C++ Programming Documentation and Copies

[frobnitzem]

The CCTBX tutorial documentation is great! However, I noticed an inaccuracy in the description of C++ and copy-optimization here:

cctbx.github.io/tour.html

Line 256 in d26097b

<p>“Ouch” indicates that the <em>entire array</em> is copied when the function

Since C++11, return value optimization removes the implicit copy in return statements for most objects (STL containers in std). So, when constructing a vector via

std::vector<int> return_vector(void) {
  std::vector<int> tmp{1,2,3};
  return tmp;
}

std::vector<int> value = return_vector();

The destructor of tmp isn't called, and the second value is actually just directly pointing to tmp.ref This works for most (all?) containers in std. The key idea is std::move and reference values (rvalues), which are used in move constructors. This makes it a bit more complex to program in C++, but stick to the rule of 3/5 and you're usually good. Note that for pointers inside classes, this requires using smart pointers (Boost smart pointers have been adopted as std::shared_ptr).