Re: [Boost-users] [boost::any] - reference vs pointer

From: Jan Van den bosch (van.schonshoven_at_[hidden])
Date: 2007-02-26 09:47:48

Hi,

There's no such thing as a vector (or any other container) of references.
Try this and see how the compiler reacts: vector<int &> v;

Jan

On 2/26/07, Damien <damien_at_[hidden]> wrote:
>
> Hi all,
>
> I've been trying out boost::any in a messaging system to package up
> function arguments.  A message
> call is basically
>
> Message::Call(std::vector<boost::any>& args)
> {
> try
> ...a few any_cast<..>
> ...fire the actual function...
> catch
> ...etc
> }
>
> It's all good, except for one niggling problem.  Say I have a
> double-precision value that I need
> changed inside the message, so it needs to be passed as a double* or a
> double& in an argument.  The
> problem is, if I create a double& inside a boost::any, it doesn't point to
> the right memory location
> once I'm inside the message, and it looks like it's copying the original
> dereferenced value and
> giving me a reference to the copy.  If I create a double* inside a
> boost::any, I get a new pointer
> inside the message like the reference, but because the pointer target is
> correct everything works.
> The code looks like this:
>
> ...
> ...
> std::vector<boost::any> args;
> double d = 1.5;
> double* dptr = &d;
> msgargs.push_back(dptr);//works fine...
> double& dref = d;
> msgargs.push_back(dref);//seems to copy d instead of a reference to d...
> ...
> msg->Call(args);
> ...
>
> I can live with this, I just have to pass "large" objects by pointer so
> they don't get copied.  BUT:
>   Can anyone tell me why references are creating a copy of the original
> and not a copy of the
> reference, or how I could avoid the copying?  Or am I doing something
> dumb?
>
> Damien
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