Extended the comma initializer to support xpr on the right side:

Matrix3i mat; Vector2i vec(33,66);
  mat << vec.transpose(), 99,
         vec, Matrix2i::random();

Eigen/src/Core/CommaInitializer.h

@@ -26,28 +26,66 @@
 #ifndef EIGEN_COMMA_INITIALIZER_H
 #define EIGEN_COMMA_INITIALIZER_H
 
+/** \internal
+  * Helper class to define the MatrixBase::operator<<
+  */
 template<typename Scalar, typename Derived>
 struct MatrixBase<Scalar, Derived>::CommaInitializer
 {
-  CommaInitializer(Derived& mat) : m_matrix(mat), m_count(1) {}
+  CommaInitializer(Derived& mat, const Scalar& s)
+    : m_matrix(mat), m_row(0), m_col(1), m_currentBlockRows(1)
+  {
+    m_matrix.coeffRef(0,0) = s;
+  }
 
-  CommaInitializer& operator,(const Scalar& s) {
-    assert(m_count<m_matrix.size() && "Too many coefficients passed to Matrix::operator<<");
-    m_matrix._coeffRef(m_count/m_matrix.cols(), m_count%m_matrix.cols()) = s;
-    m_count++;
+  template<typename OtherDerived>
+  CommaInitializer(Derived& mat, const MatrixBase<Scalar, OtherDerived>& other)
+    : m_matrix(mat), m_row(0), m_col(other.cols()), m_currentBlockRows(other.rows())
+  {
+    m_matrix.block(0, 0, other.rows(), other.cols()) = other;
+  }
+
+  CommaInitializer& operator,(const Scalar& s)
+  {
+    if (m_col==m_matrix.cols())
+    {
+      m_row+=m_currentBlockRows;
+      m_col = 0;
+      m_currentBlockRows = 1;
+    }
+    assert(m_col<m_matrix.cols() && "Too many coefficients passed to Matrix::operator<<");
+    assert(m_currentBlockRows==1);
+    m_matrix._coeffRef(m_row, m_col++) = s;
+    return *this;
+  }
+
+  template<typename OtherDerived>
+  CommaInitializer& operator,(const MatrixBase<Scalar, OtherDerived>& other)
+  {
+    if (m_col==m_matrix.cols())
+    {
+      m_row+=m_currentBlockRows;
+      m_col = 0;
+      m_currentBlockRows = other.rows();
+    }
+    assert(m_col<m_matrix.cols() && "Too many coefficients passed to Matrix::operator<<");
+    assert(m_currentBlockRows==other.rows());
+    m_matrix.block(m_row, m_col, other.rows(), other.cols()) = other;
+    m_col += other.cols();
     return *this;
   }
 
   ~CommaInitializer(void)
   {
-    assert(m_count==m_matrix.size() && "Too few coefficients passed to Matrix::operator<<");
+    assert((m_row+m_currentBlockRows)==m_matrix.rows() && m_col==m_matrix.cols() && "Too few coefficients passed to Matrix::operator<<");
   }
 
   Derived& m_matrix;
-  int m_count;
+  int m_row; // current row id
+  int m_col; // current col id
+  int m_currentBlockRows; // current block height
 };
 
-
 /** Convenient operator to set the coefficients of a matrix.
   *
   * The coefficients must be provided in a row major order and exactly match
@@ -59,9 +97,14 @@ struct MatrixBase<Scalar, Derived>::CommaInitializer
 template<typename Scalar, typename Derived>
 typename MatrixBase<Scalar, Derived>::CommaInitializer MatrixBase<Scalar, Derived>::operator<< (const Scalar& s)
 {
-  coeffRef(0,0) = s;
-  return CommaInitializer(*static_cast<Derived *>(this));
+  return CommaInitializer(*static_cast<Derived *>(this), s);
 }
 
+template<typename Scalar, typename Derived>
+template<typename OtherDerived>
+typename MatrixBase<Scalar, Derived>::CommaInitializer MatrixBase<Scalar, Derived>::operator<< (const MatrixBase<Scalar, OtherDerived>& other)
+{
+  return CommaInitializer(*static_cast<Derived *>(this), other);
+}
 
 #endif // EIGEN_COMMA_INITIALIZER_H

Eigen/src/Core/MatrixBase.h

@@ -182,6 +182,9 @@ template<typename Scalar, typename Derived> class MatrixBase
 
     CommaInitializer operator<< (const Scalar& s);
 
+    template<typename OtherDerived>
+    CommaInitializer operator<< (const MatrixBase<Scalar, OtherDerived>& other);
+
     /** swaps *this with the expression \a other.
       *
       * \note \a other is only marked const because I couln't find another way

doc/snippets/MatrixBase_set.cpp

@@ -1,8 +1,13 @@
 Matrix3i m1;
-m1 <<= 1, 2, 3,
+m1 << 1, 2, 3,
       4, 5, 6,
       7, 8, 9;
 cout << m1 << endl << endl;
 Matrix3i m2 = Matrix3i::identity();
-m2.block(0,0, 2,2) <<= 10, 11, 12, 13;
+m2.block(0,0, 2,2) << 10, 11, 12, 13;
+cout << m2 << endl << endl;
+Vector2i v1;
+v1 << 14, 15;
+m2 << v1.transpose(), 16,
+      v1, m1.block(1,1,2,2);
 cout << m2 << endl;

test/basicstuff.cpp

@@ -105,8 +105,28 @@ void EigenTest::testBasicStuff()
 
     VERIFY_RAISES_ASSERT( (m3 << 1, 2, 3, 4, 5, 6, 7, 8) );
     VERIFY_RAISES_ASSERT( (m3 << 1, 2, 3, 4, 5, 6, 7, 8, 9, 10) );
-    m3 << 1, 2, 3, 4, 5, 6, 7, 8, 9;
+
     double data[] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
+
+    m3 = Matrix3d::random();
+    m3 << 1, 2, 3, 4, 5, 6, 7, 8, 9;
+    VERIFY_IS_APPROX(m3, (Matrix<double,3,3,RowMajor>::map(data)) );
+
+    Vector3d vec[3];
+    vec[0] << 1, 4, 7;
+    vec[1] << 2, 5, 8;
+    vec[2] << 3, 6, 9;
+    m3 = Matrix3d::random();
+    m3 << vec[0], vec[1], vec[2];
+    VERIFY_IS_APPROX(m3, (Matrix<double,3,3,RowMajor>::map(data)) );
+
+    vec[0] << 1, 2, 3;
+    vec[1] << 4, 5, 6;
+    vec[2] << 7, 8, 9;
+    m3 = Matrix3d::random();
+    m3 << vec[0].transpose(),
+          4, 5, 6,
+          vec[2].transpose();
     VERIFY_IS_APPROX(m3, (Matrix<double,3,3,RowMajor>::map(data)) );
   }
 }