C++, expression can not be a function

Question

Expression can not be a function, but my expression is not a function, it is a variable.

Show my code.

class Data {

public:
    Data(int rows, int partid, vector<MatrixXd*> * res, MatrixXd * a)
    {
        _rows = rows;
        _partid = partid;
        _res = res;
        _a = a;
    }

    int _rows;
    int _partid;
    vector<MatrixXd*> * _res;
    MatrixXd * _a;
};

[snippet]    

void * partitionFunc(void * arg)
{
    Data * data = (Data *)arg;
    MatrixXd * m;
    int dim = data->_rows / data->_res->size();
    for (int i = 0; i < dim; i ++)
    {
        for (int j = 0; j < dim; j ++)
        {
            m = (*(data->_res))[data->_partid];
            (*m)(i, j) = data->_a(dim*data->_partid+i, dim*data->_partid+j);
        }
    }
    pthread_exit(NULL);
}

When compiling the code it returned

In file included from main.cpp:2:0:
solutions.h: In function ‘void* partitionFunc(void*)’:
solutions.h:86:75: error: expression cannot be used as a function
             (*m)(i, j) = data->_a(dim*data->_partid+i, dim*data->_partid+j);
                                                                           ^ 
solutions.h: In function ‘void* partitionFunc(void*)’:
solutions.h:86:75: error: expression cannot be used as a function
             (*m)(i, j) = data->_a(dim*data->_partid+i, dim*data->_partid+j);

But the m is a matrix, and (*m)(i, j) is to get the value in row i+1, column j+1.

How should I avoid such error.

Answer 1

It meant data->_a is not function (note the position of ^). You should do the same thing to data->_a as m, i.e. using operator* since it's a pointer to MatrixXd.

(*m)(i, j) = (*data->_a)(dim*data->_partid+i, dim*data->_partid+j);