How to use templates and/or using`with structs?

I have the following definitions:

template<typename T>
using refval= std::pair<int, T>;

template<typename T>
using indexedvals = std::vector<refval<T>>; 

In order to make my code more readable i wanted to use a struct instead of a pair in the definition of refval - so that in my code i could use a member name like index instead of first, or value instead of second.

So i tried

template<typename T>
typedef struct_t refval {
    refval(int iRefCell, T tValue):m_iRefCell(iRefCell),m_tValue(tValue){};
    int m_iRefCell;
    T   m_tValue;
} refval;

But here the compiler complained with "error: template declaration of ‘typedef’".

Then i tried

template<typename T>
using refval = struct refval_t {
    refval_t():m_iRefCell(-1){};
    refval_t(int iRefCell, T tValue):m_iRefCell(iRefCell),m_tValue(tValue){};
    int m_iRefCell;
    T   m_tValue;
};

template<typename T>
using indexedvals = std::vector<refval<T>>;

Here i got the following complaints

Sampling.h:12:32: error: types may not be defined in alias template declarations
   12 | using refval = struct refval_t {
      |                                ^
Sampling.h:23:7: error: conflicting declaration of template ‘template<class T> using indexedvals = std::vector<refval_t>’
   23 | using indexedvals = std::vector<refval<T>>; 
      |       ^~~~~~~~~~~

So what i want to do is to define my templated struct in such a way that i can use it in a using statement as in the code snippets at the beginning of this post.

How can i achieve this?

(probably an analogous question would be: how is std::pair implemented so that this works?)

Frankly, you are guessing how it works, and this is never going to end well.

You can define a template:

template<typename T>
struct refval_t {
    refval_t():m_iRefCell(-1){};
    refval_t(int iRefCell, T tValue):m_iRefCell(iRefCell),m_tValue(tValue){}
    int m_iRefCell;
    T   m_tValue;
};

and then you can define an alias template:

template<typename T>
using refval = refval_t<T>;

However, I don't think you actually need the alias when you can just use the template directly.

Note that you should initialize the m_tValue member also in the default constructor. As you have it currently, it is too easy to read from it before it is initialized and invoke undefined behavior.