如何在模板函数或类中实例化包含类型的容器的变量？

Question

如何在模板函数或类中实例化包含类型的容器的变量？

我在dependent scope; need typename in front;

的推荐下添加了typename

但仍然无法使其发挥作用。

#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（使用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;
}