exprops1.hpp

C++ Templates 完全导引范例

/* 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 operands
template <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 operands
template <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();
    }
};