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

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, 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::vectorboost::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::vectorboost::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 _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users

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