Type lists
- C++ course
- Expert
- Lesson 3/47
- 5 min
Learn compile-time lists with type lists.
Type lists
A type list is a data structure that stores types at compile time. It is written with a variadic template: template <typename... Ts> struct Typelist {};. Here Ts is a parameter pack of types.
The length is computed at compile time with a recursive helper. The empty list has length 0; a non-empty list has length 1 + Length<tail>::value. So for Typelist<int, double, char> the length is 3.
The Length helper
Length returns how many types a Typelist holds. The general specialization peels the head and calls Length on the rest: template <typename Head, typename... Tail> struct Length<Typelist<Head, Tail...>> { static const int value = 1 + Length<Typelist<Tail...>>::value; };.
For the empty list template <> struct Length<Typelist<>> { static const int value = 0; };. This has the same shape as the recursive Factorial; it just works on a list of types.
Practice Write Typelist and Length
Define template <typename... Ts> struct Typelist {};. Write the general Length specialization with 1 + Length<Typelist<Tail...>>::value. Add the empty specialization with value = 0. In main write std::cout << Length<Typelist<int, double, char>>::value;.
Starter code
#include <iostream>
// Define Typelist and Length templates here
int main() {
// Print the length result
return 0;
}Sample solution
#include <iostream>
template <typename... Ts>
struct Typelist {};
template <typename List>
struct Length;
template <typename Head, typename... Tail>
struct Length<Typelist<Head, Tail...>> {
static const int value = 1 + Length<Typelist<Tail...>>::value;
};
template <>
struct Length<Typelist<>> {
static const int value = 0;
};
int main() {
std::cout << Length<Typelist<int, double, char>>::value;
return 0;
}Expected output
3
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match each piece to its role
| Term | Meaning |
|---|---|
typename... Ts |
Parameter pack |
struct Typelist |
List structure |
Length<Typelist<...>> |
Compile-time length |
Typelist<int, char> |
Two types |
2 Empty list length
template <> struct Length<Typelist<>> { static const int value = ____; };template <> struct Length<Typelist<>> { static const int value = 0; };Explanation: The empty list has length 0.
3 Length of Typelist<int, double, char>?
template <typename... Ts> struct Typelist {};
template <typename Head, typename... Tail> struct Length<Typelist<Head, Tail...>> { static const int value = 1 + Length<Typelist<Tail...>>::value; };- 1
- 6
- 0
- 3 Correct answer
Explanation: int, double and char are three types; length is 3.