"no operator found" when asigning Sparse matrices results to sparse matrices - eigen3

Question

I do have a function that implements a minimization algorithm. I didn't include all the vars, just the matrices to illustrate the types:
typedef Eigen::SparseMatrix<double> SpMat;
typedef Eigen::VectorXd Vec;
int lm_solver(void (*f_dz)(Vec* x_, int m, Vec* dz_, SpMat* W_),
void (*f_H)(Vec* x_, SpMat* jac_,int n_, int m_),
Vec* x, int nx, int mm, int nnz,
double tol=1e-9, int max_iter = 100){
SpMat A(mm, nx);
SpMat H1(mm, nx);
SpMat H2(mm, nx);
SpMat H(mm, nx);
SpMat W(mm, mm);
Vec rhs(nx);
Vec dz(nx);
Vec dx(nx);
Vec a(1);
Vec b(1);
double f, f_prev, lbmda, rho, nu, tau;
bool updateH, converged;
int iter_;
// reserve matrices memory
H.reserve(nnz);
W.reserve(mm);
while (!converged && iter_ < max_iter){
// get the system matrices
if (updateH){ // if the Jacobian computation is not locked...
f_dz(x, mm, &dz, &W); // Residual increment (z-h(x)) vector creation or update: fill dz and W
f_H(x, &H, nx, mm); // Jacobian matrix creation or update: fill H
// Start forming the auxiliary matrices of A
H1 = H.transpose() * W;
H2 = H1 * H;
}
// set the first value of lmbda
if (iter_ == 1)
lbmda = tau * H2.diagonal().maxCoeff();
// form the system matrix A = H^t·W·H + lambda·I
A = H2 + lbmda * Idn;
// form the right hand side: H^t·W·dz
rhs = H1 * dz;
// Solve the increment: dx = solve(A, rhs);
solver.compute(A);
dx = solver.solve(rhs);
// calculate the objective function: Least squares function
a = 0.5 * dz * W * dz; //vector x matrix x vector -> vector of 1 element
f = a.coeffRef(0);
// calculate the gain ratio
b = 0.5 * dx * (lbmda * dx - rhs); //vector x matrix x vector -> vector of 1 element
rho = (f_prev - f) / b.coeffRef(0);
}
return 0;
}
The process does the following:
Declare sparse matrices matrices (SpMat)
reserve matrices memory
Call external functions to fill H, dz and W
Do matrices multiplications and store the results into intermediate matrices
that are sparse too.
This function is the only function in a .h file that is compiled into a static library .lib
When I compile the static library alone, it compiles flawlessly.
However when I use the library project from another project, I get the following error:
error: C2679: binary '=' : no operator found which takes a right-hand operand of type 'const Eigen::CwiseBinaryOp' (or there is no acceptable conversion)
\eigen\src/Core/Matrix.h(206): could be 'Eigen::Matrix<_Scalar,_Rows,_Cols> &Eigen::Matrix<_Scalar,_Rows,_Cols>::operator =(const Eigen::Matrix<_Scalar,_Rows,_Cols> &)'
with
[
_Scalar=double,
_Rows=-1,
_Cols=1
]
d:\proyectos\proyectos_i+d\ingrid\eigen\eigen_3_3_3\eigen\src/Core/Matrix.h(281): or 'Eigen::Matrix<_Scalar,_Rows,_Cols> &Eigen::Matrix<_Scalar,_Rows,_Cols>::operator =(Eigen::Matrix<_Scalar,_Rows,_Cols> &&)'
with
[
_Scalar=double,
_Rows=-1,
_Cols=1
]
while trying to match the argument list '(Vec, const Eigen::CwiseBinaryOp)'
This error flags the lines:
H1 = H.transpose() * W;
H2 = H1 * H;
rhs = H1 * dz;
b = 0.5 * dx * (lbmda * dx - rhs);
a = 0.5 * dz * W * dz;
I understand from this that I cannot store the result of sparse matrices multiplications in a new sparse matrix. I don't know the solution to this.
(I'm using Eigen 3.3.3)

Answer 1

I don't see what lines exactly cause your error, but it looks rather like it is caused by calculating a and b. You can't multiply a col-vector by another col-vector without transposing it, e.g.
b = 0.5 * dx.transpose() * (lbmda * dx - rhs);
However, this is actually a dot product, so you should just write
double b = 0.5 * dx.dot(lbmda * dx - rhs);

Answer 2

The problem was that I wrote all the functions in the .h.
By putting the body of the function on the .cpp all went fine.
This dicotomy of .h and .cpp is what anoys me the most about c++.
Anyway, for future reference.