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# c++
## Type deduction
Force compile error to see what `auto` is deduced to.
```cpp
auto foo = bar();
// force compile error
typename decltype(foo)::_;
```
## Variadic templates ([parameter pack][parameter-pack])
```cpp
{{#include c++/meta.cc:3:}}
```
## Example: `any_of` template meta function
```cpp
{{#include c++/meta3.cc:3:}}
```
## Example: [SFINAE][sfinae] ([enable_if][enable-if])
Provide a single entry point `Invoke` to call some `Operations`.
Use `enable_if` to enable/disable the template functions depending on the two
available traits an operation can have:
- Operation returns a result
- Operation requires a context
```cpp
{{#include c++/meta2.cc:3:}}
```
## Example: Concepts pre c++20
Prior to c++20's concepts, `SFINAE` and `std::void_t` can be leveraged to build
something similar allowing to define an interface (aka trait) for a template
parameter.
```cpp
{{#include c++/concepts-11.cc:3:}}
```
The main mechanic can be explained with the following reduced example. If one
of the `decltype(std:declval<T>...` expressions is ill-formed, the template
specialization for `is_valid` will be removed from the candidate set due to
[SFINAE][sfinae].
```cpp
template<typename T, typename = void>
struct is_valid : std::false_type {};
template<typename T>
struct is_valid<T, std::void_t<
decltype(std::declval<T>().some_fun1()),
decltype(std::declval<T>().some_fun2())
>> : std::true_type {};
```
> `std::declval<T>()` creates an instance of type T in an unevaluated context.
[gist-strict-asliasing]: https://gist.github.com/shafik/848ae25ee209f698763cffee272a58f8
[parameter-pack]: https://en.cppreference.com/w/cpp/language/parameter_pack
[enable-if]: https://en.cppreference.com/w/cpp/types/enable_if
[sfinae]: https://en.cppreference.com/w/cpp/language/sfinae
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