Re: [Boost-users] [Proto] Storing an Expression Template as a Data Member

On 1/6/2011 6:48 PM, Hossein Haeri wrote:

Hi Eric,

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MyClass mc(_1 >> &f >> &g >> &h); mc.set_input(-61.1); cout << mc.value();//returns h(g(f(-61.1)))

Here is my solution:

template<typename Expr> struct MyClass { MyClass(const Expr& e): formula_(e) {} double calculate(double d) const {return evaluate(formula_, d);} private: const Expr formula_;//*** };

#define DEFINE_MY_CLASS(name, exp) MyClass name(exp) ...

So that my customers can say:

DEFINE_MY_CLASS(mc, _1 >> &f >> &g >> &h); cout << mc.calculate(-6.1);//prints h(g(f(-61.1)))

Does the fact that I store formula_ by value (as in line ***) shoot the trouble of dangling references?

No, the problem still exists. You're storing the outer-most node by value, but all the inner nodes (temporary objects constructed while building the expression) are held by reference. At the very least, the constructor of MyClass needs to pass the incoming expression through deep_copy.

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I would suggest transforming the expression into a unary function object (see my article on cpp-next.com about function composition),

I do follow that series of yours and have to say it's superb! :) I don't see how that's connected though.

That article takes a sequence of function calls and turns them into a single callable object. You're taking a sequence of function calls and are turning them into a callable object.

So, if you think your suggested solution above would be better than that of mine, I'd be more than happy to see more explanation. Specifically, what are you exactly referring to when you use the term "expression" above? And, leave it if you'd think my solution is good enough... :D

Pros and cons. The solution I had in mind didn't involve the preprocessor or Boost.Typeof which (on some platforms) is a real, ugly hack. (But a very useful one, I must admit.) The solution I was suggesting is more work and would have slightly worse runtime performance ... type-erasure always involves an indirection somewhere. It's really a judgment call. -- Eric Niebler BoostPro Computing http://www.boostpro.com