How can I get specific std::map value with indexing while using gdb for debuggin c++ code?

Question

I use Ubuntu 20.04-LTS with WSL(Windows Subsystem Linux), GDB version is 9.2, and I builded my c++ code with c++11.

I tried to access std::map's value with index in GDB, however GDB showed error message "Invalid cast".

My code is same for below

#include <iostream>
#include <map>
#include <string>

using std::cout;
using std::map;
using std::string;
using std::to_string;

int main()
{
    map<string, int> si;

    for(int i = 0; i < 10; ++i)
    {
        si[to_string(i)] = i;
    }

    for(int i = 0; i < 10; ++i)
    {
        cout << si[to_string(i)] << "\n";
    }
}

And, in GDB

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Reading symbols from ctest...
(gdb) l main
6       using std::map;
7       using std::string;
8       using std::to_string;
9
10      int main()
11      {
12              map<string, int> si;
13
14              for(int i = 0; i < 10; ++i)
15              {
(gdb) l 15
10      int main()
11      {
12              map<string, int> si;
13
14              for(int i = 0; i < 10; ++i)
15              {
16                      si[to_string(i)] = i;
17              }
18
19              for(int i = 0; i < 10; ++i)
(gdb) b 18
Breakpoint 1 at 0x2579: file ctest.cpp, line 19.
(gdb) r
Starting program: /home/lksj/ctest

Breakpoint 1, main () at ctest.cpp:19
19              for(int i = 0; i < 10; ++i)
(gdb) p si["1"]
Invalid cast.
(gdb)

How can I access the std::map object value directly with index in GDB?

Answer 1

You first write the below code in the ".gdbinit" file.

define newstr
set ($arg0)=(std::string*)malloc(sizeof(std::string))
call ($arg0)->basic_string()
# 'assign' returns *this; casting return to void avoids printing of the 
struct.
call (void)( ($arg0)->assign($arg1) )
end

define delstr
call ($arg0)->~basic_string(0)
#                           ^
call free($arg0)
set ($arg0)=(void*)0
end

And then write the below function in your c++ code.

void indexing_map( map<string,[type]> & m, string i )
{
    cout << m[i] << "\n";
}

Now, all things are ready. You just execute gdb for debugging your code with the

newstr $foo [any string of map of the key]

For example,

example.cpp

#include <iostream>
#include <unordered_map>
#include <string>

using std::cout;
using std::unordered_map;
using std::string;

void indexing_map(unordered_map<string, int> &m, string i)
{
   cout << m[i] << "\n";
}

int main()
{
    unordered_map<string, int> st_in;
    st_in["1"] = 1;
    st_in["2"] = 2;
    st_in["3"] = 3;

    indexing_map(st_in, "1");

    return 0;
}

and gdb

(gdb) b 25
Breakpoint 1 at 0x28d6: file ctest.cpp, line 25.
(gdb) r
Starting program: /home/user/ctest
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Breakpoint 1, main () at ctest.cpp:25
25          st_in["3"] = 3;
(gdb) newstr $foo "1"
(gdb) call indexing_map(st_in , *$foo)
1