可以将派生对象转换为c ++模板类中的基本引用吗？

Question

all. I am trying to learn something about template inherit. I want to cast a temp derived object to its base reference. But I come across this problem:

typedef long size64_t;
//////Base class
template <typename _Scalar>
class MatrixBase{
public:
    virtual _Scalar operator()(size64_t rowid, size64_t colid) = 0;
};
//////Derived class 1
template <typename _Scalar>
class MatrixHolder : public MatrixBase<_Scalar>{
public:
    MatrixHolder(){};
    inline _Scalar  operator()(size64_t rowid, size64_t colid){return 0;}
};

//////Derived class 2
template <typename _Scalar>
class Matrix : public MatrixBase<_Scalar>{
public:
    Matrix(){};
    inline _Scalar  operator()(size64_t rowid, size64_t colid){return 0;}
};
//////The function with parameters as Base class reference, wanting to get derived object as input.
template <typename _Scalar>
MatrixHolder<_Scalar> apply(MatrixBase<_Scalar>& lhs, MatrixBase<_Scalar>& rhs){
    MatrixHolder<_Scalar> result;
    return result;
}

and in main, we have:

void main(){
    Matrix<double> m1;
    Matrix<double> m2;
    apply(m1, m2);//Sucess
    apply(m1, apply(m1, m2));//Fail
}

the compiler said:

note: candidate function [with _Scalar = double] not viable: no known conversion from
      'MatrixHolder<double>' to 'MatrixBase<double> &' for 2nd argument
MatrixHolder<_Scalar> apply(MatrixBase<_Scalar>& lhs, MatrixBase<_Scalar>& rhs){
                      ^
1 error generated.

Answer 1

apply(m1, apply(m1, m2));//Fail

The problem here is that the inner apply returns a temporary, which cannot bind to the non-const reference parameter of the outer apply .

If you can make the parameters MatrixBase<_Scalar> const& it would be a possible match.