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Hi All,

I accidently find that the following code works with Visual C++
compilation

#include <memory>
#include <iostream>
using namespace std;

int main()
{
unique_ptr<int> const p = new int(1);
unique_ptr<int>& q = p; // assign a constant to a non-constant
reference
q.reset(new int(2));
cout << *p; // output 2
return 0;
}

My question is from the 2nd statement of assigning a constant to a non-
constant reference. I think the compiler should report an compilation
error, but it doesn't. Is this a bug of the code of unique_ptr in the
std template library?

Thanks a lot!

Jayden

On 16.12.2011 15:28, Jayden Shui wrote:
>
> I accidently find that the following code works with Visual C++
> compilation
>
> #include<memory>
> #include<iostream>
> using namespace std;
>
> int main()
> {
> unique_ptr<int> const p = new int(1);
> unique_ptr<int>& q = p; // assign a constant to a non-constant
> reference
> q.reset(new int(2));
> cout<< *p; // output 2
> return 0;
> }
>
> My question is from the 2nd statement of assigning a constant to a non-
> constant reference. I think the compiler should report an compilation
> error, but it doesn't. Is this a bug of the code of unique_ptr in the
> std template library?

It seems to be a case PEBKAC.


<example>
[d:\dev\test]
> (cl /nologo- 2>&1) | find /i "++"
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 16.00.40219.01
for 80x86

[d:\dev\test]
> cl foo.cpp
foo.cpp
foo.cpp(7) : error C2440: 'initializing' : cannot convert from 'int *'
to 'std::unique_ptr<_Ty>'
with
[
_Ty=int
]
Constructor for class 'std::unique_ptr<_Ty>' is declared 'explicit'
with
[
_Ty=int
]
foo.cpp( : error C2440: 'initializing' : cannot convert from 'const
std::unique_ptr<_Ty>' to 'std::unique_ptr<_Ty> &'
with
[
_Ty=int
]
Conversion loses qualifiers

[d:\dev\test]
> (g++ --version 2>&1) | find /i "++"
g++ (TDM-2 mingw32) 4.4.1

[d:\dev\test]
> g++ -std=c++0x foo.cpp
foo.cpp: In function 'int main()':
foo.cpp:7: error: conversion from 'int*' to non-scalar type 'const
std::unique_ptr<int, std::default_delete<int> >' requested
foo.cpp:8: error: invalid initialization of reference of type
'std::unique_ptr<int, std::default_delete<int> >&' from expression of
type 'const std::unique_ptr<int, std::default_delete<int> >'

[d:\dev\test]
> _
</example>


Cheers & hth.,

- Alf

On Dec 16, 9:44*am, "Alf P. Steinbach" <alf.p.steinbach
+use...@gmail.com> wrote:
> On 16.12.2011 15:28, Jayden Shui wrote:
>
>
>
>
>
>
>
>
>
>
>
> > I accidently find that the following code works with Visual C++
> > compilation
>
> > #include<memory>
> > #include<iostream>
> > using namespace std;
>
> > int main()
> > {
> > * * unique_ptr<int> *const p = new int(1);
> > * * unique_ptr<int>& *q = p; // assign a constant to a non-constant
> > reference
> > * * q.reset(new int(2));
> > * * cout<< **p; *// output 2
> > * * return 0;
> > }
>
> > My question is from the 2nd statement of assigning a constant to a non-
> > constant reference. I think the compiler should report an compilation
> > error, but it doesn't. Is this a bug of the code of unique_ptr in the
> > std template library?
>
> It seems to be a case PEBKAC.
>
> <example>
> [d:\dev\test]
> *> (cl /nologo- 2>&1) | find /i "++"
> Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 16.00.40219.01
> for 80x86
>
> [d:\dev\test]
> *> cl foo.cpp
> foo.cpp
> foo.cpp(7) : error C2440: 'initializing' : cannot convert from 'int *'
> to 'std::unique_ptr<_Ty>'
> * * * * *with
> * * * * *[
> * * * * * * *_Ty=int
> * * * * *]
> * * * * *Constructor for class 'std::unique_ptr<_Ty>' is declared 'explicit'
> * * * * *with
> * * * * *[
> * * * * * * *_Ty=int
> * * * * *]
> foo.cpp( : error C2440: 'initializing' : cannot convert from 'const
> std::unique_ptr<_Ty>' to 'std::unique_ptr<_Ty> &'
> * * * * *with
> * * * * *[
> * * * * * * *_Ty=int
> * * * * *]
> * * * * *Conversion loses qualifiers
>
> [d:\dev\test]
> *> (g++ --version 2>&1) | find /i "++"
> g++ (TDM-2 mingw32) 4.4.1
>
> [d:\dev\test]
> *> g++ -std=c++0x foo.cpp
> foo.cpp: In function 'int main()':
> foo.cpp:7: error: conversion from 'int*' to non-scalar type 'const
> std::unique_ptr<int, std::default_delete<int> >' requested
> foo.cpp:8: error: invalid initialization of reference of type
> 'std::unique_ptr<int, std::default_delete<int> >&' from expression of
> type 'const std::unique_ptr<int, std::default_delete<int> >'
>
> [d:\dev\test]
> *> _
> </example>
>
> Cheers & hth.,
>
> - Alf

Sorry, There is a typo in the example code. The corrected version is:

#include <memory>
#include <iostream>
using namespace std;
int main()
{
unique_ptr<int> const p(new int(1));
unique_ptr<int>& q = p; // assign a constant to a non-constant
reference
q.reset(new int(2));
cout << *p; // output 2
return 0;
}

On 16 joulu, 16:28, Jayden Shui <jayden.s...@gmail.com> wrote:
> I accidently find that the following code works with Visual C++
> compilation

So what? Write some real code and stop worrying about non-issues.

In article <3efee69c-4258-4841-a8e5->,
Jayden Shui <> wrote:
>
>Sorry, There is a typo in the example code. The corrected version is:
>
>#include <memory>
>#include <iostream>
>using namespace std;
>int main()
>{
> unique_ptr<int> const p(new int(1));
> unique_ptr<int>& q = p; // assign a constant to a non-constant
>reference
> q.reset(new int(2));
> cout << *p; // output 2
> return 0;
>}

g++ certainly doesn't like it:

g++ uptr.cpp --std=c++0x
uptr.cpp: In function ‘int main()’:
uptr.cpp:9: error: invalid initialization of reference of type ‘std::unique_ptr<int, std::default_delete<int> >&’ from expression of type ‘const std::unique_ptr<int, std::default_delete<int> >’