[boost] [Proto] Copying expressions

From: dan marsden (danmarsden_at_[hidden])
Date: 2007-07-17 02:54:03

Hi

I'm creating lambda expressions using proto, using the extension mechanism to add my own operator() so that I can do:

(_0 += _1)(x,y)

The operator() does all the transforming of the expression tree, and evaluates the result. The above works ok, its pretty similar to your calculator examples.

If I want to do something like

function<int(int&,int&)> f(_0 + _1); // Copy lambda expression into a Boost.Function func

Now I've got problems because proto trees are built out of references (to temporaries). So func will blow up later when the temporaries are off the stack (actually the example I've tried starts evaluating junk values). I believe the above code will work today with BLL and Phoenix, as the temporaries are copied rather than handled as references.

As I cannot interfere with the Boost.Function constructors, how can I transform my expression tree to change the refs to copies, whilst retaining the syntax above? Is there some way to tell proto to build trees with copies? Or have I missed something clever in proto again :) It might make a good Calc4 example as I imagine this is a common requirement.

Cheers

Dan

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