How to std::forward class template parameter - c++

Question

I'm getting compiler error when using std::forward on a class template parameter, but not on a function template parameter. I'd like to know how to std::forward a class template parameter. This is my code:
#include <iostream>
#include <vector>
template<typename T>
class Data_List {
std::vector<T> data_list;
public:
Data_List() = default;
~Data_List() = default;
std::size_t get_list_size() {
return data_list.size();
}
void add_to_list(T&& data) {
std::cout << "\nbefore inside add_to_list: " << data.size() << std::endl;
data_list.push_back(std::forward<T>(data));
std::cout << "after inside add_to_list: " << data.size() << std::endl;
}
};
template<typename T>
void print(T&& t) {
}
int main() {
Data_List<std::vector<int>> list_of_data;
std::vector<int> data(100, 2);
std::cout << "\n1. before size: " << data.size() << std::endl;
std::cout << "1. before Data_List size: " << list_of_data.get_list_size() << std::endl;
print(data);
// list_of_data.add_to_list(data); // gives compiler error here
std::cout << "\n1. after size: " << data.size() << std::endl;
std::cout << "1. after Data_List size: " << list_of_data.get_list_size() << std::endl;
std::cout << "--------------------------------------------------------------------------\n" << std::endl;
std::cout << "\n2. before size: " << data.size() << std::endl;
std::cout << "2. before Data_List size: " << list_of_data.get_list_size() << std::endl;
print(std::move(data));
list_of_data.add_to_list(std::move(data));
std::cout << "\n2. after size: " << data.size() << std::endl;
std::cout << "2. after Data_List size: " << list_of_data.get_list_size() << std::endl;
std::cout << "--------------------------------------------------------------------------\n" << std::endl;
}
This is the error I'm getting cannot bind rvalue reference of type ‘std::vector<int>&&’ to lvalue of type ‘std::vector<int>’

Answer 1

In here:
void add_to_list(T&& data) {
std::cout << "\nbefore inside add_to_list: " << data.size() << std::endl;
data_list.push_back(std::forward<T>(data));
std::cout << "after inside add_to_list: " << data.size() << std::endl;
}
data isn't a forwarding reference, it's an rvalue reference, so you can't do something like list_of_data.add_to_list(data); because data is an lvalue.
To create a forwarding reference, the type must exist as a template parameter of the same function template (Give a reading to the forwarding references part of this cppreference.com page and look specifically at the first example):
function parameter of a function template declared as rvalue reference to cv-unqualified type template parameter of that same function template.
Basically, what you want to do is this:
// ...
template <typename U>
void add_to_list(U&& data) {
std::cout << "\nbefore inside add_to_list: " << data.size() << std::endl;
data_list.push_back(std::forward<U>(data));
std::cout << "after inside add_to_list: " << data.size() << std::endl;
}
// ...
Then data becomes a forwarding reference and does what is intended.
Demo