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From: "tejohnson at google dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/61343] New: [C++11] Missing default initialization for class with default constructor
Date: Wed, 28 May 2014 13:29:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61343

            Bug ID: 61343
           Summary: [C++11] Missing default initialization for class with
                    default constructor
           Product: gcc
           Version: 4.10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tejohnson at google dot com

The following test case does not call the default constructor when expected:

/////////////////////////////
#include <iostream>

struct Foo {
  int value;

  Foo() noexcept {
    std::cout << this << " constructed. Setting value to twelve.\n";
    value = 12;
  }
};

static thread_local Foo a{};
static thread_local Foo b;

static __attribute__((noinline)) void UseA() {
  const int value = a.value;
  std::cout << "Value of A: " << value << "\n";
}

static __attribute__((noinline)) void UseB() {
  const int value = b.value;
  std::cout << "Value of B: " << value << "\n";
}

int main(int argc, char** argv) {
  // std::cout << "Address of A: " << &a << "\n";
  // std::cout << "Address of B: " << &b << "\n";

  UseA();
  UseA();

  UseB();
  UseB();

  UseA();
  UseA();

  return 0;
}
//////////////////////////

When compiled with the current trunk compiler it gives the output:

>    Value of A: 0
>    Value of A: 0
>    0x7f2bc9150728 constructed. Setting value to twelve.
>    0x7f2bc9150724 constructed. Setting value to twelve.
>    Value of B: 12
>    Value of B: 12
>    Value of A: 12
>    Value of A: 12

I would expect something like:

>    0x7ffa4bc63720 constructed. Setting value to twelve.
>    0x7ffa4bc63724 constructed. Setting value to twelve.
>    Value of A: 12
>    Value of A: 12
>    Value of B: 12
>    Value of B: 12
>    Value of A: 12
>    Value of A: 12

It appears as though the empty brace initialization for 'a', which should
result in the user-defined default constructor being invoked, does not actually
do so. Instead the object is zero-initialized until accessing 'b', which
finally causes the default constructor for 'a' to be run as well.

Google ref: b/15250505