表达式必须具有指向对象类型的指针

Question

I'm writing a matrix program and am currently trying to multiply a point and a matrix. I keep getting an error over my objects(result and P) "Expression must have pointer to object type" in this function:

//Point Class functions
Point Matrix44::operator*(const Point & P){
    Point result;
    for (int i = 0; i < 4; i++) {
        for (int k = 0; k < 4; k++) {
            result.element[i][k] = 0;
            for (int j = 0; j < 4; j++) {
                result.element[i][k] = element[i][j] * P.element[j][k] + result.element[i][k];
            }
        }
    }
    return result;

}

My two classes are:

    //Matrix class
class Point;

class Matrix44 {
private:
    double element[4][4];
public:
    Matrix44(void);
    Matrix44 transpose(void) const;
    friend istream& operator>>(istream& s, Matrix44& t);
    friend ostream& operator<<(ostream& s, const Matrix44& t);
    Matrix44 operator *(Matrix44 b);
    Point operator*(const Point & P);
};


//Point class
class Point {
    double element[4];
    friend class Matrix44;
public:
    Point(void) {
        element[0] = element[1] = element[2] = 0;
        element[3] = 1;
    }
    Point(double x, double y, double z){
        element [0]=x;
        element [1]=y;
        element [2]=z;
        element [3]=1;
    }

};

Answer 1

In your Point class, you have the element member defined as:

double element[4];

This is a one-dimensional array. However, in your function, you're trying to access it as if it were a two -dimensional array:

result.element[i][k]
P.element[j][k]

I think you need to rethink exactly how your matrix multiplication is supposed to work.

Answer 2

result.element is a 1 dimensional array. You are using two indices with it. That will not compile. You should look at the definition of matrix multiplication .

Point Matrix44::operator*(const Point & P){
    Point result;
    for (int i = 0; i < 4; i++) {
        result.element[i] = 0;
        for (int j = 0; j < 4; j++) {
          result.element[i] += element[i][j] * P.element[j];
        }
    }
    return result;

}

Answer 3

Point::element is a double[4] . In your code, you have Point result; result.element[i][k] = 0; Point result; result.element[i][k] = 0; . Since element is not a two dimensional array, the compiler tries to convert the double to an array to use [] on it, but it can't. I would guess this is copy-pasted code from Matrix44 Matrix44::operator*(const Matrix44& M)

It always helps to tell us what line has the problem in your sample code too.

Also, the function will have the incorrect result, you set result.element[i][k] to zero, then set it to 4 different values. I think you meant to add instead of assign in the innermost loop.

Answer 4

same error might appear misleadingly if you point to a struct that is not accessible to the source, such as,

header.h

define struct myStruct;

source1.c

include header.h

myStruct structX;

source2.c

include header.h

uint8_t * ptr2Struct = &structX;

if that is a case then you get this arror with line " uint8_t * ptr2Struct = &structX;

Answer 5

You're trying to access:

result.element[i][k] = element[i][j] * P.element[j][k] + result.element[i][k];

When Point itself only have a one-level array:

double element[4];