meta.hpp

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#pragma once
 
/// @file userver/formats/common/meta.hpp
/// @brief Metaprogramming helpers for converters detection.
/// @ingroup userver_universal
 
#include <type_traits>
 
#include <userver/formats/parse/to.hpp>
#include <userver/formats/serialize/to.hpp>
#include <userver/utils/meta_light.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace formats::common {
 
namespace impl {
 
/// `kHasX` are only intended for internal diagnostic use!
///
/// `formats` doesn't support SFINAE, so e.g. `kHasParse` can return `true`
/// while a usage of `Parse` will fail to compile.
 
template <typename Value, typename T>
using HasParse = decltype(Parse(std::declval<const Value&>(), parse::To<T>{}));
 
template <typename Value, typename T>
using HasSerialize = decltype(Serialize(std::declval<const T&>(), serialize::To<Value>{}));
 
template <typename Value, typename T>
using HasConvert = decltype(Convert(std::declval<const Value&>(), parse::To<T>{}));
 
template <typename Value>
using IsFormatValue = typename Value::ParseException;
 
template <class Value, class T>
constexpr inline bool kHasParse = meta::IsDetected<HasParse, Value, T>;
 
template <class Value, class T>
constexpr inline bool kHasSerialize = meta::IsDetected<HasSerialize, Value, T>;
 
template <class Value, class T>
constexpr inline bool kHasConvert = meta::IsDetected<HasConvert, Value, T>;
 
}  // namespace impl
 
/// Used in `Parse` overloads that are templated on `Value`, avoids clashing
/// with `Parse` from string
template <class Value>
constexpr inline bool kIsFormatValue = meta::IsDetected<impl::IsFormatValue, Value>;
 
// Unwraps a transient type - tag types, for which ADL-found `Parse` returns
// another type, not the type specified in `formats::parse::To`. For example,
// there can be a `IntegerWithMin<42>` type that checks that the value contains
// a number `>= 42`, then parses and returns an `int`.
//
// For a normal type T, just returns the type T itself.
template <typename Value, typename T>
using ParseType = decltype(Parse(std::declval<Value>(), parse::To<T>()));
 
}  // namespace formats::common
 
USERVER_NAMESPACE_END