Metaprogramming(1)

C++ Templates metaprogramming

 

第二章练习

2-0. 的三种解法

// test.cpp : 定义控制台应用程序的入口点。//#include "stdafx.h"#include <boost/tr1/type_traits.hpp>#include <boost/static_assert.hpp>#include <boost/mpl/if.hpp>//struct result<class T>//{//static const bool value=boost::is_reference<T>;//}namespace test1{template<class T, bool isRef = std::tr1::is_reference<T>::value >struct add_const_ref{typedef T const & type;};template <class T>struct add_const_ref<T, true>{typedef T type;};}namespace test2{template <class T, bool isRef>struct add_const_ref_helper{typedef T const& type;};template <class T>struct add_const_ref_helper<T, true>{typedef T type;};template <class T>struct add_const_ref{typedef typename add_const_ref_helper<T, std::tr1::is_reference<T>::value>::type type; //像调用函数一样使用辅助模板类};}namespace test3{template <bool isRef>struct add_const_ref_helper//外覆类{template <class T>struct work{typedef T const & type;};};template <>struct add_const_ref_helper<true>{template <class T>struct work{typedef T type;};};template <class T>struct add_const_ref{typedef typename add_const_ref_helper<std::tr1::is_reference<T>::value>::work<T>::type type;};}namespace test4//关于外覆类的另一个例子{using boost::mpl::bool_;struct and_{template <typename a, typename b>struct work: public bool_<a::value && b::value>{};};struct or_{template <typename a, typename b>struct work: public bool_<a::value || b::value>{};}; //要求实现的功能：//par1: bool_:true return par2 and_ par3//false return par2 or_ par3template <typename arg1>struct get_op{typedef boost::mpl::if_<arg1, and_, or_> type;// arg1 ? arg2 : arg3 //这里在没有使用外覆类的时候会因为模板类没有实例化而出错 //出错例见namespace test5};template <typename op, typename arg1, typename arg2>struct apply{typedef typename op::work<arg1, arg2>::type type; };template <typename arg1, typename arg2, typename arg3>struct work{typedeftypename apply<typename get_op<arg1>::type, arg2, arg3>::type type;};}namespace test5{using boost::mpl::bool_;template<typename a,typename b>struct and_: public bool_<a::value && b::value>{};template <typename a, typename b>struct or_: public bool_<a::value || b::value>{};//par1: bool_:true return par2 and_ par3//false return par2 or_ par3template <typename arg1>struct get_op{typedef boost::mpl::if_<arg1, and_, or_> type;// arg1 ? arg2 : arg3//error C3203: “and_”: 未专用化的类 模板 不能用作 模板 变量，该变量属于 模板 参数“T2”，应为 real 类型//因此在test4中建立普通类and_和or_,其中定义模板类};template <typename op, typename arg1, typename arg2>struct apply{typedef typename op<arg1, arg2>::type type;};template <typename arg1, typename arg2, typename arg3>struct work{typedeftypename apply<typename get_op<arg1>::type, arg2, arg3>::type type;};}using namespace test3;//验证，如果is_same结果不为true，则编译时报错BOOST_STATIC_ASSERT((std::tr1::is_same<typename add_const_ref<int &>::type, int&>::type::value));BOOST_STATIC_ASSERT((std::tr1::is_same<typename add_const_ref<int const &>::type, int const &>::type::value));BOOST_STATIC_ASSERT((std::tr1::is_same<typename add_const_ref<int>::type, int const &>::type::value));int main(){}