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

From: damien_at_[hidden]
Date: 2007-02-26 14:39:04

I was just typing a somewhat sheepish response along those lines as the
message arrived.  I saw this in the any header as well:

template<typename ValueType>
        class holder : public placeholder
        {
        public: // structors

            holder(const ValueType & value)
              : held(value)
            {
            }

The ValueType is passed by reference into the holder constructor, and you
can't have a reference-reference type.  Pointers it is then...

Damien

> References can not be copied. Thus, you can't make boost::any contain a
> reference (which is what mean when you write boost::any<double&>, I
> assume).
>
> In your example, msgargs.push_back(dref) calls the any-constructor
> any(const T&) with T being double, and this copies the value, not the
> reference itself.
> When you call msgargs.push_back(dptr), it calls the any(const T&) with T
> being double*. The value is copied again, only this time, the value is a
> pointer.
>
> Benjamin Winkler
>
>>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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>>Boost-users_at_[hidden]
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>>
>
>
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