Type alias
“type alias”即“类型别名”，即给一个类型，起一个新名字。
cppreference Type alias
C++11 using keyword
c++11引入了using关键字。
cppreference using
Type alias can introduce a member typedef name
在例子中演示了这种写法：
https://en.cppreference.com/w/cpp/language/type_alias
// type alias can introduce a member typedef name
template<typename T>
struct Container { using value_type = T; };
下面是一个demo
template<class T>
class C
{
using TT = T;
};
int main(){}
Reading list
internalpointers The differences between "using" and "typedef" in modern C++
A template declaration cannot appear at block scope
Alias templates on the other hand follow the same rules of any other templated thing in C++: they cannot appear inside a block. They are actual template declarations, after all!