Fix reference return issue in MatSparse::operator().

The function prototype is imposed by the MatGeneric parent which was in its turn imposed by MatDense first function to move up in hierarchy, so we have to return a ref without having access to the buffer directly (fortunately Eigen provides the function coeffRef for that).

misc/test/src/C++/faust_matsparse_index_op.cpp.in

+#include "faust_MatSparse.h"
+#include <iostream>
+
+/** \brief unitary test for MatDense conjugate
+*/
+
+typedef @TEST_FPP@ FPP;
+
+
+using namespace Faust;
+using namespace std;
+
+int main(int argc, char* argv[])
+{
+	faust_unsigned_int dim1 = 3;
+	faust_unsigned_int dim2 = 3;
+
+	MatDense<FPP,Cpu>* M1;
+	Faust::MatSparse<FPP,Cpu> M_sp1;
+	M1 = Faust::MatDense<FPP,Cpu>::randMat(dim1,dim2);
+	M_sp1 = *M1;
+	cout << "M_sp1:" << endl;
+	M_sp1.Display();
+	cout<<"M1:"<<endl;
+	M1->Display();
+	for(int i = 0; i < M1->getNbRow(); i++)
+		for(int j = 0; j < M1->getNbCol(); j++)
+		{
+			assert(M_sp1(i,j) == (*M1)(i,j));
+		}
+	return 0;
+}

src/faust_linear_operator/CPU/faust_MatSparse.h

@@ -295,6 +295,7 @@ namespace Faust
 			matvar_t* toMatIOVarDense(bool transpose, bool conjugate) const;
 			FPP normL1(const bool transpose=false) const;
 			FPP normL1(faust_unsigned_int&, const bool transpose=false) const;
+			const FPP& operator()(faust_unsigned_int i, faust_unsigned_int j)const{return const_cast<Eigen::SparseMatrix<FPP,Eigen::RowMajor>*>(&mat)->coeffRef(i,j);}
 			Faust::Vect<FPP,Cpu> get_col(faust_unsigned_int id) const;
 			Faust::MatSparse<FPP,Cpu>* get_cols(faust_unsigned_int col_id_start, faust_unsigned_int num_cols) const;
 			Faust::MatSparse<FPP,Cpu>* get_cols(faust_unsigned_int* col_ids, faust_unsigned_int num_cols) const;