[Bug c++/25957] New: Unable to call std::map::operator[] from gdb

[Bug c++/25957] New: Unable to call std::map::operator[] from gdb

[jengelh at inai dot de]

https://sourceware.org/bugzilla/show_bug.cgi?id=25957

            Bug ID: 25957
           Summary: Unable to call std::map::operator[] from gdb
           Product: gdb
           Version: 8.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
          Assignee: unassigned at sourceware dot org
          Reporter: jengelh at inai dot de
  Target Milestone: ---

# 7-liner test program
» g++ x.cpp -v -ggdb3 -O0 -fno-inline
gcc version 9.3.1 20200406 [revision 6db837a5288ee3ca5ec504fbd5a765817e556ac2]
(SUSE Linux) 
...
» gdb a.out 
GNU gdb (GDB; openSUSE Tumbleweed) 8.3.1
(gdb) b main
Breakpoint 1 at 0x11dd: file x.cpp, line 5.
(gdb) r
Starting program: /dev/shm/a.out 

Breakpoint 1, main () at x.cpp:5
5               z[99] = 42;
(gdb) l
1       #include <map>
2       std::map<int, int> z;
3       int main()
4       {
5               z[99] = 42;
6               return 0;
7       }
(gdb) p z[99]
Attempt to take address of value not located in memory.
(gdb) p z.operator[](99)
Attempt to take address of value not located in memory.

Which value is not in memory? And why is it not in memory? What more does one
need beside -O0 -ggdb3?

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[Bug c++/25957] Unable to call std::map::operator[] from gdb

[ssbssa at sourceware dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=25957

Hannes Domani <ssbssa at sourceware dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ssbssa at sourceware dot org

--- Comment #1 from Hannes Domani <ssbssa at sourceware dot org> ---
The argument of operator[] is a reference:
_Key & operator[](const _Key &);

A simpler example is:

int const_ref_arg(const int &x)
{
  return x;
}
int ref_arg(int &x)
{
  return x;
}
int y;
int main()
{
  return const_ref_arg(y) + ref_arg(y);
}

I'm not really surprised that it can't handle literals, since it would have to
create extra space on the stack, and pass its reference to the function:

(gdb) p const_ref_arg(0)
Attempt to take address of value not located in memory.

But I did think it would work with an existing variable:

(gdb) p const_ref_arg(y)
Attempt to take address of value not located in memory.

It works with the non-const variant:

(gdb) p ref_arg(y)
$1 = 0

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