c++ vectors not like arrays

I've been getting my head round c++ for a few months, and have been directed by google to stack overflow most of the time for c++ queries. I noted frequently exhortations of the type "why don't you use a vector", and was inspired to do just that. So, primarily to get the minor benefits of automatic memory deallocation, and being able to write typed comparison functions for sorting. I switched an array of pointers to objects to being a vector. Now I thought (incorrectly it seems) that vectors could be used more or less like arrays, and hence I initialized thusly: ``` cluster clusters[LOTS]; vector<cluster *> pclust; pclust.reserve(numClust); for (int i=0; i<numClust; ++i) pclust[i] = clusters + i; ``` No complaints from the compiler. Then some time later I need to sort the vector on some attribute of the cluster object. So: ``` std::sort(pclust.begin(), pclust.end(), ClusterCompareNumSegs); ``` Again no problems compiling. Except the vector doesn't get sorted. It turns out that vector.size() is zero, and of course my initialization should have been ``` pclust.push_back(clusters + i); ``` Now that's easy to fix, but I am confused, because the initial incorrect assignment was working. I successfully iterated through the vector - using the array syntax, like so: ``` for (clustind=0; clustind < numClust; ++clustind) {<br> cluster *cl = pclust[clustind]; ...happily access *cl... ``` And it all worked fine. So I'm just wondering what's going on. Presumably in my initial assignments, I was trying to access elements not yet in the vector (I was trying to put them in), and the vector was throwing exceptions that I was ignoring. But nonetheless, when referencing the locations, the pointers were there. Can anyone provide enlightenment?