Merge pull request ethereum#10739 from ethereum/mapFetchValue

Add fetchValue utility for maps.

libsolutil/CommonData.h

@@ -232,6 +232,39 @@ std::set<K> keys(std::map<K, V> const& _map)
 	return applyMap(_map, [](auto const& _elem) { return _elem.first; }, std::set<K>{});
 }
 
+/// @returns a pointer to the entry of @a _map at @a _key, if there is one, and nullptr otherwise.
+template<typename MapType, typename KeyType>
+decltype(auto) valueOrNullptr(MapType&& _map, KeyType const& _key)
+{
+	auto it = _map.find(_key);
+	return (it == _map.end()) ? nullptr : &it->second;
+}
+
+namespace detail
+{
+struct allow_copy {};
+}
+static constexpr auto allow_copy = detail::allow_copy{};
+
+/// @returns a reference to the entry of @a _map at @a _key, if there is one, and @a _defaultValue otherwise.
+/// Makes sure no copy is involved, unless allow_copy is passed as fourth argument.
+template<
+	typename MapType,
+	typename KeyType,
+	typename ValueType = std::decay_t<decltype(std::declval<MapType>().find(std::declval<KeyType>())->second)> const&,
+	typename AllowCopyType = void*
+>
+decltype(auto) valueOrDefault(MapType&& _map, KeyType const& _key, ValueType&& _defaultValue = {}, AllowCopyType = nullptr)
+{
+	auto it = _map.find(_key);
+	static_assert(
+		std::is_same_v<AllowCopyType, detail::allow_copy> ||
+		std::is_reference_v<decltype((it == _map.end()) ? _defaultValue : it->second)>,
+		"valueOrDefault does not allow copies by default. Pass allow_copy as additional argument, if you want to allow copies."
+	);
+	return (it == _map.end()) ? _defaultValue : it->second;
+}
+
 // String conversion functions, mainly to/from hex/nibble/byte representations.
 
 enum class WhenError