multiple emplace_back calling copy constructor additionally

Question

Consider the below example :

#include <iostream>
#include <vector>

class S {
public:
  S() { puts("S()"); }
  S(int) { puts("S(int)"); }
  ~S() { puts("~S()"); }
  S(const S &) { puts("S(const S&)"); }
  S(S &&) { puts("S&&"); }
  const S &operator=(const S &s) {
    puts("=");
    return s;
  }
  S &operator=(S &&s) {
    puts("Move =");
    return s;
  }
};

int main() {
  std::vector<S> s;
  s.emplace_back();
  s.emplace_back(6);
}

O/p :

S()
S(int)
S(const S&)
~S()
~S()
~S()

When only one element is emplaced_back, the constructor/destructor pair are called exactly once. But when there are multiple emplace_back (like s.emplace_back(6);), the copy constructor is also called. Why is this behavior difference ? is there still a copy exists with emplace_back ?

score 3 · Answer 1 · edited Mar 26 '18 at 17:35

3

Take into account that std::vector sometimes enlarges it's allocated area (when you add to many elements) and must copy old values over to new positions. This cause the call of the copy constructor.

You can call reserve()

std::vector<S> s;
s.reserve(20);
s.emplace_back();
s.emplace_back(6);

to say the s vector has to reserve a larger memory area to avoid relocations of elements.

edited Mar 26 '18 at 17:35

Hatted Rooster

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answered Mar 26 '18 at 17:33

max66

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Shouldn't it use the move constructor when resizing? – Toby Speight Mar 26 '18 at 17:34
@TobySpeight The move constructor is only allowed to be called if it's marked as `noexcept`. – Hatted Rooster Mar 26 '18 at 17:36
Ah, yes; I'd forgotten that. – Toby Speight Mar 26 '18 at 17:37
@TobySpeight - good question; see the comment from Sombrero Chicken and the answer from Liliscent for an answer (and I confess I didn't know it). – max66 Mar 26 '18 at 17:38
1

@SombreroChicken - thanks for the edit-correction :-) – max66 Mar 26 '18 at 17:39

score 3 · Answer 2 · answered Mar 26 '18 at 17:36

3

This is because reallocation happens as explained by @max66. As for why it is the copy constructor being called, it is because your move constructor is not noexcept.

Change to this:

S(S &&) noexcept { puts("S&&"); }

Then it's the move constructor being called.

answered Mar 26 '18 at 17:36

llllllllll

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I am not seeing any difference in terms of construction with adding `noexcept` to move ctor (without `reserve`). Only the copy constructor is replaced with the move constructor. – Sitesh Mar 26 '18 at 17:39
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@Sitesh nothrow_move_constructible is the requirement for calling move constructor in `std::vector` reallocation. – llllllllll Mar 26 '18 at 17:42

multiple emplace_back calling copy constructor additionally

2 Answers2