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

Yes, I know. If you mean this line: msgargs.push_back(dref) that's supposed to create a boost::any<double&> directly in the vector. There's an example like that in the boost docs: typedef std::list<boost::any> many; void append_string(many & values, const std::string & value) { values.push_back(value); } The argument to the message is a reference to a vector. Damien

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@khubla.com> 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 _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users

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