this pointer gets set to null when entering function call stack - happening with Google Test under gdb

(gdb) list 95, 195
95      int BishopArranger::FillAndRecurse(int cursor)
96      {
97              if (cursor == _solutionVec.size())
98              {
99                      return 1;
100             }
101
102             unordered_set<tuple<int, int>> candidates = GetCandidates(cursor); //kn
103             if (candidates.empty())
104             {
105                     return 0;
106             }
107
108             int sum = 0;
109             for (unordered_set<tuple<int, int>>::iterator it = candidates.begin(); it != candidates.end(); it++) //n^2
110             {
111                     _solutionVec[cursor] = *it;
112                     sum += FillAndRecurse(cursor+1); //k recursions, each being kn
113             }
114             //kn^3
115             return sum;
116     }
117
118     void ConstructBoard(int k, int n)
119     {
120             BOARD = unordered_set<tuple<int, int>>();
121             for (int i=0; i<n; i++)
122             {
123                     for (int j=0; j<n; j++)
124                     {
125                             BOARD.insert(tuple<int, int>(i, j));
126                     }
127             }
128     }
129
130     int BishopArranger::Solution(int k, int n)
131     {
132             if (n == 0)
133             {
134                     return 0;
135             }
136
137             ConstructBoard(k, n);
138             _solutionVec = vector<tuple<int, int>>(k);
139             return FillAndRecurse(0);
140     }
(gdb) next
102             unordered_set<tuple<int, int>> candidates = GetCandidates(cursor); //kn
(gdb) print this
$2 = (BishopArranger * const) 0x22a5f0
(gdb) step

Program received signal SIGILL, Illegal instruction.
0x0000000100401bb7 in BishopArranger::GetCandidates (this=0x0, cursor=0)
    at ./src/bishop_arranger/BishopArranger.cc:84
84      {

I wrote some code and ran a few (Google) tests on it, only to find that it failed: the program finished and gave an incorrect output. So naturally I fired up gdb and tried to debug my tests.

Interestingly, right before I stepped into GetCandidates(int cursor) function, on line 102, the 'this' pointer was good:

(gdb) print this

$2 = (BishopArranger * const) 0x22a5f0

then I did a 'step' into the function and it immediately gave me an exception complaining 'this' was null(this=0x0):

(gdb) step

Program received signal SIGILL, Illegal instruction.
0x0000000100401bb7 in BishopArranger::GetCandidates (this=0x0, cursor=0)
    at ./src/bishop_arranger/BishopArranger.cc:84
84      {

How absurd is that? I know for sure this only happened when I debugged through my test cases written with Google Test, not when I ran them:

[ RUN      ] Solution.TinyBoardTinyK
./test/bishop_arranger/UnitTests.cc:14: Failure
Value of: bishopArranger.Solution(2, 2)
  Actual: 12
Expected: 4
[  FAILED  ] Solution.TinyBoardTinyK (0 ms)

As you see, when I ran the Google tests, an incorrect output 12 was produced. Although the program failed to pass the assertion, it finished with no exception.

Some info you might be interested in:

1, There's only one thread in my program, although Google Test makes use of pthreads to parallelize execution of tests, according to their documentation(it says there's no guarantee certain tests finish before the others).

Not sure this is relevant in my case, as each of my test has its own object and there's no common setup/teardown parts so it's not likely for the object in one test to be destructed by another test. They look like:

TEST(Solution, BoundaryCondition)
{
    BishopArranger bishopArranger;
    EXPECT_EQ(0, bishopArranger.Solution(0, 0));
}


TEST(Solution, TinyBoardTinyK)
{
    BishopArranger bishopArranger;
    EXPECT_EQ(4, bishopArranger.Solution(2, 2));
}


TEST(Solution, SmallBoardSmallK)
{
    BishopArranger bishopArranger;
    EXPECT_EQ(260, bishopArranger.Solution(4, 4));
}

2, Platform: CYGWIN_NT-6.1; g++ used to compile Google Test and this program: 4.8.3(Yes, I followed the documentation of Google Test and compiled from source using the same version of compiler for my c++ programs to avoid potential problems); gdb version: 7.8

Thanks in advance for inputs!