{"id":4529,"date":"2022-08-17T21:53:31","date_gmt":"2022-08-17T19:53:31","guid":{"rendered":"https:\/\/mariusbancila.ro\/blog\/?p=4529"},"modified":"2022-08-17T21:53:31","modified_gmt":"2022-08-17T19:53:31","slug":"using-the-cpp23-expected-type","status":"publish","type":"post","link":"https:\/\/mariusbancila.ro\/blog\/2022\/08\/17\/using-the-cpp23-expected-type\/","title":{"rendered":"Using the C++23 std::expected type"},"content":{"rendered":"\n

The C++23 standard will feature a new utility type called std::expected<\/code>. This type either contains an expected value, or an unexpected one, typically providing information about the reason something failed (and the expected value could not be returned). This feature is, at this time, supported<\/a> in GCC 12 and MSVC 19.33 (Visual Studio 2022 17.3). In this article, we’ll see what std::expected<\/code> is and how it can be used.<\/p>\n\n\n\n

\"\"<\/a><\/figure>\n\n\n\n\n\n\n\n

Why do we need std::expected?<\/h2>\n\n\n\n

Suppose you have to write a function that returns some data. It has to perform one or more operations that may fail. This function needs to return the data, but also needs to indicate failure and the cause for the failure. There are different ways to implement this. <\/p>\n\n\n\n

Alternative 1: status code + reference parameter<\/h4>\n\n\n\n

One alternative is to return a status code indicating success or the reason of failure. Additionally, the actual returned data is a parameter passed by reference.<\/p>\n\n\n\n

enum class Status\n{\n   Ok,\n   AccessDenied,\n   DataSourceError,\n   DataError,\n};\n\nbool HasAcccess() { return true; }\nint OpenConnection() { return 0; }\nint Fetch() { return 0; }\n\nStatus ReadData(std::vector<int>& data)\n{\n   if (!HasAcccess())\n      return Status::AccessDenied;\n\n   if (OpenConnection() != 0)\n      return Status::DataSourceError;\n\n   if (Fetch() != 0)\n      return Status::DataError;\n\n   data.push_back(42);\n\n   return Status::Ok;\n}<\/pre>\n\n\n\n
This is how it  can be used:<\/p>\n\n\n\n
void print_value(int const v)\n{\n   std::cout << v << '\\n';\n}\n\nint main()\n{\n   std::vector<int> data;\n   Status result = ReadData(data);\n   if (result == Status::Ok)\n   {\n      std::ranges::for_each(data, print_value);\n   }\n   else\n   {\n      std::cout << std::format(\"Error code: {}\\n\", (int)result);\n   }\n}<\/pre>\n\n\n\n
Alternative 2: using exceptions<\/h4>\n\n\n\n
Another alternative is to return the actual data but in case of failure throw an exception.<\/p>\n\n\n\n
struct status_exception : public std::exception\n{\n   status_exception(Status status) : std::exception(), status_(status) {}\n\n   status_exception(Status status, char const* const message) : std::exception(message), status_(status) {}\n\n   Status status() const { return status_; }\n\nprivate:\n   Status status_;\n};\n\nstd::vector<int> ReadData()\n{\n   if (!HasAcccess())\n      throw status_exception(Status::AccessDenied);\n\n   if (OpenConnection() != 0)\n      throw status_exception(Status::DataSourceError);\n\n   if (Fetch() != 0)\n      throw status_exception(Status::DataError);\n\n   std::vector<int> data;\n\n   data.push_back(42);\n\n   return data;\n}<\/pre>\n\n\n\n
This time, we need to try-catch the call:<\/p>\n\n\n\n
int main()\n{\n   try\n   {\n      auto data = ReadData();\n      std::ranges::for_each(data, print_value);\n   }\n   catch (status_exception const& e)\n   {\n      std::cout << std::format(\"Error code: {}\\n\", (int)e.status());\n   }      \n}<\/pre>\n\n\n\n
Choosing between one of these could be a personal choice or may depend on imposed restrictions. For instance, there could be a no-exceptions requirement, in which case the 2nd alternative cannot be used.<\/p>\n\n\n\n
Alternative 3: using std::variant<\/h4>\n\n\n\n
Another possible options, in C++17, is to use std::variant<\/code>. In this case, our function could looks as follows:<\/p>\n\n\n\n
std::variant<std::vector<int>, Status> ReadData()\n{\n   if (!HasAcccess())\n      return { Status::AccessDenied };\n\n   if (OpenConnection() != 0)\n      return {Status::DataSourceError};\n\n   if (Fetch() != 0)\n      return {Status::DataError};\n\n   std::vector<int> data;\n\n   data.push_back(42);\n\n   return data;\n}<\/pre>\n\n\n\n
However, when it comes to using it, it gets nasty. We need to visit each possible alternative of the variant type and the syntax to do so is horrendous.<\/p>\n\n\n\n
int main()\n{\n   auto result = ReadData();\n   std::visit([](auto& arg) {\n      using T = std::decay_t<decltype(arg)>;\n\n      if constexpr (std::is_same_v<T, std::vector<int>>)\n      {\n         std::ranges::for_each(arg, print_value);\n      }\n      else if constexpr (std::is_same_v<T, Status>)\n      {\n         std::cout << std::format(\"Error code: {}\\n\", (int)arg);\n      }\n   }, result);\n}<\/pre>\n\n\n\n
In my opinion, std::variant<\/code> is difficult to use, and I don’t like making use of it.<\/p>\n\n\n\n
Note<\/strong>: you can read more about std::variant<\/code> in this article: std::visit is everything wrong with modern C++<\/a>.<\/p>\n\n\n\n
Alternative 4: using std::optional<\/h4>\n\n\n\n
The std::optional<\/code> type may contain or may not contain a value. This can be used when returning no data is a valid option for a function that normally would return a value. Like in our case:<\/p>\n\n\n\n
std::optional<std::vector<int>> ReadData()\n{\n   if (!HasAcccess()) return {};\n\n   if (OpenConnection() != 0) return {};\n\n   if (Fetch() != 0) return {};\n\n   std::vector<int> data;\n\n   data.push_back(42);\n\n   return data;\n}<\/pre>\n\n\n\n
We can use this as follows:<\/p>\n\n\n\n
int main()\n{\n   auto result = ReadData();\n   if (result)\n   {\n      std::ranges::for_each(result.value(), print_value);\n   }\n   else\n   {\n      std::cout << \"No data\\n\";\n   }\n}<\/pre>\n\n\n\n
The std::optional type has several members for checking and accessing the value, including:<\/p>\n\n\n\n