Is this a bug of std::unique_ptr?

[Jayden Shui]

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

 

[Alf P. Steinbach]

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.

(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(8) : 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

 

[Jayden Shui]

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(8) : 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;
}

 

[Krice]

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

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

 

[none]

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> >’