Storing expression template functions

Question

I'm really interested in expression patterns at the moment, and you want to code a library for writing and differentiating mathematical functions using the lambda-style syntax. At the moment I can write (_x * _x)(2);and get the correct result 4. But I would really like to do something like MathFunction f = _x * _x; f(2);that, but I have no ideas on how to deal with the patterns of recursive expressions on the right side. Is it possible to achieve this without using the “auto” keyword instead of the MathFunction or by forcing the operator () virtual?

Thank you for your help!

Answer 1

, , . , , .

Answer 2

.

template<class T, class R>
struct MathFuncBase
{
   virtual R operator()(const T & v) = 0;
   virtual ~MathFuncBase() {}
};

tempate<class T, class R, class Func>
struct MathFunc : MathFuncBase<T, R>
{
   MathFunc(Func func) : func(func) {}
   virtual R operator()(const T & v) {
       return func(v);           
   }
private:
   Func func;
};

tempate<class T, class R, class Func>
boost::shared_ptr<MathFuncBase<T, R> > GetMathFunc(Func func) {
    return boost::shared_ptr<MathFuncBase<T, R> >(new MathFunc<T, R, Func> (func));
}

int main () {
    boost::shared_ptr<MathFuncBase<int, int> > f = GetMathFunc<int,int> (_x * _x);
    return (*f)(2);   
}

Answer 3

, Boost , , .

, :
http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Expression-template
http://www.angelikalanger.com/Articles/Cuj/ExpressionTemplates/ExpressionTemplates.htm http://www.flipcode.com/archives/Faster_Vector_Math_Using_Templates.shtml

- !

.

Answer 4

, . boost:: lambda, , , int, :

#include <boost/lambda/lambda.hpp>

int main()
{
    using namespace boost::lambda;
    //int x = _1 + _2;
    boost::lambda::lambda_functor<boost::lambda::lambda_functor_base<
    boost::lambda::arithmetic_action<boost::lambda::plus_action>, 
    boost::tuples::tuple<boost::lambda::lambda_functor<boost::lambda::placeholder<1> >, 
    boost::lambda::lambda_functor<boost::lambda::placeholder<2> >, 
    boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, 
    boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, 
    boost::tuples::null_type, boost::tuples::null_type> > > x = _1 + _2;
}

, , boost::function.

#include <boost/lambda/lambda.hpp>
#include <boost/function.hpp>   
int main()
{
    using namespace boost::lambda;
    boost::function<int(int, int)> x = _1 + _2;
    return x(-1, 1);
}

Answer 5

, . , auto . , , .

I suspect that without autoyou you cannot do this.