派生类操作符=错误

在编译以下代码时出现错误。我想创建一个叫做SqMatrix的派生类，方阵，作为matrix的派生类型。然而，编译器不允许我实例化一些自定义运算符。下面是我的代码为matrix.h

#ifndef matrix_h
#define matrix_h
#include <string>
#include <vector>
#include <typeinfo>
template<class T>
class Matrix {
private:
  size_t nrow;
  size_t ncol;
  std::string typenm;
  std::vector<T> *vec;
public:
  Matrix(): nrow(0), ncol(0), vec(), typenm("") {}
  Matrix(size_t nr, size_t nc, T value=T()): nrow(nr), ncol(nc), typenm(std::string(typeid(T).name()))
  { vec = new std::vector<T>(nrow*ncol, value); }
  ~Matrix()
  { delete vec; }
  size_t getnrow() const
  { return nrow; }
  size_t getncol() const
  { return ncol; }
  std::string gettypenm() const
  { return typenm; }
  T *getdata()
  { return vec->data(); }
  T *getdata() const
  { return vec->data(); }
  T &at(size_t ir, size_t ic)
{ return vec->at(ic*nrow+ir); }
  T &at(size_t ir, size_t ic) const
  { return vec->at(ic*nrow+ir); }
  virtual void resize(size_t nr, size_t nc, T value=T())
  {
    vec->resize(nr*nc, value);
    nrow = nr;
    ncol = nc;
  }
  Matrix &operator =(const Matrix &mat);
  Matrix operator +(const Matrix &mat) const;
  Matrix operator -(const Matrix &mat) const;
};
template<class T>
class SqMatrix: public Matrix<T> {
private:
  size_t ndim;
public:
  SqMatrix(): Matrix<T>(), ndim(0) {}
  SqMatrix(size_t nd, T value=T()): Matrix<T>(nd, nd, value), ndim(nd) {}
  size_t getndim() const
  { return ndim; }
  virtual void resize(size_t nd, T value=T())
  {
    Matrix<T>::resize(nd, nd, value);
    ndim = nd;
  }
  SqMatrix &operator =(const SqMatrix &sqmat)
  {
    Matrix<T>::operator =(sqmat);
    return *this;
  }
};
#endif

然后是我的方法实现，matrix。cpp

#include <iostream>
#include <cstring>
#include <string>
#include <typeinfo>
#include "precision.h"
#include "matrix.h"
#include "lapack.h"
template<class T>
Matrix<T> &Matrix<T>::operator =(const Matrix<T> &mat)
{
  try {
    if(nrow != mat.nrow) throw "Matrix::operator =: Invalid mat.nrow";
    if(ncol != mat.ncol) throw "Matrix::operator =: Invalid mat.ncol";
  }
  catch(const char *str) {
    std::cout << str << std::endl;
  }
  memcpy(vec->data(), mat.vec->data(), sizeof(T)*nrow*ncol);
  return *this;
}
template<class T>
Matrix<T> Matrix<T>::operator +(const Matrix<T> &mat) const
{
  try {
    if(nrow != mat.nrow) throw "Matrix::operator +: Invalid mat.nrow";
    if(ncol != mat.ncol) throw "Matrix::operator +: Invalid mat.ncol";
  }
  catch(const char *str) {
    std::cout << str << std::endl;
  }
  Matrix<T> matsum(nrow, ncol);
  for(size_t i = 0; i < nrow*ncol; ++i)
    matsum.vec->at(i) = vec->at(i) + mat.vec->at(i);
  return matsum;
}
template<class T>
Matrix<T> Matrix<T>::operator -(const Matrix<T> &mat) const
{
  try {
    if(nrow != mat.nrow) throw "Matrix::operator -: Invalid mat.nrow";
    if(ncol != mat.ncol) throw "Matrix::operator -: Invalid mat.ncol";
  }
  catch(const char *str) {
    std::cout << str << std::endl;
  }
  Matrix<T> matdif(nrow, ncol);
  for(size_t i = 0; i < nrow*ncol; ++i)
    matdif.vec->at(i) = vec->at(i) - mat.vec->at(i);
  return matdif;
}
template Matrix<dreal> &Matrix<dreal>::operator =(const Matrix<dreal> &);
template Matrix<dreal> Matrix<dreal>::operator +(const Matrix<dreal> &) const;
template Matrix<dreal> Matrix<dreal>::operator -(const Matrix<dreal> &) const;
template Matrix<dcmplx> &Matrix<dcmplx>::operator =(const Matrix<dcmplx> &);
template Matrix<dcmplx> Matrix<dcmplx>::operator +(const Matrix<dcmplx> &) const;
template Matrix<dcmplx> Matrix<dcmplx>::operator -(const Matrix<dcmplx> &) const;

#include <iostream>
#include <typeinfo>
#include "constants.h"
#include "matrix.h"
using namespace std;
using namespace constants;
int main()
{
  const size_t Ndim(7);
  SqMatrix<dreal> sqmatA(Ndim,Ndim), sqmatB(Ndim,Ndim), sqmatC(Ndim,Ndim);
  for(size_t i = 0; i < Ndim; ++i)
    for(size_t j = 0; j < Ndim; ++j) {
      sqmatA.at(j, i) = (i+1) * (j+1);
      sqmatB.at(j, i) = i + j;
    }
  sqmatC = sqmatA + sqmatB;
  return 0;
}

奇怪的是编译器报错:

test.cpp: In function ‘int main()’:
test.cpp:20: error: no match for ‘operator=’ in ‘sqmatC = Matrix<T>::operator+(const Matrix<T>&) const [with T = double](((const Matrix<double>&)((const Matrix<double>*)(& sqmatB.SqMatrix<double>::<anonymous>))))’
matrix.h:62: note: candidates are: SqMatrix<T>& SqMatrix<T>::operator=(const SqMatrix<T>&) [with T = double]

显然，这是由于派生类SqMatrix的操作符=。我实际上为SqMatrix实现了operator =，为什么它仍然抱怨，或者我犯了什么实际的错误?

提前感谢!