Question

我试图在C ++上创建虚拟运算符

class Data
{
    virtual Matrix operator* (Matrix &_matrix);
    virtual Scalar operator* (Scalar &_scalar);
};

class Matrix : public Data
{
private:
    vector<vector<double>> data;
public:
    // ...
    Matrix operator* (Matrix &_matrix);
};

class Scalar : public Data
{
private:
    double data;
public:
    // ...
    Scalar operator* (Scalar &_scalar);
};

问题是，当我创建Data * Array时，如下所示

Data* arr[10];
arr[0] = new Matrix(3,3);
arr[1] = new Matrix(3,3);

arr[0]->operator*(arr[1]);

我无法在这两个矩阵之间进行乘法运算，因为我不能将Data作为参数传递。但问题是，我不能使函数的参数采用Data *类型，因为它将无法访问Matrix或Scalar对象的私有成员。

如何应对这种奇怪的情况？

Answer 1

班级双重调度问题 - 见梅耶（忘了哪一个）。您需要操作员在lhs和rhs上都是虚拟的，因此您需要两个虚拟调用：

class Matrix;
class Scalar;

class Data
{
public:
    virtual Data* operator* (Data& data) = 0;
    virtual Data* operator* (Matrix &matrix) = 0;
    virtual Data* operator* (Scalar &scalar) = 0;
};

class Matrix : public Data
{
private:
    std::vector<std::vector<double>> data;

    // ...
public:
    Matrix* operator* (Matrix &_matrix)
    {
        // implement
    }
    Matrix* operator* (Scalar& scalar)
    {
        // implement
    }
    Data* operator* (Data &data)
    {
        // Magic here - *this is now Matrix, not Data
        return data * (*this);
    }
};

class Scalar : public Data
{
private:
    double data;
public:
    // ...
    Data* operator* (Data& data)
    {
        // Magic here - *this is now Scalar, not Data
        return data * (*this);
    }
    Scalar* operator* (Scalar &scalar)
    {
        // implement
    }

    Matrix* operator* (Matrix &matrix)
    {
        // Note how we need to allow for parameter reveral during the double dispatch
        Matrix& lhs = matrix;
        Matrix& rhs = *this;
        // Compute matrix product lhs * rhs
    }

};

我已经忽略了返回类型和内存管理的问题。与运算符一样，您可能最好将*=定义为基元。然后，这可以返回对*this的引用。然后，*运算符可以*=来定义。再次，这是“有效的C ++”。

Answer 2

对此的解决方案称为Double Dispatch，这在C ++中有点奇怪。以下代码显示了这个技巧的工作原理：

#include <iostream>
#include <stdexcept>
#include <vector>


class Matrix;
class Scalar;

class Data {
public:
    virtual ~Data() {}
    virtual Data const& operator* (Data const& other) const = 0;
    virtual Data const& multiplyBy (Data const& other) const {
        throw std::runtime_error("bad function call");
    }
    virtual Data const& multiplyBy (Matrix const& other) const = 0;
    virtual Data const& multiplyBy (Scalar const& other) const = 0;
};

class Matrix : public Data {
private:
    std::vector< std::vector<double> > data;
public:
    virtual Data const& operator* (Data const& other) const {
        return other.multiplyBy(*this);
    }
    virtual Data const& multiplyBy (Matrix const& other) const {
        std::cout << "Matrix * Matrix" << std::endl;
        return *this;
    }
    virtual Data const& multiplyBy (Scalar const& other) const {
        std::cout << "Matrix * Scalar" << std::endl;
        return *this;
    }
};

class Scalar : public Data {
private:
    double data;
public:
    virtual Data const& operator* (Data const& other) const {
        return other.multiplyBy(*this);
    }
    virtual Data const& multiplyBy (Matrix const& other) const {
        std::cout << "Scalar * Matrix" << std::endl;
        return *this;
    }
    virtual Data const& multiplyBy (Scalar const& other) const {
        std::cout << "Scalar * Scalar" << std::endl;
        return *this;
    }
};

int
main() {
    Matrix m;
    Scalar s;
    Data* ary[] = { &m, &s };
    m * s;
    s * m;
    *(ary[0]) * *(ary[1]);
    *(ary[1]) * *(ary[0]);
    return 0;
}

关于C ++上虚拟运算符*的问题

2 个答案: