[honkhonkpants]

I have a feeling 100% of the c++ time is being spent in some silliness like setting up the locale of the ostream, because my compiler totally eliminates that loop.

[e12e]

Probably. I glanced at the asm to make sure the loop was still there (which it was, possibly because it loops over an int, and sums doubles?) and couldn't see anything that stood out. Still a little surprised that pypy without warmup is faster than c++ for this silly thing.

[ed: On this system, eliminating the loop by hand looks like:

  #include<iostream>
  int main() {
    double s = 100000000;

    std::cout << s << std::endl;
    return 0;
  }

  $ g++ --std=c++14 -O3 float2.cpp -o float2 \
    && time ./float2
  1e+08

  real    0m0.001s
  user    0m0.000s
  sys     0m0.000s

Just for completeness.]