Does reserving capacity incur two allocations or just one?

Question

std::vector<T> vec;   // line #1
vec.reserve(100);     // line #2

I am wondering if line #1 triggers a small allocation (say, memory for 10 Ts), or if the first allocation happens on line #2. Does the standard say anything about that?

Answer 1

It's implementation defined. The default-constructor for vector does not need to allocate anything, but it's permissible for an implementation to do so.