std::vector::reserve performance penalty

Question

inline void add(const DataStruct& rhs) {
   using namespace boost::assign;
   vec.reserve(vec.size() + 3);
   vec += rhs.a, rhs.b, rhs.c;
}

The

Answer 1

You only use reserve() if you know in advance the number of elements. In that case reserve() space for all elements at once.

Otherwise just use push_back() and rely on the default strategy - it will reallocate exponentially and greatly reduce the number of reallocations at a cost of slightly suboptimal memory consumption.