[C + + templates] Length and typeat in Loki use c++11 properties to improve

C + + template parameter implementation of length and typeat in Loki

First, the length and typeat simulations in the original Loki are implemented as follows

1, template file

/**********************************                                                                ** author : szyu* * date : 2017.1.7***************************************/#ifndef  __szyu_loki__#define  __SZYU_LOKI__#include <iostream> #include  <vector>template <typename t , typename u>class typelist{public:    typedef t head;     typedef U Tail;}; class nulltype; #define &NBSP;TYPELIST_1 (type1)                 TypeList<type1, NullType> #define  typelist_2(type1, type2)         typelist<type1, typelist_1 (type2) > #define &NBSP;TYPELIST_3 (type1, type2, type3)  typelist<type1, typelist_2 (type2, &NBSP;TYPE3) >/********************************************* loki length************************** /template <typename t> class length;template <typename  T, typename U>class Length<TypeList<T, U> >{public:     enum { value = 1 + Length<U>::value };}; Template <>class length<nulltype>{public:    enum { value  = 0 };};/  loki typeat******************************************* /template <typename t, unsigned int index> class typeat;template  <typEname t, typename u, unsigned int index>class typeat<typelist<t,  u>, index>{public:    typedef typename typeat<u, index  - 1>::Result Result;}; Template <typename t, typename u>class typeat<typelist<t, u>,  0>{public:    typedef T Result;}; #endif

2, test case

/**********************************                                                                 ** author :  szyu** date : 2017.1.7***************************************/#include   "loki.h" Voidtest1 () {    std::cout <<  "length<typelist_1 (int) ..."  < < length<typelist_1 (int) >::value << std::endl;    std::cout  <<  length<typelist_2 (int, int)  << length<typelist_2 (int,  int) >::value << std::endl;    std::cout <<  " Length<typelist_3 (Int, int, int)  << length<typelist_3 (int, int, int) >::value <<  std::endl;    std::cout <<  "************************************"  << std::endl;    std::vector<int> IntVector;     std::cout << typeid (Typeat<typelist_3 ( double, std::string, std:: vector<int>),  0>::result). Name ()  << std::endl;    std::cout  << typeid (Typeat<typelist_3 ( double, std::string, std::vector<int>),  1>::result). Name ()  << std::endl;    std::cout <<  typeID (Typeat<typelist_3 ( double, std::string, std::vector<int>),  2>::result). Name ()  << std::endl;        typeat<typelist_3 (  double, std::string, std::vector<int>),  0>::result var1;    var1 = 10.10 ;         typeat<typelist_3 (&NBSP;DOUBLE,&NBSP;STD::STRING,&NBSP;STD ::vector<int>), 1>::result var2;    var2 =  "abc";     typeat<typelist_3 ( double, std::string, std::vector<int>),  2>:: Result var3;    var3 = intvector;} Intmain ( int argc, char *argv[] ) {    test1 ();     return 0;}

3, the execution results are as follows:

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Second, improve length and typeat using c++11 template parameter characteristics

There are complex typelist nesting issues when you don't use templates, although using a macro definition can increase code readability, but you also need to define a large number of macros to support. When hundreds of parameters (extreme) are present, it is necessary to define hundreds of macros. Obviously more troublesome. C++11 provides support for the template's variable parameters, so the variable parameter implementation length and Typeat are as follows:

1, header file

/**********************************                                                                 ** author :  szyu** date : 2017.1.6************************************/#ifndef  __szyu_loki__# Define __szyu_loki__#include <iostream> #include  <vector>/**************************  loki length****************************************/template <typename t,  typename... args>class length{public:    enum { value =  1 + Length<Args...>::value };}; Template <typename t>class length<t>{public:    enum { value = 1 };};/  loki typeat****************************************/template  <unsigned int index, typename t, typename... args>class typeat{ Public:    typedef typename typeat<index - 1, args...>:: result result;}; Template <typename t, typename... args>class typeat<1, t, args ... >{public:    typedef T Result;}; #endif

2, test case

/**********************************                                                                 ** author :  szyu** date : 2017.1.6************************************/#include   "TypeList.h" Voidtest1 () {    std::cout <<  "Length<int&gt ..."  <<  length<int>::value << std::endl;    std::cout <<  " Length<int, int&gt ... "&NBSP;&LT;&LT;&NBSP;LENGTH&LT;INT,&NBSP;INT&GT;::VALUE&NBSP;&LT;&LT;&NBSP;STD:: endl;    std::cout <<  "Length<int, int, char&gt ..."   << Length<int, int, char>::value << std::endl;    std::cout <<  "* " << std::endl;    std::vector<int>  intvector;    std::cout << typeid (TYPEAT&LT;1,&NBSP;INT,&NBSP;STD:: String, std::vector<int> >::result). Name ()  << std::endl;     std::cout << typeid (typeat<2, int, std::string, std::vector<int>  >::result). Name ()  << std::endl;    std::cout <<  typeID (Typeat<3, int, std::string, std::vector<int> >::result). Name ()   << std::endl;    TypeAt<1, int, std::string, std::vector< int> >::result var1;    var1 = 10;         typeat<2, int, std::string, std::vector<int> >::result var2;     var2 =  "SSSs"; &NBSP;&NBSP;&NBSP;&NBSP;TYPEAT&LT;3,&NBSP;INT,&NBSP;STD::STRING,&NBSP;STD: : Vector<int> >::result var3;    var3 = intvector;} Intmain ( int argc, char *argv[] ) {    test1 ();     return 0;}

3, execution results

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In the implementation of Typeat, the position parameter is moved to the first parameter of the template because the parameter is to be located in the last argument.

Live:

Details of length and typeat in Loki see "new thinking on C + + design"

This article is from the "11988200" blog, please be sure to keep this source http://11998200.blog.51cto.com/11988200/1889957

[C + + templates] Length and typeat in Loki use c++11 properties to improve