Reading from an istream to a bool vector

[Nomak]

Hello,

With this code:

$ cat -n ifs.cc
1 #include <vector>
2 #include <iostream>
3
4 using std::vector;
5 using std::cin;
6
7 int
8 main()
9 {
10 vector< bool > vb(1);
11 vector< int > vi(1);
12 bool b;
13 int i;
14
15 cin >> b;
16 cin >> i;
17 cin >> (vb[0]);
18 cin >> (vi[0]);
19 return 0;
20 }

I have the following errors:

$ g++-2.95.3 -Wall ifs.cc
ifs.cc: In function `int main()':
ifs.cc:17: initialization of non-const reference type `bool &'
ifs.cc:17: from rvalue of type `bool'
/.../iostream.h:217: in passing argument 1 of `istream::opera tor

(bool &)'

$ g++-3.3.2 -Wall ifs.cc
ifs.cc: In function `int main()':
ifs.cc:17: error: no match for 'operator>>' in 'std::cin >>
std::vector<boo l, _Alloc>::operat or[](unsigned int) [with _Alloc =
std::allocator< bool>](0)'

$ g++-3.4.0 -Wall ifs.cc
ifs.cc: In function `int main()':
ifs.cc:17: error: no match for 'operator>>' in 'std::cin >>
(&vb)->std::vector<bo ol, _Alloc>::operat or[] [with _Alloc =
std::allocator< bool>](0u)'

$ g++-3.4.1 -Wall ifs.cc
ifs.cc: In function `int main()':
ifs.cc:17: error: no match for 'operator>>' in 'std::cin >>
(&vb)->std::vector<bo ol, _Alloc>::operat or[] [with _Alloc =
std::allocator< bool>](0u)'

So i guess this is the good behavior, but does anybody have a
rationale for that? Why can i read into an int vector but not a bool
one??

--
Nomak

[Ali Cehreli]

On Sat, 07 Aug 2004 12:37:30 -0700, Nomak wrote:

$ cat -n ifs.cc
1 #include <vector>
2 #include <iostream>
3
4 using std::vector;
5 using std::cin;
6
7 int
8 main()
9 {
10 vector< bool > vb(1);
11 vector< int > vi(1);
12 bool b;
13 int i;
14
15 cin >> b;
b is an lvalue, we can pass it as a non-const reference to
operator>>.
16 cin >> i;
17 cin >> (vb[0]);
vb[0] is an rvalue because vector<bool>::o perator[] returns 'bool'
(the value of the first object, by-value).
18 cin >> (vi[0]);
vi[0] is an lvalue because vector<int>::op erator[] returns 'int&'
(the first object by-reference).
19 return 0;
20 }

I have the following errors:

$ g++-2.95.3 -Wall ifs.cc
ifs.cc: In function `int main()':
ifs.cc:17: initialization of non-const reference type `bool &'
ifs.cc:17: from rvalue of type `bool' /.../iostream.h:217: in passing
argument 1 of `istream::opera tor

(bool &)'

[...]
So i guess this is the good behavior, but does anybody have a rationale
for that? Why can i read into an int vector but not a bool one??

There is a specialization for vector<bool> where the return type
of operator[] differs from the return value of operator[] of the
general vector implementation.

vector<bool> is special in that it uses single bits to store the
bool objects. In a typical implementation, an unsigned int may be
used as the underlying data type. On a 32-bit machine this might
have the first 32 objects of the container share one unsigned
int.

Since it is not possible to return a reference to a single bit,
operator[] has to return by-value in the vector<bool>
specialization.

Ali

[Nomak]

Ali Cehreli wrote:

[...]

So i guess this is the good behavior, but does anybody have a
rationale for that? Why can i read into an int vector but not a
bool one??

There is a specialization for vector<bool> where the return type
of operator[] differs from the return value of operator[] of the
general vector implementation.

vector<bool> is special in that it uses single bits to store the
bool objects. In a typical implementation, an unsigned int may be
used as the underlying data type. On a 32-bit machine this might
have the first 32 objects of the container share one unsigned
int.

Since it is not possible to return a reference to a single bit,
operator[] has to return by-value in the vector<bool>
specialization.

Thanks. It makes my loading code ugly...

--
Nomak

[Sylvain Audi]

>> 16 cin >> i;

17 cin >> (vb[0]);
vb[0] is an rvalue because vector<bool>::o perator[] returns 'bool'
(the value of the first object, by-value).

[...]
Since it is not possible to return a reference to a single bit,
operator[] has to return by-value in the vector<bool>
specialization.

That might not be that simple, otherwise it would be impossible to
change any value in that bool array : if vb[i] is rvalue, we can't
assign anything to it.

If I'm not mistaken, the vector<>::opera tor[] method returns an object
of type named "reference" , which (in the case of bool vector) is
probably some kind of proxy object for the targetted array element.

Not too sure though, but I can't think of anything else that would do
the job...

Sylvain.

[Ali Cehreli]

On Fri, 13 Aug 2004 17:47:11 -0700, Sylvain Audi wrote:

16 cin >> i;
17 cin >> (vb[0]);
vb[0] is an rvalue because vector<bool>::o perator[] returns 'bool' (the
value of the first object, by-value).

[...]

Since it is not possible to return a reference to a single bit,
operator[] has to return by-value in the vector<bool> specialization.

That might not be that simple, otherwise it would be impossible to
change any value in that bool array : if vb[i] is rvalue, we can't
assign anything to it.

Thank you for the correction.
If I'm not mistaken, the vector<>::opera tor[] method returns an object
of type named "reference" , which (in the case of bool vector) is
probably some kind of proxy object for the targetted array element.

Even when writing the above I was thinking about the posibility of such
a proxy but didn't pay much attention to it. A quick test of mine made
me believe that the operator>> for the proxy can do what the OP
needed. I am not sure why it's not implemented that way.

In any case, googling for older posts show that vector<bool> does not
satisfy some of the container requirements anyway.

Ali