# Variable templates (C++17)
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## Why?

Eg. different precision
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```cpp
template<class T>
constexpr T pi = T(3.1415926535897932385L);

template<class T>
T circular_area(T r) { return pi<T> * r * r; }
```
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But it is really rarely used.
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## Specialization example #2 - field values

Remember this code?
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```c++ []
#include <iostream>

template<typename T>   // primary template
struct is_int {
    static constexpr bool value = false;
};

template<>  // explicit specialization for T = int
struct is_int<int> {
    static constexpr bool value = true;
};

template<typename T>
constexpr bool is_int_v = is_int<T>::value;

int main() {
    std::cout << is_int_v<char> << '\n';  // prints 0 (false)
    std::cout << is_int_v<int> << '\n';   // prints 1 (true)
    return 0;
}
```
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We mainly use template variables as helpers to class template field values.
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Check out [`type_traits` on cppreference.com](https://en.cppreference.com/w/cpp/header/type_traits)

Every trait has a corresponding helper variable template.

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## Exercise

Write a variable template `is_int_key_v`. It should return a value of the `is_int_key` field in a given template type.