How to call operator<< functions?

Michael Veksler
2006-08-30T10:11:50Z

Hello,

I have searched the web for an answer without success. The following
problem does not seem to exist. Either my configuration is broken in a
unique way or my requirements are considered weird and outrageous (so no
one tries to do similar things). Read on and tell me what you think.

I have a complex data structure MyClass (e.g. a parse tree) which I want
to examine in GDB. It is impractical to traverse and print it node by
node from GDB. For that I have written both MyClass::Print(ostream&)
and operator<< that invokes this Print. These method and operator can
print the data structure in a very nice and readable form. I can use
them to print the data into some stream when my application operates in
verbose mode.

Now I want to invoke the operator<<(ostream& , MyClass const &) function,
or alternatively the MyClass::Print(ostream&) const -- from GDB;

This does not work even with gcb-6.5, not to mention older versions. The
compiler RedHat's gcc-3.4.

Passing 'std::cout' seems impossible. Any attempt to pass std::cout
crashes. To overcome this I define my own global
ostream gecLog(cout.rdbuf());
(or something similar) and recompile the code. This lets me pass gecLog
instead of cout, and it _sometimes_ works:

(gdb) p pd.Print(gecLog)
Cannot resolve method (null)Print to any overloaded instance
-----------
Now, going through function pointers helps for some weird reason:

(gdb) p pd.Print
$13 = &MyClass::Print(std::ostream&) const
(gdb) p $13(&pd, gecLog)
Enter Print()
(gdb) p $15(&pd, std::cout)

Program received signal SIGSEGV, Segmentation fault.
0x0042cf88 in std::ostream::sentry::sentry () from /usr/lib/libstdc++.so.6
The program being debugged was signaled while in a function called from GDB.
GDB remains in the frame where the signal was received.
To change this behavior use "set unwindonsignal on"
Evaluation of the expression containing the function
(MyClass::Print(std::ostream&) const) will be abandoned.

--------------------------------------
I wanted to do the same for operator<<:
(gdb) p 'operator<<(std::ostream&, MyClass const&)'
$17 = {ostream &(ostream &, const class MyClass
&)} 0x8068a00 <operator<<(std::ostream&, MyClass const&)>
(gdb) p $17(gecLog, *pd)
Program received signal SIGSEGV, Segmentation fault.
-----------------------

Now, I wanted to write a GDB macro to call Print for me. When I try to use
p $-1(&pd, gecLog)
I get a SIGSEGV (when with p $14(....) it would "just" work).
---------------------

Is there any way to overcome any of the above problems? All I want is to
pass a stream such as cerr/cout/clog to a printing function, is that
unreasonable?

Thanks
Michael

P.S.
To overcome this, I am writing a method MyClass::DebugPrint() that calls
Print(cerr). Sometimes, while debugging a piece of code, that a vital
object has <<, but no DebugPrint, and *that* really frustrates me -
especially when I loose a 60 minutes debugging session to a SIGSEGV.