Why custom error object can not be handled when leaf::result is lay in the std::vector

[pwrliang]

Hi there,

I'd like to use std::vector to store a series of leaf::result, because both of a valid value or an error can be represent by leaf::result. Like, std::vector<boost::leaf::result<int>> result_vector.

Assuming I have an error object MyError:

struct MyError {
  MyError(int ecode, std::string emsg)
      : error_code(ecode), error_msg(std::move(emsg)) {}
  int error_code;
  std::string error_msg;
};

I'm trying to push_back a normal value and an error object into the vector, then iterate the vector. I expect MyError object can be handled, but it hits the unmatched branch.

  std::vector<boost::leaf::result<int>> result;
  result.emplace_back(123);
  result.emplace_back(boost::leaf::new_error(MyError(-1, "Invalid number")));

  boost::leaf::try_handle_all(
      [&result]() -> boost::leaf::result<int> {
        for (auto& e : result) {
          BOOST_LEAF_AUTO(val, e);
          std::cout << "Val: " << val << std::endl;
        }
        return 0;
      },
      [](const MyError& e) {
        std::cerr << "Found error: " << e.error_code << ", " << e.error_msg
                  << std::endl;
        return 1;
      },
      [](const boost::leaf::error_info& unmatched) {
        std::cerr << "Unmatched error: " << unmatched << std::endl;
        return 1;
      });

The output:

Val: 123
Unmatched error: leaf::error_info: Error ID = 1

Thanks, Liang

[zajo]

Yes, this is by design. In case of success, a leaf::result<T> contains a T object, but in case of an error, by default, it does not contain error objects -- they get transported directly to the error handling scope.

If you need to collect results and then handle errors later, you need to make a leaf::polymorphic_context, which can then be stored in a leaf::result. This example is similar to your use case, except that the leaf::result objects are computed asynchronously by worker threads: https://github.com/boostorg/leaf/blob/master/examples/capture_in_result.cpp.

Tutorial: https://boostorg.github.io/leaf/#tutorial-async_result.