bug #826: document caveats in 1x1 and 2x1 constructors.

Eigen/src/Core/Matrix.h

@@ -257,9 +257,15 @@ class Matrix
 
     /** \brief Constructs a vector or row-vector with given dimension. \only_for_vectors
       *
-      * Note that this is only useful for dynamic-size vectors. For fixed-size vectors,
-      * it is redundant to pass the dimension here, so it makes more sense to use the default
-      * constructor Matrix() instead.
+      * This is useful for dynamic-size vectors. For fixed-size vectors,
+      * it is redundant to pass these parameters, so one should use the default constructor
+      * Matrix() instead.
+      * 
+      * \warning This constructor is disabled for fixed-size \c 1x1 matrices. For instance,
+      * calling Matrix<double,1,1>(1) will call the initialization constructor: Matrix(const Scalar&).
+      * For fixed-size \c 1x1 matrices it is thefore recommended to use the default
+      * constructor Matrix() instead, especilly when using one of the non standard
+      * \c EIGEN_INITIALIZE_MATRICES_BY_{ZERO,\c NAN} macros (see \ref TopicPreprocessorDirectives).
       */
     EIGEN_STRONG_INLINE explicit Matrix(Index dim);
     /** \brief Constructs an initialized 1x1 matrix with the given coefficient */
@@ -268,9 +274,17 @@ class Matrix
       *
       * This is useful for dynamic-size matrices. For fixed-size matrices,
       * it is redundant to pass these parameters, so one should use the default constructor
-      * Matrix() instead. */
+      * Matrix() instead.
+      * 
+      * \warning This constructor is disabled for fixed-size \c 1x2 and \c 2x1 vectors. For instance,
+      * calling Matrix2f(2,1) will call the initialization constructor: Matrix(const Scalar& x, const Scalar& y).
+      * For fixed-size \c 1x2 or \c 2x1 vectors it is thefore recommended to use the default
+      * constructor Matrix() instead, especilly when using one of the non standard
+      * \c EIGEN_INITIALIZE_MATRICES_BY_{ZERO,\c NAN} macros (see \ref TopicPreprocessorDirectives).
+      */
     EIGEN_DEVICE_FUNC
     Matrix(Index rows, Index cols);
+    
     /** \brief Constructs an initialized 2D vector with given coefficients */
     Matrix(const Scalar& x, const Scalar& y);
     #endif

doc/PreprocessorDirectives.dox

@@ -27,10 +27,23 @@ are doing.
    Defaults to the %IOFormat constructed by the default constructor IOFormat::IOFormat().
  - \b EIGEN_INITIALIZE_MATRICES_BY_ZERO - if defined, all entries of newly constructed matrices and arrays are
    initialized to zero, as are new entries in matrices and arrays after resizing. Not defined by default.
+   \warning The unary (resp. binary) constructor of \c 1x1 (resp. \c 2x1 or \c 1x2) fixed size matrices is
+   always interpreted as an initialization constructor where the argument(s) are the coefficient values
+   and not the sizes. For instance, \code Vector2d v(2,1); \endcode will create a vector with coeficients [2,1],
+   and \b not a \c 2x1 vector initialized with zeros (i.e., [0,0]). If such cases might occur, then it is
+   recommended to use the default constructor with a explicit call to resize:
+   \code
+   Matrix<?,SizeAtCompileTime,1> v;
+   v.resize(size);
+   Matrix<?,RowsAtCompileTime,ColsAtCompileTime> m;
+   m.resize(rows,cols);
+   \endcode
  - \b EIGEN_INITIALIZE_MATRICES_BY_NAN - if defined, all entries of newly constructed matrices and arrays are
    initialized to NaN, as are new entries in matrices and arrays after resizing. This option is especially
    useful for debugging purpose, though a memory tool like <a href="http://valgrind.org/">valgrind</a> is
    preferable. Not defined by default.
+   \warning See the documentation of \c EIGEN_INITIALIZE_MATRICES_BY_ZERO for a discussion on a limitations
+   of these macros when applied to \c 1x1, \c 1x2, and \c 2x1 fixed-size matrices.
  - \b EIGEN_NO_AUTOMATIC_RESIZING - if defined, the matrices (or arrays) on both sides of an assignment 
    <tt>a = b</tt> have to be of the same size; otherwise, %Eigen automatically resizes \c a so that it is of
    the correct size. Not defined by default.