C++ operator>> overloading issue: change object only once - c++

Question

i have the following Vector3D code, and for some reason cin is able to change the object values only once. do you have any idea why?
i guess it's something related to the 'const' definition but i'm not sure.
adding both code (partial) and test
// code:
class Vector3D
{
private:
double _x, _y, _z;
public:
Vector3D() : _x(0), _y(0), _z(0) {};
Vector3D(double x, double y, double z) : _x(x), _y(y), _z(z) {};
Vector3D(const double parm[DIMENSION]) : _x(parm[0]), _y(parm[1]),
_z(parm[2]) {};
const Vector3D operator+(const Vector3D &other) const;
Vector3D &operator+=(const Vector3D &other);
const double &operator[](int idx) const;
double &operator[](int idx);
friend ostream &operator<<(ostream &out, const Vector3D &c);
friend istream &operator>>(istream &in, Vector3D &c);
};
const Vector3D Vector3D::operator+(const Vector3D &other) const {
return Vector3D(*this) += other;
}
Vector3D &Vector3D::operator+=(const Vector3D &other) {
(*this)._x += other._x;
(*this)._y += other._y;
(*this)._z += other._z;
return *this;
}
const double &Vector3D::operator[](int idx) const {
assert(idx >= 0 && idx < DIMENSION);
switch (idx)
{
case 0:
return _x;
case 1:
return _y;
case 2:
return _z;
default:
return _x; // not reachable
}
}
double &Vector3D::operator[](int idx)
{
assert(idx >= 0 && idx < DIMENSION);
switch (idx)
{
case 0:
return _x;
case 1:
return _y;
case 2:
return _z;
default:
return _x; // not reachable
}
}
ostream &operator<<(ostream &out, const Vector3D &c)
{
out << c._x << " " << c._y << " " << c._z;
return out;
}
istream &operator>>(istream &in, Vector3D &c)
{
in >> c._x >> c._y >> c._z;
return in;
}
// test:
void readFromStreamTest(int &tests) {
std::stringstream os;
os << 8.0 << " " << 3.0 << " " << 4.0;
Vector3D a;
os >> a;
os << a; // "8 3 4" as should be
os.str(std::string());
os << 2.1 << " " << 3.1 << " " << 4.1;
os >> a;
os.str(std::string());
os << a;
if (os.str()!=("2.1 3.1 4.1"))
{
std::cerr << "failed. should be 2.1 3.1 4.1 but was " << a;
}
}
test results: "failed. should be 2.1 3.1 4.1 but was 8 3 4" (old values!!)

Answer 1

You cannot use a stringstream like that, after extracting the contain throw os >> you cannot again modify it by os.str(value) nor os << if you don't call os.clear() before, for instance
void readFromStreamTest(int &tests) {
std::stringstream os;
os << 8.0 << " " << 3.0 << " " << 4.0;
Vector3D a;
os >> a;
os.clear();
os << a; // "8 3 4" as should be
os.str(std::string());
os << 2.1 << " " << 3.1 << " " << 4.1;
os >> a;
os.str(std::string());
os.clear();
os << a;
if (os.str()!=("2.1 3.1 4.1"))
{
std::cerr << "failed. should be 2.1 3.1 4.1 but was " << a;
}
}
P.S. I modified your original code to make it compilable