5 ms·

for vectors it makes std::move a very fast operation

• AstralStorm 8y ago
  Vector has a good move assignment which also works. Why swap when you can move? Swap is additionally risky because someone can ADL specialize it for their own purposes. (which is the point) And suddenly your move assignment is broken. So people work around it by adding using std::swap clause there... And then you're calling all those destructors.
• olliej 8y ago
  std::move on vector zeroes out the source element array. e.g. (pseudo-ish code) operator=(vector&& foo) { delete [] elements; elements = foo.elements; foo.elements = nullptr; } what the example code they have as an answer does is something morally equivalent to this: operator=(vector&& foo) { swap(elements, foo.elements);' } relying on the knowledge that because it's being moved the ~vector is going to be called immediately afterwards. This strikes me as being "clever" in the "potentially confusing code, for no real reason" sense
  • gpderetta 8y ago
    actually that move assignment operator is not ideal. The foo parameter won't be destroyed immediately but only at the end of whatever scope (if any) it was at the call site. The idiomatic copy and swap is like this: T& operator =(T x) // note pass by value { std::swap(this, x); return this; } This acts both as a normal and move assignment operator (if T is not copy constructible); edit: how do you get HN to show asterisks? There should be one before each 'this'
    • arh68 8y ago
      Prefix each line with 4 spaces: T& operator =(T x) // note pass by value { std::swap(*this, x); return *this; }