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Q&A How should one match a specialized version of a variant?

This can be achieved using SFINAE, making the default overload invalid for non-numeric types: template <class T, class... Types> constexpr bool in_variant(const std::variant<Types...>...

posted 1y ago by Baum mit Augen‭ · edited 1y ago by Baum mit Augen‭

Answer

#2: Post edited by

Baum mit Augen‭ · 2023-06-09T20:47:12Z (over 1 year ago)

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This can be achieved using SFINAE, making the default overload invalid for non-numeric types:
```
template <class T, class... Types>
constexpr bool in_variant(const std::variant<Types...> &) {
const std::array hit{std::is_same_v<T, Types>...};
return std::any_of(std::begin(hit), std::end(hit),
[](auto v) { return v; });
}
void test(Value something) {
std::visit(
overloaded{
[](auto a,
~~std::enable_if_t<in_variant<decltype(a)>(Numeric{}), int> = 0) {~~
std::cout << "Number" << std::endl; },
[](auto a,
~~std::enable_if_t<!in_variant<decltype(a)>(Numeric{}), int> = 0) {~~
std::cout << "Other" << std::endl;
},
},
something);
}
```
[See it live](https://coliru.stacked-crooked.com/a/c3dd91d37285211b)

This can be achieved using SFINAE, making the default overload invalid for non-numeric types:
```
template <class T, class... Types>
constexpr bool in_variant(const std::variant<Types...> &) {
const std::array hit{std::is_same_v<T, Types>...};
return std::any_of(std::begin(hit), std::end(hit),
[](auto v) { return v; });
}
void test(Value something) {
std::visit(
overloaded{
[](auto a,
std::enable_if_t<in_variant<decltype(a)>(Numeric{}),
int> = 0) {
std::cout << "Number" << std::endl; },
[](auto a,
std::enable_if_t<!in_variant<decltype(a)>(Numeric{}),
int> = 0) {
std::cout << "Other" << std::endl;
},
},
something);
}
```
[See it live](https://coliru.stacked-crooked.com/a/c3dd91d37285211b)

#1: Initial revision by

Baum mit Augen‭ · 2023-06-09T20:46:23Z (over 1 year ago)

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This can be achieved using SFINAE, making the default overload invalid for non-numeric types:

```
template <class T, class... Types>
constexpr bool in_variant(const std::variant<Types...> &) {
  const std::array hit{std::is_same_v<T, Types>...};
  return std::any_of(std::begin(hit), std::end(hit),
                     [](auto v) { return v; });
}

void test(Value something) {
  std::visit(
      overloaded{
          [](auto a,
             std::enable_if_t<in_variant<decltype(a)>(Numeric{}), int> = 0) {
            std::cout << "Number" << std::endl; },
          [](auto a,
             std::enable_if_t<!in_variant<decltype(a)>(Numeric{}), int> = 0) {
            std::cout << "Other" << std::endl;
          },
      },
      something);
}
```

[See it live](https://coliru.stacked-crooked.com/a/c3dd91d37285211b)

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