# Mundane std::tuple tricks: Finding a type in a tuple

Raymond

Given a tuple, you can extract the type of the `N`th element in the tuple with the help of `std::tuple_element_t<N, Tuple>`

:

// example = char using example = std::tuple_element_t<1, std::tuple<int, char, float>>;

The index is zero-based, so element 0 is `int`

, element 1 is `char`

, and element 2 is `float`

.

What about going in reverse? Given a tuple, find the index of a specific type.

template<typename T, typename Tuple> struct tuple_element_index_helper;

We start by writing a `tuple_`

`element_`

`index_`

`helper`

which does the heavy lifting. It sets `value`

equal to the index of the first element whose type matches, or equal to the number of elements (*i.e.*, one greater than the maximum legal element index) if the type was not found.

template<typename T> struct tuple_element_index_helper<T, std::tuple<>> { static constexpr std::size_t value = 0; };

This is the base case. The type is not found in the empty tuple, so we set the `value`

to zero.

template<typename T, typename... Rest> struct tuple_element_index_helper<T, std::tuple<T, Rest...>> { static constexpr std::size_t value = 0; using RestTuple = std::tuple<Rest...>; static_assert( tuple_element_index_helper<T, RestTuple>::value == std::tuple_size_v<RestTuple>, "type appears more than once in tuple"); };

This is the success case. The type is the first element in the tuple. Therefore, the `value`

is zero (index zero). We also validate that the type is not present in the remaining types of the tuple. If the `value`

for `std::tuple<Rest...>`

is not equal to the size of the tuple, then that means that the type was found among the remaining types, so we raise a compile-time assertion failure.

template<typename T, typename First, typename... Rest> struct tuple_element_index_helper<T, std::tuple<First, Rest...>> { using RestTuple = std::tuple<Rest...>; static constexpr std::size_t value = 1 + tuple_element_index_helper<T, RestTuple>::value; };

And then we have the failure case. If the type does not match the first element in the tuple, then we search for the type in the remaining elements and add one to account for the fact that we removed the first type.

template<typename T, typename Tuple> struct tuple_element_index { static constexpr std::size_t value = tuple_element_index_helper<T, Tuple>::value; static_assert(value < std::tuple_size_v<Tuple>, "type does not appear in tuple"); };

Now that the helper is written, we can write the real template class. It asks the helper to do the work and validates that the resulting `value`

is less than the size of the tuple, meaning that the type was found. If not, then we complain with a compile-time assertion.

template<typename T, typename Tuple> inline constexpr std::size_t tuple_element_index_v = tuple_element_index<T, Tuple>::value;

Finally, we create an alias type to reduce future typing.

Let’s take it out for a spin.

// index = 1 constexpr std::size_t index = tuple_element_index_v<int, std::tuple<char, int, float>>; // error: type does not appear in tuple constexpr std::size_t index = tuple_element_index_v<double, std::tuple<char, int, float>>; // error: type appears more than once in tuple constexpr std::size_t index = tuple_element_index_v<int, std::tuple<char, int, int>>;

All of these mundane tuple tricks will come in handy soon.

**Bonus chatter**: You might be tempted to try something like this:

template<typename T, typename Tuple> constexpr int tuple_element_index_helper() { for (int i = 0; i < std::tuple_size_v<Tuple>; i++) { if constexpr (std::is_same_v< T, std::tuple_element_t<i, Tuple>>) { return i; } } return std::tuple_size_v<Tuple>; }

Unfortunately, it doesn’t work because a variable `i`

is not valid as a template non-type parameter, not even in a `constexpr`

context.

## 10 comments

Does anything happen at runtime in a C++ program any more?

this comment has been deleted.

The question is different: will C++ compilation times be reasonable anymore?

or will we get normal IDE experience anymore?

I think

`for constexpr`

is coming with the static reflection proposal.Exercise for the reader: Create an index sequence for those tuple elements of a particular type.

2 weeks ago, I needed this code, and I came up with a similar solution to you. But I really wanted a solution that would take advantage of a fold expression. Sadly, I couldn’t find a solution. AFAIK, both our solutions instantiate a struct for each T and each element of each tuple type, and that is expensive.

If you don’t need to worry about tuples containing the same type multiple times, you can drop the static_asserts, which should reduce the number of instantiated classes that the compiler generates.

The nice thing about tuples that contain same type multiple times is that you can introduce strong typing, e.g. instead of

you can write

.

Just posted a solution that avoids compile time recursion by using a fold expression here https://www.reddit.com/r/cpp/comments/hoasne/avoiding_compile_time_recursion/?utm_source=share&utm_medium=web2x

variable “i” not valid as a template non-type parameter but you can add recursion over index: