如何在模板函数或类中实例化一个包含类型的容器变量？

Question

如何在模板函数或类中实例化一个包含类型的容器变量？

我说对dependent scope; need typename in front;

推荐类型名称但还是没能使其发挥作用。

#include <iostream> 
#include <array> 
#include <type_traits> 
#include <typeinfo> 
#include <vector> 

using namespace std; 

template<class T> 
void crazy_func_byvalue(T data){ 
    typename decltype(data)::value_type var; //typename added here 
    cout<< typeid(var).name(); 
    cout<<endl; 
    return; 
} 

template<class T> 
void crazy_func_byaddress(T& data){ 
    typename decltype(data)::value_type var; 
    cout<< typeid(var).name(); 
    cout<<endl; 
    return; 
} 

class test{ 
}; 

int main(){ 
    std::array<double,12> var1={1593,-88517,14301,3200,6,-15099,3200,5881,-2593,11,57361,-92990}; 
    decltype(var1)::value_type x; 
    std::vector<test> var2; 
    crazy_func_byvalue(var1); 
    crazy_func_byvalue(var2); 
    crazy_func_byaddress(var1); //error: 'std::array<double, 12ul>&' is not a class, struct, or union type 
    crazy_func_byaddress(var2); //error: 'std::vector<test>&' is not a class, struct, or union type 

    //cout<<"It is working here"; 
    return 0; 
} 

Answer 1

您必须删除引用（与std::remove_reference）

template<class T> 
void crazy_func_byaddress(T& data){ 
    typename std::remove_reference<decltype(data)>::type::value_type var; 
    cout<< typeid(var).name(); 
    cout<<endl; 
    return; 
}