Nice way to append a vector to itself

by

Wow. So many answers that are close, none with all the right pieces. You need both resize (or reserve) and copy_n, along with remembering the original size.

auto old_count = xx.size();
xx.resize(2 * old_count);
std::copy_n(xx.begin(), old_count, xx.begin() + old_count);

or

auto old_count = xx.size();
xx.reserve(2 * old_count);
std::copy_n(xx.begin(), old_count, std::back_inserter(xx));

When using reserve, copy_n is required because the end() iterator points one element past the end… which means it also is not “before the insertion point” of the first insertion, and becomes invalid.

23.3.6.5 [vector.modifiers] promises that for insert and push_back:

Remarks: Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid. If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of T or by any InputIterator operation there are no effects. If an exception is thrown by the move constructor of a non-CopyInsertable T, the effects are unspecified.

More Related Contents:

  1. push_back vs emplace_back
  2. Concatenating two std::vectors
  3. Is std::unique_ptr required to know the full definition of T?
  4. Why can I not push_back a unique_ptr into a vector?
  5. Why do I get the same sequence for every run with std::random_device with mingw gcc4.8.1?
  6. Why can’t simple initialize (with braces) 2D std::array? [duplicate]
  7. Is the size of std::array defined by standard
  8. Why is the C++ initializer_list behavior for std::vector and std::array different?
  9. What does the standard library guarantee about self move assignment?
  10. std::map emplace without copying value
  11. C++ valarray vs. vector
  12. vector::at vs. vector::operator[]
  13. How to declare a vector of atomic in C++
  14. std::vector, default construction, C++11 and breaking changes
  15. std::optional specialization for reference types
  16. Why is value taking setter member functions not recommended in Herb Sutter’s CppCon 2014 talk (Back to Basics: Modern C++ Style)?
  17. Non-copyable elements in vector
  18. what is the best modern c++ approach to construct and manipulate a 2d array [closed]
  19. What is the default hash function used in C++ std::unordered_map?
  20. How can I use Standard Library (STL) classes in my dll interface or ABI?
  21. May std::vector make use of small buffer optimization?
  22. Converting between C++ std::vector and C array without copying
  23. Can std::vector emplace_back copy construct from an element of the vector itself?
  24. stl random distributions and portability
  25. Are lambdas inlined like functions in C++?
  26. Why vector access operators are not specified as noexcept?
  27. error for hash function of pair of ints
  28. STL algorithms: Why no additional interface for containers (additional to iterator pairs)?
  29. Is there a standard way of moving a range into a vector?
  30. Can the ‘type’ of a lambda expression be expressed?

Leave a Comment