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diff --git a/src/development/c++.md b/src/development/c++.md index b42a263..1cc4003 100644 --- a/src/development/c++.md +++ b/src/development/c++.md @@ -32,9 +32,35 @@ available traits an operation can have: ```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 |