/* The following code example is taken from the book * "C++ Templates - The Complete Guide" * by David Vandevoorde and Nicolai M. Josuttis, Addison-Wesley, 2002 * * (C) Copyright David Vandevoorde and Nicolai M. Josuttis 2002. * Permission to copy, use, modify, sell and distribute this software * is granted provided this copyright notice appears in all copies. * This software is provided "as is" without express or implied * warranty, and with no claim as to its suitability for any purpose. */#include <stddef.h>#include <cassert>// include helper class traits template to select whether to refer to an// ``expression template node'' either ``by value'' or ``by reference.''#include "exprops1a.hpp"// class for objects that represent the addition of two operandstemplate <typename T, typename OP1, typename OP2>class A_Add {  private:    typename A_Traits<OP1>::ExprRef op1;    // first operand    typename A_Traits<OP2>::ExprRef op2;    // second operand  public:     // constructor initializes references to operands    A_Add (OP1 const& a, OP2 const& b)     : op1(a), op2(b) {    }    // compute sum when value requested    T operator[] (size_t idx) const {        return op1[idx] + op2[idx];    }    // size is maximum size    size_t size() const {        assert (op1.size()==0 || op2.size()==0                || op1.size()==op2.size());        return op1.size()!=0 ? op1.size() : op2.size();    }};// class for objects that represent the multiplication of two operandstemplate <typename T, typename OP1, typename OP2>class A_Mult {  private:    typename A_Traits<OP1>::ExprRef op1;    // first operand    typename A_Traits<OP2>::ExprRef op2;    // second operand  public:    // constructor initializes references to operands    A_Mult (OP1 const& a, OP2 const& b)     : op1(a), op2(b) {    }    // compute product when value requested    T operator[] (size_t idx) const {        return op1[idx] * op2[idx];    }    // size is maximum size    size_t size() const {        assert (op1.size()==0 || op2.size()==0                || op1.size()==op2.size());        return op1.size()!=0 ? op1.size() : op2.size();    }};