c++ - can not print global objects in gdb

Question

I have this simple c++ code :

#include<bits/stdc++.h>

using namespace std;
vector<string> q;

int main()
{
    q.push_back("test1");
    q.push_back("test2");
    cout<<q.front();
    return 0;
}

When I use gdb to print variable q I get following error:

No symbol "q" in current context.

I compile my program using g++ like this:

 g++ -g a.cpp

And here is my gdb commands:

gdb a.out 
GNU gdb (GDB) 7.12
Copyright (C) 2016 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 "x86_64-pc-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
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For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from a.out...done.
(gdb) b 6
Breakpoint 1 at 0x400c6f: file a.cpp, line 6.
(gdb) r
Starting program: /home/mohammad/Desktop/a.out 

Breakpoint 1, main () at a.cpp:9
9       q.push_back("test");
(gdb) print q
No symbol "q" in current context.

Answer 1

This issue is somehow related to new gcc Dual ABI, introduced in gcc 5. gdb has poor support of these new C++11 ABI tags. See these bugs for example:

• https://sourceware.org/bugzilla/show_bug.cgi?id=19436
• https://sourceware.org/bugzilla/show_bug.cgi?id=18601

As a workaround you may disable this new gcc ABI by compiling with -D_GLIBCXX_USE_CXX11_ABI=0:

g++ -D_GLIBCXX_USE_CXX11_ABI=0 -g a.cpp