Printing the first element of a C++ std::set of strings in gdb?

Question

As Element at index in a std::set? explains, there is no "direct" random access by index in a std::set - so, here I'm trying to use it's .begin() method that returns an iterator... Here is a simple minimal example:

// g++ --std=c++11 -g test.cpp -o test.exe

#include <iostream>
#include <set>

int main()
{
  std::set<std::string> my_set;
  my_set.insert("AA");
  my_set.insert("BB");
  std::cout << "Hello World!" << std::endl;
  return 0;
}

What I ultimately want to do is use a gdb dprintf type break in my actual problem - I would like to not change the code, and thus not add additional iterator variables. Since dprintf uses format specifiers, where %s is for a C-style string, I ultimately need not just a reference to the first element of my_set, but I'd also need to call .c_str() on it - and all of this in a one-liner. (in my actual problem, I have a std::set of a custom String class, which has a custom method for getting a C-style string).

The problem is, I cannot find the right syntax to access the first element:

$ gdb --args ./test.exe
GNU gdb (Ubuntu 7.7.1-0ubuntu5~14.04.2) 7.7.1
...
Reading symbols from ./test.exe...done.
(gdb) b test.cpp:11
Breakpoint 1 at 0x8048bf8: file test.cpp, line 11.
(gdb) r
Starting program: /tmp/test.exe 

Breakpoint 1, main () at test.cpp:11
11    std::cout << "Hello World!" << std::endl;
(gdb) p my_set
$1 = std::set with 2 elements = {[0] = "AA", [1] = "BB"}
(gdb) p my_set.begin()
Cannot evaluate function -- may be inlined
(gdb) p my_set->begin()
Cannot resolve method std::set<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >::begin to any overloaded instance
(gdb) printf "%s", my_set.begin()
Cannot evaluate function -- may be inlined

Is it possible to print the first value of a std::set in a context like this using gdb's printf, without having to change the code (for instance, to add iterator variables)?

it seems like you may adapt script to dump STL containers content, https://gist.github.com/skyscribe/3978082 — Andrey Starodubtsev, Nov 16 '15 at 18:22
Many thanks for that, @AndreyStarodubtsev - however, I still cannot access the first element, `.begin()` still gives "Cannot evaluate function -- may be inlined", `my_set[0]` still gives "No symbol "operator[]" in current context." I see there is a `pset` command, but cannot get it to work either. Any suggestions? — sdaau, Nov 16 '15 at 18:30
I could be miserably wrong but I guess compiler needs to see a function usage in oder to put it inside the executable. Try calling my_set.begin() anywhere in the code to see if it works — Amadeus, Nov 16 '15 at 18:36
Thanks, @Amadeus - you are, in fact, absolutely right: I added some usage as `std::set::iterator my_iter; my_iter = my_set.begin(); cout << &my_iter;` and that indeed made `my_set.begin()` print in `gdb`! But that seemingly requires code changes, which I'd like to avoid... — sdaau, Nov 16 '15 at 18:47

score 0 · Answer 1 · answered Nov 16 '15 at 19:03

Well, I got somewhere, thanks to both comments from @AndreyStarodubtsev and @Amadeus.

For one, as @Amadeus noted - it turns out, that indeed you must have some sort of a reference to a type in the program, otherwise it may be optimised away. So the OP example, as it is, when debugged will give this:

(gdb) p ((std::string*)(my_set._M_t._M_impl._M_header._M_left+1))->c_str()
A syntax error in expression, near `)(my_set._M_t._M_impl._M_header._M_left+1))->c_str()'.
(gdb) p std::string
No symbol "string" in namespace "std".

... however, just a variable reference seems enough to fix that:

// g++ --std=c++11 -g test.cpp -o test.exe

#include <iostream>
#include <set>

std::string aa; // just to have reference to std::string

int main()
{
  std::set<std::string> my_set;
  my_set.insert("AA");
  my_set.insert("BB");
  std::cout << "Hello World!" << std::endl;
  return 0;
}

... and while this is code change, I do not have to add new lines of actual code referring to iterators inside. Then, a gdb session looks like this:

$ gdb --args ./test.exe
...
Reading symbols from ./test.exe...done.
(gdb) b 13
Breakpoint 1 at 0x8048c38: file test.cpp, line 13.
(gdb) r
Starting program: /tmp/test.exe 

Breakpoint 1, main () at test.cpp:13
13    std::cout << "Hello World!" << std::endl;
(gdb) p ((std::string*)(my_set._M_t._M_impl._M_header._M_left+1))->c_str()
$1 = 0x804d014 "AA"
(gdb) printf "'%s'\n", ((std::string*)(my_set._M_t._M_impl._M_header._M_left+1))->c_str()
'AA'

And the syntax to access the first node of my_set (which is apparently a "red-black tree" of nodes), I consulted the https://gist.github.com/skyscribe/3978082 gdb script noted by @AndreyStarodubtsev, in particular the function pset (which for some reason tended to give me syntax errors - possibly because of the optimized reference to std::string)

You can try putting `std::string aa;` inside a new file, compiles it, and make an executable linking both files, this new one and the test.cpp. In this way, you will not need to change test.cpp — Amadeus, Nov 16 '15 at 19:14

Printing the first element of a C++ std::set of strings in gdb?

1 Answers1