//...
private:
    string _firm;
    string _type;
    int _memoryGB;
    int _quantity;
    string _date;
};

class FlashList
{
public:
    FlashList(vector<Flashka> list);
//...
private:
    vector<Flashka> _list;
};

//...
    vector<Flashka> FirmSort(string firm)
    {
        vector<Flashka> result;

        for (auto i = 0; i < _list.size(); ++i)
            if (firm == _list[i].getFirm())
                result.push_back(_list[i]);
        return result;
    }

TEST(FlashList, Sort)
{
    Flashka a("HP", "NOR", 32, 56, "21.11.2017");
    Flashka b("HP", "NAND", 64, 48, "01.11.2017");
    Flashka c("Kingston", "NOR", 16, 24, "02.11.2017");
    Flashka d("Transcend", "NAND", 16, 48, "21.11.2017");

    vector<Flashka> v;
    v.push_back(a);
    v.push_back(b);
    v.push_back(c);
    v.push_back(d);
    FlashList List(v);

    vector<Flashka> trew;
    trew.push_back(a);
    trew.push_back(b);

    EXPECT_TRUE(trew == List.FirmSort("HP"));
}