# C++ Tuple元组实现 Metaprogramming
核心玩法：模板递归、偏特化
template<typename ... Tn>struct Tuple;
template<typename T0,typename ... Tn>
struct Tuple<T0,Tn...>{
T0 head;
Tuple<Tn...>tail;
constexpr static int size = sizeof...(Tn)+1;
Tuple():head(0){
}
Tuple(const T0 &h,const Tuple<Tn...> &t):head(h),tail(t){
}
Tuple(const Tuple<T0,Tn...> & a):head(a.head),tail(a.tail){
}
template<typename _T0,typename ... _Tn,typename =std::enable_if_t<sizeof...(Tn)==sizeof...(_Tn)>>
Tuple( _T0&& a, _Tn && ... n):head(std::forward<_T0>(a)),tail(std::forward<_Tn>(n)...){
}
};
template<typename T>
struct Tuple<T>{
constexpr static int size = 1;
T head;
Tuple():head(0){
}
template<typename _T>
Tuple( _T&& a):head(a){
}
};
## 1、类型搜索
从Tuple中搜索某个类型，只要存在一个，则返回true,否则返回false 例如：Tuple<char,short,double> 类型中是否包含 某个类型（short）？只要包含返回true,否则返回false
```
template<typename Patten,typename ...T>struct Search;
template<typename Patten,typename H,typename ...T>
struct Search<Patten,Tuple<H,T...>>: std::conditional_t<std::is_same_v<Patten, H>,std::true_type, Search<Patten, Tuple<T...>>> {
};
template<typename Patten>
struct Search<Patten,Tuple<>>: std::false_type {
};
///Test
int main(int argc, const char * argv[]) {
Tuple<char,short,double> a('s',5,6.8);
bool x0x = Search<short, decltype(a)>::value;/// true
bool x2x = Search<double, decltype(a)>::value;/// true
bool x3x = Search<unsigned short, decltype(a)>::value;/// false
bool x4x = Search<int, decltype(a)>::value;/// false
return 0;
}
```
## 2、类型搜索（并统计个数）
或者，我们可以更近一步，从Tuple中搜索某个类型，并且统计出有多少个该类型？
```
template<typename Patten,typename ...T>struct Count;
template<typename Patten,typename H,typename ...T>
struct Count<Patten,Tuple<H,T...>>:std::integral_constant<int, std::is_same_v<Patten, H> + Count<Patten, Tuple<T...>>::value > {
};
template<typename Patten>
struct Count<Patten,Tuple<>>: std::integral_constant<int, 0> {
};
///Test
int main(int argc, const char * argv[]) {
Tuple<char,short,double,short,int,double,double> a;
int c0 = Count<char, decltype(a)>::value;///1
int c1 = Count<short, decltype(a)>::value;///2
int c2 = Count<double, decltype(a)>::value;///3
int c3 = Count<bool, decltype(a)>::value;///0
int c4 = Count<unsigned char, decltype(a)>::value;///0
int c5 = Count<int, decltype(a)>::value;///1
return 0;
}
```
有了上述统计类型个数的方法，判断Tuple中是否包含某个类型还可以修改成如下：
原理：该类型个数大于0返回true,否则返回false，不过效率相对低点，因为不管是否包含都会完全遍历完整个类型列表。
```
///判断是否存在某个类型？
template<typename Patten,typename ...T>
using search_t = std::conditional_t< (Count<Patten, T...>::value > 0) , std::true_type, std::false_type>;
///Test
int main(int argc, const char * argv[]) {
Tuple<char,short,double,short,double,double> a;
bool x0x = search_t<short, decltype(a)>::value;/// true
bool x2x = search_t<double, decltype(a)>::value;/// true
bool x3x = search_t<unsigned short, decltype(a)>::value;/// false
bool x4x = search_t<int, decltype(a)>::value;/// false
return 0;
}
```
## 3、类型搜索（返回首次匹配到该类型时的位置）
或者，我们可以更更再近一步，从Tuple中搜索某个类型，并返回第一次匹配到该类型时的位置（从1开始），如果没有匹配到返回0
```
template<typename Patten,typename ...T>struct SearchHIdx;
template<typename Patten,typename H,typename ...T>
struct SearchHIdx<Patten,Tuple<H,T...>>: std::integral_constant<int,(std::is_same_v<Patten, H> ? sizeof...(T) : SearchHIdx<Patten, Tuple<T...>>::value )>{
};
template<typename Patten>
struct SearchHIdx<Patten,Tuple<>>: std::integral_constant<int, 0> {
};
template<typename Patten,typename ...T>struct SearchH;
template<typename Patten,typename ...T>
struct SearchH<Patten,Tuple<T...>> :std::integral_constant<int,((SearchHIdx<Patten, Tuple<T...>>::value == 0) ? 0 : (sizeof...(T) - SearchHIdx<Patten, Tuple<T...>>::value))>{
};
///Test
int main(int argc, const char * argv[]) {
Tuple<char,short,double,short,double,double> a;///6
int idx0 = SearchH<char, decltype(a)>::value;///1
int idx1 = SearchH<short, decltype(a)>::value;///2
int idx2 = SearchH<double, decltype(a)>::value;///3
int idx3 = SearchH<int, decltype(a)>::value;///0， Not found
return 0;
}
```
## 4、萃取Tuple指定位置的类型
我们可以从Tuple中萃取指定位置的类型，例如第一个、最后一个、或者第n个...
可以例如模版函数声明，代码如下：
```
template<typename Head,typename ...Trail>
Head ForntT(Tuple<Head,Trail...>);///获取第一个类型
///template<typename ...Head,typename Trail>///获取最后一个类型
///Trail BackT(Tuple<Head...,Trail>);///【无法使用该模板】：因为Tuple<Head...,Trail>类型作为函数参数编译器推断不出来
template<typename Head,typename ...Trail>
Tuple<Trail...> PopForntT(Tuple<Head,Trail...>);///移除第一个类型
template<typename Head,typename ...Trail>
Tuple<Head,Trail...> PushForntT(Head ,Tuple<Trail...>);///插入类型到Tuple的首部
template<typename ...Head,typename Trail>
Tuple<Head...,Trail> PushBackT(Tuple<Head...>,Trail);///插入类型到Tuple的尾部
///Test
int main(int argc, const char * argv[]) {
Tuple<char,short,double> a;
decltype(ForntT(a)) x1;/// char x1;
decltype(PopForntT(a)) x2;/// Tuple<short,double> x2;
decltype(PushBackT(a,false)) x3;/// Tuple<char,short,double,bool> x3;
return 0;
}
```
通过上述函数模板的声明，可以利用编译器推断出来Tuple的部分位置的情况，但是上述方法无法推断出Tuple的最后一个类型，或者移除最后一个类型，我们得想其他办法。我们尝试用类模板偏特化试试看：
```
template<typename T>struct FrontC;
template<typename Head,typename ...Trail>
struct FrontC<Tuple<Head,Trail...>>{///获取第一个类型
using type = Head;
};
//template<typename T>struct BackC;
//
//template<typename ...Head,typename Trail>
//struct BackC<Tuple<Head...,Trail>>{///获取最后一个类型，无法编译通过
// using type = Trail;
//};
template<typename T>struct PopFrontC;
template<typename Head,typename ...Trail>
struct PopFrontC<Tuple<Head,Trail...>>{///移除第一个类型
using type = Tuple<Trail...>;
};
template<typename Head,typename ...Trail>struct PushFrontC;
template<typename Head,typename ...Trail>
struct PushFrontC<Head,Tuple<Trail...>>{///插入类型到Tuple首部
using type = Tuple<Head,Trail...>;
};
template<typename Trail,typename ...Head>struct PushBackC;
template<typename Trail,typename ...Head>
struct PushBackC<Trail,Tuple<Head...>>{///插入类型到Tuple尾部
using type = Tuple<Head...,Trail>;
};
int main(int argc, const char * argv[]) {
Tuple<char,short,double> a;
FrontC<decltype(a)>::type x1;/// char x1;
PopFrontC<decltype(a)>::type x2;/// Tuple<short,double> x2;
PushFrontC<bool, decltype(a)>::type x3;/// Tuple<bool,char,short,double> x3;
PushBackC<int, decltype(a)>::type x4;/// Tuple<char,short,double,int> x4;
return 0;
}
```
发现类模板也无法萃取出Tuple的最后一个类型，或者移除最后一个类型。因为Tuple<Head...,Trail>这样的类型，无法被偏特化。
### 萃取Tuple的指定位置的类型
通过Supscript可以萃取指定位置的类型，代码如下：
```
template <int idx, typename ...T>
struct Supscript;
template <int idx,typename H, typename ...T>
struct Supscript<idx,Tuple<H,T...>>{
private:
using last = typename Supscript<idx-1,Tuple<T...>>::type;
public:
using type = std::conditional_t<idx==0, H,last>;
};
template <typename H,typename ...T>
struct Supscript<0,Tuple<H,T...>>{
using type = H;
};
///为何不能这样写？
///template <typename ...T>
///using last_t = typename Supscript<sizeof...(T)-1, T...>::type;
template<typename ...T>struct LastType;
template<typename ...T>
struct LastType<Tuple<T...>>{
using type = typename Supscript<sizeof...(T)-1, Tuple<T...>>::type;
};
int main(int argc, const char * argv[]) {
Tuple<char,short,double,bool> a;
Tuple<short,double> av;
Tuple<char> ab;
Supscript<0, decltype(a)>::type xx4;
Supscript<1, decltype(a)>::type xx43;
Supscript<2, decltype(a)>::type x4x4;
Supscript<3, decltype(a)>::type xx554;
Supscript<0, decltype(av)>::type xx444;
Supscript<1, decltype(av)>::type xx334;
LastType<decltype(a)>::type xaa2a;///bool xaa2a;
LastType<decltype(av)>::type xaa3a;///double xaa3a;
LastType<decltype(ab)>::type xa4aa;///char xa4aa;
return 0;
}
```
上述代码，也可以用另外一种写法实现【好像换汤不换药，逻辑是一样的】，修改后如下：
```
template <int idx, typename ...T>
struct Supscript;
template <int idx,typename H, typename ...T>
struct Supscript<idx,Tuple<H,T...>>{
using type = typename Supscript<idx-1,Tuple<T...>>::type;
};
template <typename H, typename ...T>
struct Supscript<0,Tuple<H,T...>>{
using type = H;
};
template <int idx, typename ...T>
using supscript_t = typename Supscript<idx,T...>::type;
///test
int main(int argc, const char * argv[]) {
Tuple<char,short,double,bool> a;
Tuple<short,double> av;
Tuple<char> ab;
supscript_t<0, decltype(a)> xxx0;/// char xxx0;
supscript_t<1, decltype(a)> xxx1;/// short xxx1;
supscript_t<2, decltype(a)> xxx2;/// double xxx2;
supscript_t<3, decltype(a)> xxx3;///bool xxx3;
supscript_t<0, decltype(av)> xxx4;///short xxx4
supscript_t<1, decltype(av)> xxx5;///double xxx5;
supscript_t<0, decltype(ab)> xxx6;/// char xxx6;
supscript_t<1, decltype(ab)> xxx7;/// Error
return 0;
}
```
经过上述代码改进后，Supscript依然可以萃取到任意指定位置的类型。
## 5、翻转Tuple的类型列表
借助上面PushBackC这个方法可以轻松实现Tuple类型反转。
```
template <int idx, typename ...T>
struct Reverse;
template <int idx,typename H, typename ...T>
struct Reverse<idx,Tuple<H,T...>>{
using type = typename PushBackC<H,typename Reverse<idx-1,Tuple<T...>>::type>::type;
};
template <typename H, typename ...T>
struct Reverse<0,Tuple<H,T...>>{
using type = Tuple<H>;
};
template <int idx, typename ...T>
using reverse_t = typename Reverse<idx, T...>::type;
int main(int argc, const char * argv[]) {
Tuple<char,short,double,bool> a;
Tuple<short,double> av;
Tuple<char> ab;
reverse_t<0, decltype(a)> xfdas0;/// Tuple<char> xfdas0;
reverse_t<1, decltype(a)> xfdas1;/// Tuple<short,char> xfdas1;
reverse_t<2, decltype(a)> xfdas2;/// Tuple<double,short,char> xfdas2;
reverse_t<3, decltype(a)> xfdas3;/// Tuple<bool,double,short,char> xfdas3;
reverse_t<0, decltype(av)> xfdas4;/// Tuple<short> xfdas4;
reverse_t<1, decltype(av)> xfdas5;/// Tuple<double,short> xfdas5;
return 0;
}
```
## 6、两个Tuple合并
可以想办法把两个Tuple内部的类型合并成到一个Tuple中：
```
template<typename T,typename ...T1> struct Merge;
template<typename T,typename T1,typename ...T2>
struct Merge<T,Tuple<T1,T2...>>{
private:
using Head = decltype(PushBackT(T(),T1()));
using Trail = Tuple<T2...>;
public:
using type = typename Merge<Head,Trail>::type;
};
template<typename T,typename T1>
struct Merge<T,Tuple<T1>>{
public:
using type = decltype(PushBackT(T(),T1()));
};
template<typename T,typename ...T1>
using merge_t = typename Merge<T, T1...>::type;
int main(int argc, const char * argv[]) {
Tuple<char,short,double,bool> a;
Tuple<short,double> av;
Tuple<char> ab;
merge_t<decltype(ab), decltype(av)> x11;///Tuple<char,short,double> x11;
merge_t<decltype(av), decltype(av)> x12;/// Tuple<short,double,short,double> x12;
merge_t<decltype(av), decltype(ab)> x13;/// Tuple<short,double,char> x13;
return 0;
}
```
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