$}
+	\begin{itemize}
+		\item{Problem: \texttt{std::string s = "Hello, World";}}
+		\begin{block}{Printing ``s'' inside \gdb{}}
+			\tiny
+			\begin{lstlisting}[language=sh,showstringspaces=false]
+(gdb) print s
+$1 = {static npos = <optimized out>,
+      _M_dataplus = {<std::allocator<char>> =
+      {<__gnu_cxx::new_allocator<char>> = {<No data fields>},
+      <No data fields>}, _M_p = 0x602028 ``Hello, World''}}
+			\end{lstlisting}
+		\end{block}
+		\pause
+		\item{\gdb{} knows about \verb|_M_dataplus -> _M_p|.  How to tell that this is the only field you are interested in?}
+		\pause
+		\item{Python!}
+	\end{itemize}
+\end{frame}
+
+\begin{frame}[fragile]
+	\frametitle{\python{}$^6$}
+		\begin{block}{print-std-string.py}
+		\tiny
+		\begin{lstlisting}[language=python,showstringspaces=false]
+class StdStringPrinter:
+    "Print a std::basic_string of some kind"
+
+   def __init__(self, typename, val):
+        self.val = val
+
+    def to_string(self):
+        # ... snip ...
+
+        # Calculate the length of the string so that to_string returns
+        # the string according to length, not according to first null
+        # encountered.
+        (*\textcolor{red}{ptr = self.val ['\_M\_dataplus']['\_M\_p']}*)
+        realtype = type.unqualified ().strip_typedefs ()
+        reptype = gdb.lookup_type (str (realtype) + '::_Rep').pointer ()
+        header = ptr.cast(reptype) - 1
+        len = header.dereference ()['_M_length']
+        if hasattr(ptr, "lazy_string"):
+            return ptr.lazy_string (length = len)
+        return ptr.string (length = len)
+		\end{lstlisting}
+		\end{block}
+	\begin{itemize}
+		\pause
+		\item{Present on \verb|libstdcxx| upstream}
+	\end{itemize}
+\end{frame}
+
 \section{Conclusion}
 \begin{frame}[fragile]
 	\frametitle{Conclusion}