"Cannot evaluate function -- may be in-lined" error in GDB for STL template container

I want to be able to get the address and print a single pair from an STL container using GDB.

E.g., given the following toy program:

#include <map>

int main() 
{
  std::map<int,int> amap;
  amap.insert(std::make_pair(1,2));

}

which I compile as:

g++ -ggdb3 -O0 -std=c++11 -Wall -Wextra -pedantic -o main.out main.cpp

Then, when I try to examine a single element of map, for example:

p amap.begin()

I get:

"Cannot evaluate function -- may be in-lined" 

Why is this happening and how do I work around it?

Tested in Ubuntu 20.04, GCC 9.3.0, 2.34.

Solution 1:

This is because amap.begin() does not exist in resulting binary. This is how C++ templates work: if you don't use or explicitly instantiate some template method it is not generated in resulting binary.

If you do want to call amap.begin() from gdb you have to instantiate it. One way to do it is to instantiate all methods of std::map:

#include <map>

template class std::map<int,int>;

int main()
{
  std::map<int,int> amap;
  amap.insert(std::make_pair(1,2));
}

gdb session:

(gdb) p amap.begin()
$1 = {first = 1, second = 2}

Solution 2:

@ks1322 has the correct answer. Here are some additional information that may be useful in the future.

Only the constructor, destructor and insert methods on std::map are in the debuginfo:

(gdb) info functions std::map
All functions matching regular expression "std::map":

File /usr/include/c++/6/bits/stl_map.h:
std::pair<std::_Rb_tree_iterator<std::pair<int const, int> >, bool> std::map<int, int, std::less<int>, std::allocator<std::pair<int const, int> > >::insert<std::pair<int, int>, void>(std::pair<int, int>&&);
void std::map<int, int, std::less<int>, std::allocator<std::pair<int const, int> > >::map();
void std::map<int, int, std::less<int>, std::allocator<std::pair<int const, int> > >::~map();

Still, we can call both the size and empty methods:

(gdb) p amap.size()
$1 = 1
(gdb) p amap.empty()
$2 = false

That's because gdb has something called xmethods, a python API for calling mockup functions meant to work identically to the functions that have not been instantiated. The libstdc++ xmethods can be found here. If we disable them, then the same error message appears:

(gdb) disable xmethod
(gdb) p amap.size()
Cannot evaluate function -- may be inlined
(gdb) p amap.empty()
Cannot evaluate function -- may be inlined
(gdb)