There seems to be a problem with the @clear()@ clear() method and all methods that call @clear()@: clear(): I noticed that the iterator on a just cleared sparse mat is not empty. Looking into the debugger revealed that the hash table is not cleared. Trying to trace into the @clear()@ clear() method revealed that the actual method is never called. I observed this with 2.3.1 on a Mac OS X 10.7.2 withh gcc 4.2.

The following code works if and only if the commented line remains commented out:
<pre><code class="cpp">
int main (int argc, const char * argv[])
{
int sizesr2 = {10, 20};
cv::SparseMat_<float> v(2,sizes);
v.ref(2,4) = 7.0f;
//v.clear();
cv::SparseMatIterator_<float> i = v.begin();
cv::SparseMatIterator_<float> ie = v.end();
for (; i != ie; ++i)
std::cout << *i;
std::cout << '\n';
return 0;
}
</code></pre>

Commenting out @v.clear()@, v.clear(), I would expect the hash table to be cleared and the iterator to be on an empty set, i.e., never executed. This is not the case and any iterator on @SparseMat@ SparseMat needs to be embraced by
<pre><code class="cpp">
if (my_matrix.nzcount()) {
...
}
</code></pre>