etl/docs/utilities/parameter-pack.md

---
title: "parameter_pack"
---

{{< callout type="info">}}
  Header: `parameter_pack.h`  
  For C++11 or above
{{< /callout >}}

{{< callout type="warning">}}
**This class is deprecated.**  
Use `etl::type_list` as a replacement.
{{< /callout >}}

Compile time mapping of index to type and type to index for a template parameter pack.  

## parameter_pack

```cpp
template <typename... TTypes>
class parameter_pack
```

## Constants
```cpp
static constexpr size_t size = sizeof...(TTypes);
```

## Template alias

```cpp
template <size_t Index, typename... TTypes>
using parameter_pack_t 
    = typename etl::parameter_pack<TTypes...>::template type_from_index_t<Index>;
```
Type from index and type list.

**For C++17 or above**  
```cpp
template <typename T, typename... TTypes>
inline constexpr size_t parameter_pack_v 
    = etl::parameter_pack<TTypes...>::template index_of_type<T>::value;
```
Index from type and type list.

**The class contains two nested templates.**  

## index_of_type

```cpp
template <typename T>
class index_of_type
```

**For C++17 or above**  
```cpp
template <typename T>
static constexpr size_t index_of_type_v = index_of_type<T>::value;
```

## type_from_index

```cpp
template <size_t Index>
class type_from_index
```

---

```cpp
template <size_t Index>
using type_from_index_t = typename type_from_index<Index>::type;
```

## Examples

```cpp
using Pack = etl::parameter_pack<char, short, int>;

constexpr size_t size = Pack::size; // size == 3

size_t indexChar  = Pack::index_of_type_v<char>;   // indexChar  == 0
size_t indexShort = Pack::index_of_type_v<short>;  // indexShort == 1
size_t indexInt   = Pack::index_of_type_v<int>;    // indexInt   == 2
size_t indexLong  = Pack::index_of_type_v<long>;   // Static assert
                                                   // "T is not in parameter pack"

using type0 = typename Pack::type_from_index_t<0>; // type0 = char
using type1 = typename Pack::type_from_index_t<1>; // type1 = short
using type2 = typename Pack::type_from_index_t<2>; // type2 = int
using type3 = typename Pack::type_from_index_t<3>; // Static assert 
                                                   // "Index out of bounds of parameter pack"
```