Question

I have a class with a printDebug method. Its not used anywhere in the code but I would like to use it when I am debugging with gdb (using call). This is basically to print the contents of the object in a nicely formatted way, for instance I may have a vector of sets. What is the g++ option to use for this? I have tried -O0 but that does not work.

The work around I used was to make a psuedo call in the constructor to debugPrint and provide a bool indicating if you actually want to print or do nothing. This works fine but there has to be a better way to do this.

If I understand correctly -O0 should not do any optimisations so dead code should not be eliminated but perhaps I am wrong.

Answer 1

If you have a method that is not used anywhere on the code gcc smart features can identify this and ignore it while compiling your application. That’s why when you display the symbols (using nm) of the application that method doesn’t show on the results.

However, if you want to force that method to be compiled anyway you need to specify the _attribute_ used on the method declaration. For instance:

  1 
  2 #include <iostream>
  3 #include <stdio.h>
  4 
  5 
  6 class aClass
  7 {
  8     public:
  9         void __attribute__ ((used)) publicPrint()
 10         {
 11             std::cout << "public method" << std::endl;
 12         }
 13 };
 14 
 15 
 16 int main()
 17 {
 18     aClass my_obj;
 19 
 20     getchar();
 21 }

For testing purposes I compiled this source code with -g:

g++ -g print_dbg.cpp -o print_dbg

What I’m about to say is probably unnecessary, but I’ll do it anyway: notice that my_obj is declared as a local variable inside main(). This means I only have access to the method publicPrint() while I’m debugging code inside this scope. When the code execution jumps to the beginning of getchar(), code execution will be at another scope, i.e. another stack frame, and my_obj will no longer exist in this new context. This is just a heads up.

On gdb, if you set a breakpoint where my_obj is valid, you can execute the method publicPrint() through: call my_obj.publicPrint()

$ gdb print_dbg 
GNU gdb (GDB) 7.1-ubuntu
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i486-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /home/karl/workspace/gdb/print_dbg...done.

(gdb) list main
12              }
13      };
14
15
16      int main()
17      {
18          aClass my_obj;
19
20          getchar();
21      }
(gdb) break main
Breakpoint 1 at 0x804871d: file print_dbg.cpp, line 20.

(gdb) run
Starting program: /home/karl/workspace/gdb/print_dbg 
Breakpoint 1, main () at print_dbg.cpp:20
20          getchar();

(gdb) call my_obj.publicPrint()
public method

(gdb)