[Bug c++/19544] New: Difference in behaviour if default constructor added

[Bug c++/19544] New: Difference in behaviour if default constructor added

[chris at bubblescope dot net]

While looking at PR 19510, I noticed the following strangeness.

Consider:
--
#include<vector>
struct ptr
{
int* a;
ptr() {}
};
std::vector<ptr> v(1);
--
At -O3 -Wall
This code produces the error message:
...../include/c++/4.0.0/bits/stl_construct.h: In function 'void
__static_initialization_and_destruction_0(int,int)':
...../include/c++/4.0.0/bits/stl_construct.h:81: warning: 'SR.186' is used
uninitalized in this function

This warning is not generated if I declare ptr without a default constructor,
although 12.1/7 seems to imply that the default generated constructor should be
exactly what I have written down.

-- 
           Summary: Difference in behaviour if default constructor added
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: chris at bubblescope dot net
                CC: caj at cs dot york dot ac dot uk,gcc-bugs at gcc dot gnu
                    dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19544

[Bug libstdc++/19544] Difference in behaviour if default constructor added

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-01-20 14:56 -------
This is a library bug.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |libstdc++
           Keywords|                            |diagnostic


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19544

[Bug libstdc++/19544] Difference in behaviour if default constructor added

[chris at bubblescope dot net]

------- Additional Comments From chris at bubblescope dot net  2005-01-20 15:14 -------
Heres another test-case...

#include<algorithm>

struct ptr
{
int* a;
ptr() {}
};

struct foo
{
ptr array[1];
foo() { std::uninitalized_fill_n(array,1,ptr()); }
};

foo f;

Which shows it isn't limited to libstdc++ (assuming uninitalized_fill_n is
correct, which I'm fairly sure it is).

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19544

[Bug libstdc++/19544] Difference in behaviour if default constructor added

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-01-20 15:21 -------
Invalid, as you default constructor is not doing its job of initializing:
ptr() {}



-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19544

[Bug libstdc++/19544] Difference in behaviour if default constructor added

[gdr at integrable-solutions dot net]

------- Additional Comments From gdr at integrable-solutions dot net  2005-01-20 19:19 -------
Subject: Re:  New: Difference in behaviour if default constructor added

"chris at bubblescope dot net" <gcc-bugzilla@gcc.gnu.org> writes:

| While looking at PR 19510, I noticed the following strangeness.
| 
| Consider:
| --
| #include<vector>
| struct ptr
| {
| int* a;
| ptr() {}
| };
| std::vector<ptr> v(1);
| --
| At -O3 -Wall
| This code produces the error message:
| ...../include/c++/4.0.0/bits/stl_construct.h: In function 'void
| __static_initialization_and_destruction_0(int,int)':
| ...../include/c++/4.0.0/bits/stl_construct.h:81: warning: 'SR.186' is used
| uninitalized in this function
| 
| This warning is not generated if I declare ptr without a default constructor,
| although 12.1/7 seems to imply that the default generated constructor should be
| exactly what I have written down.

This is a bug in your code.  Can you explain me why it is suddenly a
bug in the library?

-- Gaby


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19544

[Bug libstdc++/19544] Difference in behaviour if default constructor added

[chris at bubblescope dot net]

------- Additional Comments From chris at bubblescope dot net  2005-01-20 19:25 -------
I never thought it was a bug in the library :)

I however throught (incorrectly) that copying an unassigned pointer was valid,
mainly as some other test case was considering constructing
std::vector<std::list<int>::iterator> v(1). Now I realise that in fact copying
an unassigned pointer is undefined behaviour, so of course the error is fine :)
Sorry

I'm still curious as to why the error doesn't appear if let g++ generate a
default constructor, as from my reading of 12.1/7, the default generated
constructor should be "ptr() {}", so the error should still appear. Am I reading
it wrong?

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19544

[Bug libstdc++/19544] Difference in behaviour if default constructor added

[gdr at integrable-solutions dot net]

------- Additional Comments From gdr at integrable-solutions dot net  2005-01-20 19:42 -------
Subject: Re:  Difference in behaviour if default constructor added

"chris at bubblescope dot net" <gcc-bugzilla@gcc.gnu.org> writes:

| I'm still curious as to why the error doesn't appear if let g++ generate a
| default constructor, as from my reading of 12.1/7, the default generated
| constructor should be "ptr() {}", so the error should still appear. Am I reading
| it wrong?

The syntax "ptr()" invokes value initialization, which has the effect of
initializing ptr to zero.  See the relevant section in the standard.

-- Gaby


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19544

[Bug libstdc++/19544] Difference in behaviour if default constructor added

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2005-01-20 20:46 -------
To be completely clear, the compiler generated default constructor is 
  ptr() : a(0) {} 
not 
  ptr() {} 
Thus, it _does_ initialize 'a'. 
 
W. 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19544

[Bug libstdc++/19544] Difference in behaviour if default constructor added

[caj at cs dot york dot ac dot uk]

------- Additional Comments From caj at cs dot york dot ac dot uk  2005-01-20 21:04 -------
Subject: Re:  Difference in behaviour if default constructor
 added

bangerth at dealii dot org wrote:

>------- Additional Comments From bangerth at dealii dot org  2005-01-20 20:46 -------
>To be completely clear, the compiler generated default constructor is 
>  ptr() : a(0) {} 
>not 
>  ptr() {} 
>Thus, it _does_ initialize 'a'. 
> 
>W. 
>
>  
>
Sorry to be dense, but I want to follow this.. Is this a choice g++ 
makes, or is it mandated by the standard? Is this defined by something 
other than 12.1/7, or does 12.1/7 actually require this and I'm 
misreading it?

Thank you




-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19544

[Bug libstdc++/19544] Difference in behaviour if default constructor added

[gdr at integrable-solutions dot net]

------- Additional Comments From gdr at integrable-solutions dot net  2005-01-21 02:25 -------
Subject: Re:  Difference in behaviour if default constructor added

"bangerth at dealii dot org" <gcc-bugzilla@gcc.gnu.org> writes:

| To be completely clear, the compiler generated default constructor is 
|   ptr() : a(0) {} 
| not 
|   ptr() {} 
| Thus, it _does_ initialize 'a'. 

I don't follow you here.

If one writes

    ptrwrapper() : ptr() { }

with the above syntax, ptr is *value initialized*, e.g. it is set to
zero.  That is not to be confused with callig the default constructor
(which is trivial) for ptr.

-- Gaby


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19544

[Bug libstdc++/19544] Difference in behaviour if default constructor added

[gdr at integrable-solutions dot net]

------- Additional Comments From gdr at integrable-solutions dot net  2005-01-21 02:28 -------
Subject: Re:  Difference in behaviour if default constructor added

"caj at cs dot york dot ac dot uk" <gcc-bugzilla@gcc.gnu.org> writes:

| Subject: Re:  Difference in behaviour if default constructor
|  added
| 
| bangerth at dealii dot org wrote:
| 
| >------- Additional Comments From bangerth at dealii dot org  2005-01-20 20:46 -------
| >To be completely clear, the compiler generated default constructor is 
| >  ptr() : a(0) {} 
| >not 
| >  ptr() {} 
| >Thus, it _does_ initialize 'a'. 
| > 
| >W. 
| >
| >  
| >
| Sorry to be dense, but I want to follow this.. Is this a choice g++ 
| makes, or is it mandated by the standard? Is this defined by something 
| other than 12.1/7, or does 12.1/7 actually require this and I'm 
| misreading it?

You mean when you write "ptr()" in the member initializer list?  Check
for value initialization in 8.5.

-- Gaby


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19544