Number of elements in a list of list

[giorgik]

Hello everyone, I have a problem like this: I do not know how to count the number of elements in a list of lists. Let me explain:
I have the following list of list

std::vector< std::vector<GLdouble> > contours;
...
std::vector< std::vector<GLdouble> >::iterator contour;
for(contours.begin(), ite = contours.end(); contour != ite; ++contour)
{
    for( size_t v = 0; v < ????; v += 3 )
}

std::vector< std::vector<GLdouble> > contours;
...
std::vector< std::vector<GLdouble> >::iterator contour;
for(contours.begin(), ite = contours.end(); contour != ite; ++contour)
{
    for( size_t v = 0; v < ????; v += 3 )
}

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I do not know how to write the part indicated with ???? that represents the size (ie the number of elements of the list std::vector<GLdouble>).
Can you help me, I'm going crazy.