Unintended std::vector blues

std::vector sometimes has rather obscure semantics. For example, when you're writing generic code with MPI and you use vector to store some local data that needs to be sent/received by another processor, the usual trick of retrieving the beginning address of the internal array by &my_vector[0] will fail miserably when T=bool

This post is about another obscurity. Namely the reserve() method. Unless you use push_back()'s, this is rarely useful. Here is why:

vector < T > v, w;
v.reserve(n);
for(int i=0; i<n; ++i) v[i] = (T) i;
w.swap(v);

Now, guess what? You have two completely empty vectors. The reserve only changed capacity, and assignments with operator[] did not need to resize as well since capacity was enough (imho, it should update size whenever an unused location in the vector starts being used, but that's hard to ensure in practice). Therefore, v is zeros-sized even after the for-loop.