How efficient is moving a vector? - c++

Question

I'm trying to gain a better grip as to when should I implement move semantics on my code or just delegate that to the compiler, and while performing some basic profiling/benchmarking I got confused as to why moving a vector is actually taking longer than copying it:
#include <chrono>
#include <vector>
#include <utility>
#include <iostream>
#define REPS 10000000
int main(int argc, char const *argv[]) {
// ----------------------------------------------------
std::chrono::high_resolution_clock clock;
auto start = clock.now();
for (int i = 0; i < REPS; i++) {
std::vector< float >(700, 1.54343);
}
auto end = clock.now();
std::cout << "Instantiating vector [x" << std::to_string(REPS) << "]: " << std::chrono::duration_cast< std::chrono::milliseconds >(end - start).count() << " ms" << std::endl;
// ----------------------------------------------------
std::vector< float > src(700, 1.54343);
start = clock.now();
for (int i = 0; i < REPS; i++) {
std::vector< float > dst = src;
}
end = clock.now();
std::cout << "Copying from rvalue [x" << std::to_string(REPS) << "]: " << std::chrono::duration_cast< std::chrono::milliseconds >(end - start).count() << " ms" << std::endl;
// ----------------------------------------------------
start = clock.now();
for (int i = 0; i < REPS; i++) {
std::vector< float > dst = std::vector< float >(700, 1.54343); // vector< float >(700, 1.54343);
}
end = clock.now();
std::cout << "Moving from a temporary [x" << std::to_string(REPS) << "]: " << std::chrono::duration_cast< std::chrono::milliseconds >(end - start).count() << " ms" << std::endl;
}
I get the follwing output:
Instantiating vector [x10000000]: 1160 ms
Copying from rvalue [x10000000]: 1167 ms
Moving from a temporary [x10000000]: 1491 ms
But shouldn't moving it, at worst, be as efficient as copying it (or in this case, initializing a new one)? Is there anything wrong with how I'm profiling this code?
Weirdly enough, I also noticed that commenting the first loop had a significant impact on the time:
Instantiating vector [x10000000]: 0 ms
Copying from rvalue [x10000000]: 1176 ms
Moving from a temporary [x10000000]: 1049 ms
Which may have an obvious explanation, but seems quite strange to me.

Answer 1

You're not moving correctly! In your snippets the line when you want to move, consists of one creation operation and one assigning operation, that take much longer than a single assignment.
If you want to calculate move operation, you can write something like this:
std::vector< float > dst = std::move(src);
My test result without this modification:
Instantiating vector [x10000000]: 22000 ms
Copying from rvalue [x10000000]: 19912 ms
Moving from a temporary [x10000000]: 22694 ms
My test result with this modification:
Instantiating vector [x10000000]: 23187 ms
Copying from rvalue [x10000000]: 20267 ms
Moving from a temporary [x10000000]: 593 ms