Question

我使用static_cast时遇到问题。这是我的计划：

#include <iostream>
using namespace std;

class Mtx { // base matrix
private:
    // refer to derived class
    Mtx& ReferToDerived() {
        return static_cast<Mtx&>(*this);
    }
    // entry() uses features of derived class
    virtual double& entry(int i, int j){
    return ReferToDerived() (i,j);    // error appears here
    }
protected:
    int dimn; // dimension of matrix
public:
    // define common functionality in base class that can
    // be called on derived classes
    double sum() { // sum all entries
        double d = 0;
        for (int i = 0; i < dimn; i++)
            for (int j = 0; j < dimn; j++) d += entry(i,j);
        return d;
    }
};

class FullMtx: public Mtx {
    double** mx;
public :
    FullMtx(int n) {
        dimn = n;
        mx = new double* [dimn] ;
        for (int i=0; i<dimn; i++) mx[i] = new double [dimn];
        for (int i=0; i<dimn; i++)
            for (int j=0; j<dimn; j++)
                mx[i][j] = 0; // initialization
    }
    double& operator() (int i, int j) { return mx[i] [j]; }
};

class SymmetricMtx : public Mtx {
    // store only lower triangular part to save memory
    double** mx ;
public :
    SymmetricMtx(int n) {
        dimn = n;
        mx = new double* [dimn];
        for (int i=0; i<dimn; i++) mx[i] = new double [i+1];
        for (int i=0; i<dimn; i++)
            for (int j=0; j <= i; j++)
                mx[i][j] = 0; // initialization
    }
    double& operator() (int i, int j) {
        if (i >= j ) return mx[i][j] ;
        else return mx[j][i]; // due to symmetry
    }
};

int main() 
{
    FullMtx A(1000);
    for (int i=0; i<1000; i++)
        for (int j=0; j<1000; j++)
        A(i,j)=1;
    cout << "sum of full matrix A = " << A.sum() << '\n';

    SymmetricMtx S(1000); // just assign lower triangular part
    for (int i=0; i<1000; i++)
        for (int j=0; j<1000; j++)
        S(i,j)=1;
    cout << "sum of symmetric matrix S = " << S.sum() << '\n';
}

当我运行它时，它说：不匹配调用'（Mtx）（int＆amp;，int＆amp;）' 我不明白什么是错的，我应该如何修改呢？它应该使用虚函数编写，但我不知道如何正确编写它。有人能帮我吗？该程序应计算FullMatrix和SymmetricMatrix的所有元素的总和。

Answer 1

在此处删除虚拟

virtual double& entry(int i, int j){
    return (*this)() (i,j);    // error appears here
}

在那里添加虚拟

class Mtx {
...
    virtual double& operator() (int i, int j) = 0;
}

让派生类重载该运算符和voala。

class FullMtx: public Mtx {
    ...
    virtual double& operator() (int i, int j) override { return mx[i] [j]; }
}

这也只是废话，正如已经指出的那样。你转换为相同的类型，而不是派生的类型。除此之外，你不能转换为派生类型，因为在基础上你没有关于完全继承的信息 - 如果有人在你不知情的情况下从FullMtx派生出来怎么办？

Mtx& ReferToDerived() {
    return static_cast<Mtx&>(*this);
}

C ++给了我一个错误：没有匹配的调用

1 个答案: