C++ constructor performance

Question

In C++17 if we design a class like this:

class Editor {
public:
  // "copy" constructor
  Editor(const std::string& text) : _text {text} {}

  // "move" constructor
  Editor(std::string&& text) : _text {std::move(text)} {}

private:
  std::string _text;
}

It might seem (to me at least), that the "move" constructor should be much faster than the "copy" constructor.

But if we try to measure actual times, we will see something different:

  int current_time()
  {
    return chrono::high_resolution_clock::now().time_since_epoch().count();
  }

 int main()
 {
   int N = 100000;

   auto t0 = current_time();
   for (int i = 0; i < N; i++) {
     std::string a("abcdefgh"s);
     Editor {a}; // copy!
   }
   auto t1 = current_time();
   for (int i = 0; i < N; i++) {
     Editor {"abcdefgh"s};
   }
   auto t2 = current_time();

   cout << "Copy: " << t1 - t0 << endl;
   cout << "Move: " << t2 - t1 << endl;
 }

Both copy and move times are in the same range. Here's one of the outputs:

Copy: 36299550
Move: 35762602

I tried with strings as long as 285604 characters, with the same result.

Question: why is "copy" constructor Editor(std::string& text) : _text {text} {} so fast? Doesn't it actually creates a copy of input string?

Update I run the benchmark given here using the following line: g++ -std=c++1z -O2 main.cpp && ./a.out

Update 2 Fixing move constructor, as @Caleth suggests (remove const from the const std::string&& text) improves things!

Editor(std::string&& text) : _text {std::move(text)} {}

Now benchmark looks like:

Copy: 938647
Move: 64

Answer 1

It also depends on your optimization flags. With no optimization, you can (and I did!) get even worse results for the move:

Copy: 4164540
Move: 6344331

Running the same code with -O2 optimization gives a much different result:

Copy: 1264581
Move: 791

See it live on Wandbox.

That's with clang 9.0. On GCC 9.1, the difference is about the same for -O2 and -O3 but not quite as stark between copy and move:

Copy: 775
Move: 508

I'm guessing that's a small string optimization kicking in.

In general, the containers in the standard library work best with optimizations on because they have a lot of little functions that the compiler can easily inline and collapse when asked to do so.

Also in that first constructor, per Herb Sutter, "Prefer passing a read-only parameter by value if you’re going to make a copy of the parameter anyway, because it enables move from rvalue arguments."

---

Update: For very long strings (300k characters), the results are similar to the above (now using std::chrono::duration in milliseconds to avoid int overflows) with GCC 9.1 and optimizations:

Copy: 22560
Move: 1371

and without optimizations:

Copy: 22259
Move: 1404

Answer 2

const std::string&& looks like a typo.

You can't move from it, so you get a copy instead.

Answer 3

So your tests is really looking at the number of times we have to "build" a string object.

So in the fist test:

for (int i = 0; i < N; i++) {
  std::string a("abcdefgh"s);    // Build a string once.
  Editor {a}; // copy!           // Here you build the string again.
}                                // So basically two expensive memory
                                 // allocations and a copying the string

While in the second test:

for (int i = 0; i < N; i++) {
  Editor {"abcdefgh"s};         // You build a string once.
                                // Then internally you move the allocated
                                // memory (so only one expensive memory
                                // allocation and copying the string
}

So the difference between the two loops is one extra string copy.

The problem here. I as a human can spot one easy peephole optimization (and the compiler is better than me).

for (int i = 0; i < N; i++) {
  std::string a("abcdefgh"s);   // This string is only used in a single
                                // place where it is passed to a
                                // function as a const parameter

                                // So we can optimize it out of the loop.

  Editor {a};
}

So if we do a manually yanking of the string outside the loop (equivalent to a valid compiler optimization).

So this loop has the same affect:

std::string  a("abcdefgh"s); 
for (int i = 0; i < N; i++) {
  Editor {a};
}

Now this loop only has 1 allocation and copy.
So now both loops look the same in terms of the expensive operations.

Now as a human I am not going to spot (quickly) all the optimization possible. I am just trying to point out here that your quick test here you will not spot a lot of optimizations that the compiler will do and thus estimations and doing timings like this are hard.

Answer 4

On paper you're right, but in practice this is quite easily optimisable so you'll probably find the compiler has ruined your benchmark.

You could benchmark with "optimisations" turned off, but that in itself holds little real-world benefit. It may be possible to trick the compiler in release mode by adding some code that prevents such an optimisation, but off the top of my head I can't imagine what that would look like here.

It's also a relatively small string that can be copied really quickly nowadays.

I think you should just trust your instinct here (because it's correct), while remembering that in practice it might not actually make a lot of difference. But the move certainly won't be worse than the copy.

Sometimes we can and should write obviously "more efficient" code without being able to prove that it'll actually perform better on any particular day of the week with any particular phase of the moon/planetary alignment, because compilers are already trying to make your code as fast as possible.

People may tell you that this is therefore a "premature optimisation", but it really isn't: it's just sensible code.