CODEWITHSUNDEEP
c++
Andrei_Furtuna
Andrei_Furtuna

Reputation: 21

C++ segment line intersection

i would like to ask where is the problem?

I got an error: invalid type argument of ‘unary *’

I am new in c++ programming, i use java style. Pointer and dereference is a big problem for me.

My application got input values and save as point object, after this i should find intersection of 2 lines.

I thought to return a Point object in which i will compute x and y value.

.h file

class Point {
public:
    double x_val, y_val;

    Point(double, double);

    double x();
    double y();

    double dist(Point other);

    Point add(Point b);
    Point sub(Point b);

    void move(double a, double b);



};



 class Triungle {
public:


        Triungle(std::string);
        void compute_length();
        void lines_intersect(Point a, Point b, Point c, Point d, Point *intersection);

        Point a, b, c;
};

.cpp file

Point::Point(double x = 0.0, double y = 0.0) {
    x_val = x;
    y_val = y;
}

double Point::x() {
    return x_val;
}

double Point::y() {
    return y_val;
}

double Point::dist(Point other) {
    double xd = this->x() - other.x();
    double yd = this->y() - other.y();
    return sqrt(xd * xd + yd * yd);
}

Point Point::add(Point b) {
    return Point(x_val + b.x_val, y_val + b.y_val);
}

Point Point::sub(Point b) {
    return Point(x_val - b.x_val, y_val - b.y_val);
}

void Point::move(double a, double b) {
    x_val += a;
    y_val += b;
}

    void Triungle::lines_intersect(Point a, Point b, Point c, Point d, Point *intersection) {

        double x, y;
        double A1 = b.y_val - a.y_val;
        double B1 = b.x_val - a.x_val;
        double C1 = a.y_val - (A1 / B1) * a.x_val;

        double A2 = d.y_val - c.y_val;
        double B2 = d.x_val - c.x_val;
        double C2 = c.y_val - (A2 / B2) * c.x_val;

        double det = (A1 / B1) - (A2 / B2);
        if (det == 0) {
            // lines are paralel
        } else {
            x = (C2 - C1) / det;
            y = (A1 * C2 - A2 * C1) / det;
        }
        *intersection->x_val = x;  // here i got error
        *intersection->y_val = y;  // here i got error
       }

Triungle::Triungle(std::string s) {

    cout << "enter first point of " << s << " triangle: ";
    cin >> a.x_val;
    cin >> a.y_val;
    if (!(cin)) {
        cout << "input error." << endl;
        exit(1);
    }
    cin.clear();
    cout << "enter second point of " << s << " triangle: ";
    cin >> b.x_val;
    cin >> b.y_val;
    if (!(cin)) {
        cout << "input error." << endl;
        exit(1);
    }
    cin.clear();
    cout << "enter 3 point of " << s << " triangle: ";
    cin >> c.x_val;
    cin >> c.y_val;
    if (!cin) {
        cout << "input error." << endl;
        exit(1);
    }
    cin.clear();

}

and i call function in this way

 int main(int argc, char** argv) {
    Triungle a("first");
    Triungle b("second");
 Point p;

    a.lines_intersect(a.a, a.b, a.c, a.a, &p);

}

Upvotes: 1

Views: 676

Answers (3)

divanov
divanov

Reputation: 6339

What you do in your code

*intersection->x_val = x;

is an equivalent to 

(*(intersection->x_val)) = x;

because operator of selection through pointer -> has higher precedence than dereference operator *, and the latter has higher precedence than assignment operator =.

So first your select member double x_val of class Point. Second you try to apply unary dereference operator * to the result. And because x_val is double, not a pointer, which is expected by dereference operator, compiler reports an error.

Thus, dereference operator is excessive here and it's enough to do the following

intersection->x_val = x;

Upvotes: 1

TZHX
TZHX

Reputation: 5398

Assuming that the error you get is a compilation error on the two lines:

*intersection->x_val = x;  // here i got error 
*intersection->y_val = y;  // here i got error

The problem is that you are de-referencing a pointer, and then using the derefencing operator -> on it.

Instead, you should either do:

intersection->x_val = x; 
intersection->y_val = y;  // leave it as a pointer

or

*intersection.x_val = x; 
*intersection.y_val = y;  // use it as an object

Upvotes: 0

Phil H
Phil H

Reputation: 20151

intersection->member

will dereference a pointer intersection. This is the same as

(*intersection).member

you don't need to dereference it twice.

Upvotes: 1

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