chromium/third_party/dawn/src/dawn/common/MatchVariant.h

// Copyright 2024 The Dawn & Tint Authors
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#ifndef SRC_DAWN_COMMON_MATCHVARIANT_H_
#define SRC_DAWN_COMMON_MATCHVARIANT_H_

#include <variant>

namespace dawn {

// This is the `Overloaded` template in chromium/src/base/functional/Overloaded.h.
// std::visit() needs to be called with a functor object, such as
//
//  struct Visitor {
//    std::string operator()(const PackageA& source) {
//      return "PackageA";
//    }
//
//    std::string operator()(const PackageB& source) {
//      return "PackageB";
//    }
//  };
//
//  std::variant<PackageA, PackageB> var = PackageA();
//  return std::visit(Visitor(), var);
//
// `Overloaded` enables the above code to be written as:
//
//  std::visit(
//     Overloaded{
//         [](const PackageA& pack) { return "PackageA"; },
//         [](const PackageB& pack) { return "PackageB"; },
//     }, var);
//
// Note: Overloads must be implemented for all the variant options. Otherwise, there will be a
// compilation error.
//
// This struct inherits operator() method from all its base classes. Introduces operator() method
// from all its base classes into its definition.
template <typename... Callables>
struct Overloaded : Callables... { … };

// Uses template argument deduction so that the `Overloaded` struct can be used without specifying
// its template argument. This allows anonymous lambdas passed into the `Overloaded` constructor.
template <typename... Callables>
Overloaded(Callables...) -> Overloaded<Callables...>;

// With this template we can simplify the call of std::visit(Overloaded{...}, variant).
template <typename Variant, typename... Callables>
auto MatchVariant(const Variant& v, Callables... args) { … }

}  // namespace dawn

#endif