timestring class

/*** HPP ***/
class TimeString {
private:
    vector<string> str_vec;
    int _h = 0;
    int _m = 0;
    double _s = 0;
    double total_sec = 0;
    bool is_valid = false;
    string timestr;

    void TimeString::to_timestr(const string& str);
    void TimeString::to_timestr(double sec);
    void TimeString::build_string();

public:
    // constructor
    TimeString();
    TimeString(const string& str);
    TimeString(double sec);

    // assignment
    TimeString operator=(const char* str);
    bool operator()(string str);

    // conversion
    operator const string();
    operator const double();
    operator const bool();

    // comparison
    bool operator<(double b);
    bool operator>(double b);

    // getters
    double h();
    double m();
    double s();
};

/*** CPP ***/
void TimeString::to_timestr(const string& str) {
    if (str.empty()) return;
    if (timestr == str) return;
    timestr = str;
    is_valid = true;
    str_vec = split(str, ':');
    auto i = str_vec.end();

    // get seconds
    if (is_number(*--i)) {
        _s = ToI(*i);
        total_sec += _s;
    }
    else is_valid = false;

    if (is_valid == false || i == str_vec.begin()) return;

    // get minutes
    if (is_number(*--i)) {
        _m = ToI(*i);
        total_sec += _m * 60;
    }
    else is_valid = false;

    if (is_valid == false || i == str_vec.begin()) return;

    // get hours
    if (is_number(*--i)) {
        _h = ToI(*i);
        total_sec += _h * 3600;
    }
    else is_valid = false;

    return;
}

void TimeString::to_timestr(double sec) {
    is_valid = true;
    str_vec.resize(3, "0");
    total_sec = sec;

    _h = sec / 3600;
    _s = sec - _h * 3600;
    _m = _s / 60;
    _s = sec - _m * 60;
}

void TimeString::build_string() {
    if (!is_valid) return;
    if (timestr.size()) return;
    if (_h) timestr += ToS(_h) + ":";
    if (_m) timestr += ToS(_m) + ":";
    if (_s) timestr += ToS(_s) + ":";
    timestr.pop_back();
}

TimeString TimeString::operator=(const char * str) {
    to_timestr(str);
    return *this;
}

bool TimeString::operator()(string str) {
    to_timestr(str);
    return is_valid;
}

TimeString::operator const string() { build_string(); return timestr; }
TimeString::operator const double() { return total_sec; }
TimeString::operator const bool() { return is_valid; }

bool TimeString::operator<(double b) { return total_sec < b; }
bool TimeString::operator>(double b) { return total_sec > b; }

TimeString::TimeString() {};
TimeString::TimeString(const string& str) { to_timestr(str); }
TimeString::TimeString(double sec) { to_timestr(sec); }

double TimeString::h() { return _h; }
double TimeString::m() { return _m; }
double TimeString::s() { return _s; }