如何檢查參數包中的每種類型是否都是唯一的？

Question

對於固定數量的模板參數很容易，儘管手動書寫檢查的數量以二次方式增長。

#include <type_traits> 

template < 
    typename T1, 
    typename T2, 
    typename T3, 
    typename T4> 
struct unique_types 
{ 
    static_assert(!std::is_same<T1, T2>::value, "Types must be unique"); 
    static_assert(!std::is_same<T1, T3>::value, "Types must be unique"); 
    static_assert(!std::is_same<T1, T4>::value, "Types must be unique"); 
    static_assert(!std::is_same<T2, T3>::value, "Types must be unique"); 
    static_assert(!std::is_same<T2, T4>::value, "Types must be unique"); 
    static_assert(!std::is_same<T3, T4>::value, "Types must be unique"); 
}; 

int main() 
{ 
    // OK. 
    unique_types<int, double, char, float> foo; 

    // Should not compile. 
    unique_types<int, double, char, double> bar; 
} 

對於任意數量類型的參數包，這怎麼實現？

Answer 1

你可以做這樣的：

#include <type_traits> 

template <class ... Trest> 
struct unique_types; 

template <class T1, class T2, class ... Trest> 
struct unique_types<T1, T2, Trest ...> 
: unique_types<T1, T2>, unique_types<T1, Trest ...>, unique_types<T2, Trest ...> {}; 

template <class T1, class T2> 
struct unique_types<T1, T2> 
{ 
    static_assert(!std::is_same<T1, T2>::value, "Types must be unique"); 
}; 

int main() 
{ 
    // OK. 
    unique_types<int, double, char, float> foo; 

    // Should not compile. 
    unique_types<int, double, char, double> bar; 
}