# Can I use one variation template parameter when creating a new one?

I am writing a hoist that is supposed to hoist a variable arity function into some std :: vectors which is like a SQL table with an index. I want to apply the f parameter to every set of elements that have the same id across all vectors. I had a problem with the template output and I overtook it here (minus the iterator logic). I wrote what I think is a reasonable recursive affair, but boilerplate inference considers it not viable.

```
// Simple container type
template <typename T>
struct A {
T x;
T y;
T z;
};
// Wrapper type to differentiate an extracted value from a container
// in template deduction
template <typename T>
struct B {
T value;
};
// Base case. All A<Ts> have been extracted to B<Ts>.
template <typename F, typename... Ts>
void lift (F f, B<Ts> ...bs) {
// Call f with extracted values
f(bs.value...);
}
// Recursive case
template <typename F, typename T, typename... Ts, typename Us>
void lift (F f, A<T> a, A<Ts> ...as, B<Us> ...bs) {
// Copy a value from the beheaded container A<T>.
B<T> b = {a.x};
// Append this B<T> to args and continue beheading A<Ts>.
lift(f, as..., bs..., b);
}
// Test function
template <typename... Ts>
struct Test {
void operator () (Ts...) {}
};
int main () {
B<int> b = {1};
// No errors for the base case
lift(Test<>());
lift(Test<int, int>(), b, b);
// error: no matching function for call to 'lift'
// The notes refer to the recursive case
A<int> a = {1,0,0};
lift(Test<int>(), a); // note: requires at least 3 arguments, but 2 were provided
lift(Test<int>(), a, a); // note: requires 2 arguments, but 3 were provided
lift(Test<int>(), a, a, b); // note: requires 2 arguments, but 4 were provided
}
```

What's wrong with this code?

Ignoring that everything is passed by value here for read / write convenience. Why doesn't it compile?

+3

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1 answer

Why not just just extract the A and B values?

```
template <typename T>
T extractValue(A<T> a)
{
return a.x;
}
template <typename T>
T extractValue(B<T> b)
{
return b.value;
}
template <typename F, typename... T>
void lift (F f, T... values) {
f(extractValue(values)...);
}
```

Also, your last 2 test cases fail to compile because the number of arguments is wrong.

```
lift(Test<int,int>(), a, a);
lift(Test<int,int,int>(), a, a, b);
```

+1

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