Lambda capture is deleted too many times

Question

Lambda capture is deleted too many times

M3 116

In the following code I get the unexpected output:
s(1)
~s(1)
~s(0)
~s(-1)

I'm using VS 2022 preview 7.0 with /std:c++latest and /experimental:module
I was expecting the constructor and destructor to be called the same number of times.
I get expected output with gcc (godbolt).

#include <iostream>

using namespace std;
struct S {
  S() {
    ++count;
    cout << "\ns(" << count << ")";
  }
  ~S() {
    cout << "\n~s(" << count << ")";
    --count;
  }
  static int count;
};
int S::count = 0;
auto my_test(S const s) {
  return [s](int i) { return true; };
}

int main(int, char **) { auto f = my_test(S()); }

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Answer 1

RLWA32 52,571

And what do you see if you provide a copy constructor that increments the count variable?

M3 116 Reputation points

2021-10-31T00:28:51.387+00:00

I think the copy constructor increments the count on line 6, but either way the text in the output makes the imbalanced number of calls clear
RLWA32 52,571 Reputation points

2021-10-31T00:34:36.313+00:00

No, line 6 is not in a copy constructor. struct S gets the default compiler generated copy constructor that doesn't touch the count variable and that doesn't print anything to cout.

Just for fun, provide the copy constructor that increments the count and prints to cout.
M3 116 Reputation points

2021-10-31T00:54:06.2+00:00

I wasn't paying attention to the word "copy". I added this copy constructor
S( const S &s ) {
count = s.count;
++count;
cout << "\ncopy s(" << count << ")";
}

And got the expected answer
s(1)
copy s(2)
~s(2)
~s(1)

Thanks. That solves that.
RLWA32 52,571 Reputation points

2021-10-31T01:18:28.847+00:00

You're welcome.

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Lambda capture is deleted too many times

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