BigInt Problem

/* Class used for testing my own BigInt library
* Stores tested numbers as string,
* because some tests are on numbers bigger than int
* result vector is empty when test is auto-checked
* which happens when num1 and num2 aren't bigger than int,
* else you need to input exactly the required number of
* expected results into the vector result
* The format of the text for the operator>> is:
*   { label num1 num2 autoChecked{ result+ result- result* } }
*   { label num1 num2 autoChecked{ } }
* There have to be all operators' results given, or none
* The order of tests:
*   num1 + num2
*   num2 + num1
*   num1 - num2
*   num2 - num1
*   num1 * num2
*   num2 * num1
*/
struct TestBigInt
{
    void doTests(); // apply the operators (with BigInts)
    void autoFillExR(); // fills exResult for autochecking (i.e. convert to int, do calculations, and than back to string)

    static constexpr int amountOfResults = 6; // exactly this meany results have to be read to exResult
    std::string label;
    std::string num1;
    std::string num2;
    bool autoChecked;
    std::vector<std::string> exResult; // expected results
    // Expected results of operations in this order: +, -, *
    std::vector<std::string> outResult; // results output by big int calculations
};

std::istream& operator>>(std::istream& is, TestBigInt& t);

// Test the BigInt with the given test case
std::ostream& performTest(std::ostream& os, TestBigInt& t);

// Needed for auto-checking
int stringToI(const std::string& s);
std::string iToString(int a);

void TestBigInt::doTests()
{
    BigInt firstNum{ num1 };
    BigInt secNum{ num2 };
    // THESE FOUR LINES CAUSE THE BUG
//  outResult.push_back((firstNum + secNum).toString());
//  outResult.push_back((secNum + firstNum).toString());
//  outResult.push_back((firstNum - secNum).toString());
//  outResult.push_back((secNum - firstNum).toString());
    outResult.push_back((firstNum * secNum).toString());
    outResult.push_back((secNum * firstNum).toString());
    std::cout << "pushed_back outResults\n";
}

void TestBigInt::autoFillExR()
{
    int firstNum = stringToI(num1);
    int secNum = stringToI(num2);

    exResult.push_back(iToString(firstNum + secNum));
    exResult.push_back(iToString(secNum + firstNum));
    exResult.push_back(iToString(firstNum - secNum));
    exResult.push_back(iToString(secNum - firstNum));
    exResult.push_back(iToString(firstNum * secNum));
    exResult.push_back(iToString(secNum * firstNum));
    std::cout << "pushed_back exResults\n";
}

std::istream& operator>>(std::istream& is, TestBigInt& t)
/* Read input in the form:
*   { label num1 num2 autoChecked{ result+ result- result* } }
*   { label num1 num2 autoChecked{ } }
*/
{
    char ch1;
    is >> ch1;
    if (ch1 != '{') { // no tests anymore
        is.unget();
        is.clear(std::ios_base::failbit);
        return is;
    }

    char ch2;
    std::string lab;
    std::string firstNum;
    std::string secNum;
    bool check;
    is >> lab >> firstNum >> secNum >> check >> ch2;
    if ((ch2 != '{'))
        throw std::runtime_error("invalid formatting brace1");

    t.label = lab;
    t.num1 = firstNum;
    t.num2 = secNum;
    t.autoChecked = check;

    if (!(check)) { // not auto-checking; need to input expected results
        for (int i = 0; i < t.amountOfResults; ++i) {
            std::string s;
            is >> s;
            if (s == "}")
                throw std::runtime_error("invalid number of expected results");
            t.exResult.push_back(s);
        }
    }

    char ch3, ch4;
    is >> ch3 >> ch4;
    if ((ch3 != '}') || (ch4 != '}'))
        throw std::runtime_error("invalid formatting brace2");

    return is;
}

std::ostream& performTest(std::ostream& os, TestBigInt& t)
{
    if (t.autoChecked) // if the results are to be autochecked, than fill the exResult -- else it is already full
        t.autoFillExR();

    t.doTests();

    for (auto itE = t.exResult.cbegin(), itO = t.outResult.cbegin(); itE != t.exResult.cend() && itO != t.outResult.cend(); ++itE, ++itO)
        if (*itE != *itO)
            os << "Test not passed: " << t.label  << ", " << distance(t.exResult.cbegin(), itE) << "\n\tExpected: " << *itE << ", got " << *itO << "\n";

    return os;
}

int stringToI(const std::string& s)
{
    std::istringstream is{ s };
    int res;
    is >> res;
    return res;
}

std::string iToString(int a)
{
    std::ostringstream os;
    os << a;
    return (os.str());
}

int main()
{
    /* "BigIntTestCases.txt" - format { label num1 num2 autoChecked { } }, contents:
    * { 1 3 2 1 { } }
    * { 2 10 7 1 { } }
    * { 3 21 9 1 { } }
    * ...
    */
    ifstream ifs{ "BigIntTestCases.txt" };
    vector<TestBigInt> tests;
    for (TestBigInt t; ifs >> t; )
        tests.push_back(t);
    ofstream ofs{ "TestOutput.txt" };
    for (auto it = tests.begin(); it != tests.end(); ++it) {
        cout << "Read: " << it->label << ' ' << it->num1 << ' ' << it->num2 << ' ' << it->autoChecked << '\n';
        performTest(ofs, (*it));
    }
    std::cout << "Tests Complete! \n";

    return 0;
}