How to initialize SparseMatrix of Eigen in a class - c++

Question

I am trying initializing sparseMatrix of Eigen, but it does not work when the initialization is in a class description. In the case of initialization in a function, not in a class, it works.
I am writting codes by C++ and using Visual Studio 2017.
I added
public:
EIGEN_MAKE_ALIGNED_OPERATOR_NEW , but the problem remains.
#include <iostream>
#include <vector>
#include <Eigen/SparseCore>
#include <Eigen/Sparse>
#include "pch.h"
using namespace Eigen;
namespace A {
class A
{
std::size_t max_doc_id = 4;
std::size_t max_term_id = 4;
SparseMatrix<float, Eigen::RowMajor, int64_t> smat(max_term_id, max_doc_id);
public:
EIGEN_MAKE_ALIGNED_OPERATOR_NEW
};
I want to decide the size of smat matrix (col=4, row=4) , but the error message is like this (actually it is written in Japanese so it may not correct)
"the member A::A::max_term_id is not the name of the type."
I appreciate if you can help me.

Answer 1

The compiler thinks that you are declaring a member function not a member variable (see here for more info on initialization). The following compiles. I am using Index instead of size_t to get rid of some warnings (narrowing conversion). You can play around with the code here: https://godbolt.org/z/yV1NUL
#include <iostream>
#include <vector>
#include <Eigen/SparseCore>
#include <Eigen/Sparse>
using namespace Eigen;
namespace A {
class A
{
Index max_doc_id = 4;
Index max_term_id = 4;
SparseMatrix<float, Eigen::RowMajor, int64_t> smat{max_term_id, max_doc_id};
public:
EIGEN_MAKE_ALIGNED_OPERATOR_NEW
};
}
Note: you should not use using namespace in header files, see "using namespace" in c++ headers
Edit: please also consider what user #ggael says in the comments, most likely you do not need EIGEN_MAKE_ALIGNED_OPERATOR_NEW because the SparseMatrix is not fixed-size vectorizable