variant.hpp

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#pragma once
 
/// @file userver/formats/parse/variant.hpp
/// @brief Ineffective but generic parser for std::variant type
///
/// Parsing is performed for each of the N alternatives in variant, N-1
/// exceptions is thrown and caught during the parsing.
///
/// @ingroup userver_universal userver_formats_parse
 
#include <optional>
#include <typeinfo>
#include <variant>
 
#include <fmt/format.h>
 
#include <userver/compiler/demangle.hpp>
#include <userver/formats/parse/to.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace formats::json {
class Value;
}
 
namespace formats::parse {
 
template <typename ParseException, typename Variant, typename TypeA>
[[noreturn]] void ThrowVariantAmbiguousParse(std::string_view path, std::type_index type_b) {
    throw ParseException(fmt::format(
        "Value of '{}' is ambiguous, it is parseable into multiple variants of '{}', at least '{}' and '{}'",
        path,
        compiler::GetTypeName<Variant>(),
        compiler::GetTypeName<TypeA>(),
        compiler::GetTypeName(type_b)
    ));
}
 
template <class ParseException, typename Variant>
[[noreturn]] void ThrowVariantParseException(std::string_view path) {
    throw ParseException(fmt::format("Value of '{}' cannot be parsed as {}", path, compiler::GetTypeName<Variant>()));
}
 
namespace impl {
template <class T, class Value, typename Result>
void ParseVariantSingle(const Value& value, std::optional<Result>& result) {
    if (result) {
        const auto old_type = std::visit([](const auto& v) -> std::type_index { return typeid(v); }, *result);
        try {
            value.template As<T>();
        } catch (const std::exception&) {
            return;
        }
 
        ThrowVariantAmbiguousParse<typename Value::ParseException, Result, T>(value.GetPath(), old_type);
    } else {
        // No result yet
        try {
            result = value.template As<T>();
        } catch (const std::exception&) {
        }
    }
}
 
}  // namespace impl
 
template <class Value, typename... Types>
std::variant<decltype(Parse(std::declval<Value>(), To<Types>{}))...>
Parse(const Value& value, formats::parse::To<std::variant<Types...>>) {
    std::optional<std::variant<decltype(Parse(std::declval<Value>(), To<Types>{}))...>> result;
 
    if constexpr (std::is_same_v<Value, formats::json::Value>) {
        Value value2{value};
        value2.DropRootPath();
        (impl::ParseVariantSingle<Types>(value2, result), ...);
    } else {
        (impl::ParseVariantSingle<Types>(value, result), ...);
    }
 
    if (!result) {
        ThrowVariantParseException<typename Value::ParseException, std::variant<Types...>>(value.GetPath());
    }
 
    return std::move(*result);
}
 
}  // namespace formats::parse
 
USERVER_NAMESPACE_END