core: fix .begin()/.end() of empty Mat

modules/core/include/opencv2/core/mat.inl.hpp

@@ -1304,6 +1304,8 @@ const _Tp& Mat::at(const Vec<int, n>& idx) const
 template<typename _Tp> inline
 MatConstIterator_<_Tp> Mat::begin() const
 {
+    if (empty())
+        return MatConstIterator_<_Tp>();
     CV_DbgAssert( elemSize() == sizeof(_Tp) );
     return MatConstIterator_<_Tp>((const Mat_<_Tp>*)this);
 }
@@ -1311,6 +1313,8 @@ MatConstIterator_<_Tp> Mat::begin() const
 template<typename _Tp> inline
 MatConstIterator_<_Tp> Mat::end() const
 {
+    if (empty())
+        return MatConstIterator_<_Tp>();
     CV_DbgAssert( elemSize() == sizeof(_Tp) );
     MatConstIterator_<_Tp> it((const Mat_<_Tp>*)this);
     it += total();
@@ -1320,6 +1324,8 @@ MatConstIterator_<_Tp> Mat::end() const
 template<typename _Tp> inline
 MatIterator_<_Tp> Mat::begin()
 {
+    if (empty())
+        return MatIterator_<_Tp>();
     CV_DbgAssert( elemSize() == sizeof(_Tp) );
     return MatIterator_<_Tp>((Mat_<_Tp>*)this);
 }
@@ -1327,6 +1333,8 @@ MatIterator_<_Tp> Mat::begin()
 template<typename _Tp> inline
 MatIterator_<_Tp> Mat::end()
 {
+    if (empty())
+        return MatIterator_<_Tp>();
     CV_DbgAssert( elemSize() == sizeof(_Tp) );
     MatIterator_<_Tp> it((Mat_<_Tp>*)this);
     it += total();
@@ -2690,6 +2698,7 @@ MatConstIterator::MatConstIterator(const Mat* _m)
 {
     if( m && m->isContinuous() )
     {
+        CV_Assert(!m->empty());
         sliceStart = m->ptr();
         sliceEnd = sliceStart + m->total()*elemSize;
     }
@@ -2703,6 +2712,7 @@ MatConstIterator::MatConstIterator(const Mat* _m, int _row, int _col)
     CV_Assert(m && m->dims <= 2);
     if( m->isContinuous() )
     {
+        CV_Assert(!m->empty());
         sliceStart = m->ptr();
         sliceEnd = sliceStart + m->total()*elemSize;
     }
@@ -2717,6 +2727,7 @@ MatConstIterator::MatConstIterator(const Mat* _m, Point _pt)
     CV_Assert(m && m->dims <= 2);
     if( m->isContinuous() )
     {
+        CV_Assert(!m->empty());
         sliceStart = m->ptr();
         sliceEnd = sliceStart + m->total()*elemSize;
     }

modules/core/test/test_mat.cpp

@@ -2082,4 +2082,12 @@ TEST(Mat, regression_12943)  // memory usage: ~4.5 Gb
     cv::flip(src, dst, 0);
 }
 
+TEST(Mat, empty_iterator_16855)
+{
+    cv::Mat m;
+    EXPECT_NO_THROW(m.begin<uchar>());
+    EXPECT_NO_THROW(m.end<uchar>());
+    EXPECT_TRUE(m.begin<uchar>() == m.end<uchar>());
+}
+
 }} // namespace