Re: Is this a bug of std::unique_ptr?
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-cons=
tant
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 declar=
ed '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;
}