---
title: "type-lookup"
---

{{< callout type="info">}}
  Header: `type_lookup.h`  
{{< /callout >}}

Compile time mapping of id to type, type to id and type to type.  
This file is generated from `type_lookup_generator.h` and can be created by running `generate_type_lookup.bat`.  

For a less complex solution for simple id to type lookup, see `etl::type_select`.

## type_id_pair
Support structure for `type_id_lookup`.  

```cpp
etl::type_id_pair<typename T, size_t ID>
```
**Defines**  
`type` The type.  
`ID`   The id associated with the type.

## type_type_pair
Support structure for `type_type_lookup`.  

```cpp
etl::type_type_pair<typename T1, typename T2>
```

**Defines**  
`type1` The indexing type.
`type2` The associated type.

## type_id_lookup
Creates a class for compile time id to type and type to id mappings.  

```cpp
etl::type_id_lookup<typename T1, ...>
```
The number of types that can be handled is determined by the generator.  
Each template parameter is a `type_id_pair`.

---

The class contains two nested templates.

### type_from_id
Gets the type from the id.
```cpp
type_from_id<size_t ID>
```

**Defines**  
`type` The associated type.  

**C++14 and above**  
```cpp
template <int ID>
using type_from_id_t = typename type_from_id<ID>::type;
```

### id_from_type
```cpp
id_from_type<typename T>
```
Gets the id from the type.  

**Defines**  
ID The associated id.

**C++ 17 and above**  
```cpp
template <typename T>
constexpr size_t id_from_type_v = id_from_type<T>::value;
```

## Static functions
```cpp
template <typename T>
static unsigned int get_id_from_type(const T&)
```
Gets the id from the type.

```cpp
template <typename T>
static unsigned int get_id_from_type()
```
Gets the id from the type.

## type_type_lookup

Creates a class for compile time type to type mapping.  

```cpp
etl::type_id_lookup<typename T1, ...>
```
The number of types that can be handled is determined by the generator.  
Each template parameter is a `type_id_pair`.

---

The class contains one nested template.

### type_from_type
```cpp
type_from_type<typename T>
```

**Defines**  
`type` The associated type.

**C++14 and above**  
```cpp
template <typename T>
using type_from_type_t = typename type_from_type<T>::type;
```

## nth_type
Gets the nth type in a variadic type list.  

```cpp
template <size_t N, typename... TTypes>
struct nth_type
```
Defines member type as the nth type in the variadic type list.

```cpp
template <size_t N, typename... TTypes>
using nth_type_t = typename nth_type<N, TTypes...>::type;
```
Type alias defined as the nth type in the variadic type list.

## Examples

```cpp
enum
{
  INT32  = 32,
  STRING = 1,
  UINT8  = 8,
  OTHER  = 0
};

typedef etl::type_id_pair<int32_t,     INT32>  TypeIdInt32;
typedef etl::type_id_pair<uint8_t,     UINT8>  TypeIdUint8;
typedef etl::type_id_pair<std::string, STRING> TypeIdString;

// Type from Id / Id from Type.
typedef etl::type_id_lookup<TypeIdInt32, TypeIdString, TypeIdUint8> TypeIdLookup;

// int32_t
typename TypeIdLookup::type_from_id_t<INT32>

// uint8_t
typename TypeIdLookup::type_from_id_t<UINT8>

// std::string
typename TypeIdLookup::type_from_id_t<STRING>

// Compile error
typename TypeIdLookup::type_from_id_t<OTHER>

// INT32
TypeIdLookup::id_from_type_v<int32_t>

// UINT8
TypeIdLookup::id_from_type_v<uint8_t>

// STRING
TypeIdLookup::id_from_type_v<std::string>

// Compile error
TypeIdLookup::id_from_type_v<double>
---
type_type_lookup
// Get the next larger type.
typedef etl::type_type_pair<int16_t, int32_t>  TypeInt16;
typedef etl::type_type_pair<uint8_t, uint16_t> TypeUint8;
typedef etl::type_type_pair<float,   double>   TypeFloat;

// Type from Type.
etl::type_type_lookup<TypeInt16, TypeUint8, TypeFloat> TypeTypeLookup;

// int32_t
typename TypeTypeLookup::type_from_type_t<int16_t>

// uint16_t
typename TypeTypeLookup::type_from_type_t<uint8_t>

// double
typename TypeTypeLookup::type_from_type_t<float>

// Compile error
typename TypeTypeLookup::type_from_type_t<int32_t>
```