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

GCC implementation of reserve() will allocate exact number of elements, while push_back() will grow internal buffer exponentially by doubling it, so you are defeating the exponential growth and forcing reallocation/copy on each iteration. Run your test under ltrace or valgrind and see the number of malloc() calls.