Re: reference to vector element

From:

Stuart Golodetz <blah@blah.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 29 Mar 2010 14:34:41 +0100

Message-ID:

<Fs-dnSp9BcTuMS3WnZ2dnUVZ8mCdnZ2d@pipex.net>

On 29/03/2010 10:48, Ralf Goertz wrote:

Hi,

why doesn't this compile?

#include<vector>

int main() {
std::vector<bool> v(10,true);
bool& br=v[3];
}

g++ complains:

error: invalid initialization of non-const reference of type ???bool&???
from a temporary of type ???std::_Bit_reference???

But there are two overloads of [] for std::vector, one reference and one
const reference. So why doesn't the compiler pick the nonconst
reference? Also, neither using at nor dereferencing begin() works.

Here's one possible workaround for the problem (can't promise I haven't
overlooked something as I just hacked it up quickly - but this is the
gist anyway):

#include <vector>

class Bool
{
private:
    bool m_b;

public:
    Bool() {}

    Bool(bool b) : m_b(b)
    {}

    Bool& operator=(bool b) // "a minor performance optimization"
    { m_b = b; return *this; }

    operator bool&()
    { return m_b; }

    operator bool() const
    { return m_b; }
};

int main()
{
    std::vector<Bool> v(10, true);
    bool& b = v[3];
    b = false;
    return 0;
}

Alternatively, you can use something like boost::dynamic_bitset.

Regards,
Stu