leaf-examples/src/leaf_on_error_example.cpp

// leaf_on_error_example.cpp
#include <boost/leaf.hpp>
#include <ctime>
#include <iostream>
#include <string>
#include <utility>

namespace leaf = boost::leaf;

struct error_tag {};
struct open_error {};
struct write_error {};

struct file_context {
    std::string path;
};

struct user_context {
    std::string name;
    int privileges;
};

struct chunk_context {
    int chunk;
    std::size_t size;
    std::time_t timestamp;
};

leaf::result<void> write_chunk(int chunk, std::size_t size)
{
    chunk_context cc{chunk, size, std::time(nullptr)};
    // Registers cc to be captured only if an error unwinds this scope.
    // On success the registration is discarded.
    [[maybe_unused]] auto ctx = leaf::on_error(cc);

    if (chunk == 3)
        // The context registered above rides along with this error
        // even though handlers live several frames up.
        return leaf::new_error(write_error{});

    return {};
}

leaf::result<void> process_file(std::string path, std::string user)
{
    error_tag tag;
    file_context fc{std::move(path)};
    user_context uc{std::move(user), 0600};

    [[maybe_unused]] auto tag_ctx = leaf::on_error(tag);
    [[maybe_unused]] auto file_ctx = leaf::on_error(fc);
    [[maybe_unused]] auto user_ctx = leaf::on_error(uc);

    // write_chunk knows nothing about the contexts registered here;
    // LEAF attaches them automatically to any error it propagates.
    for (int i = 0; i != 5; ++i) {
        auto r = write_chunk(i, 4096);
        if (!r)
            return r;
    }

    return {};
}

int main()
{
    return leaf::try_handle_all(
        []() -> leaf::result<int> {
            // Try block and handlers must return the same type.
            BOOST_LEAF_CHECK(process_file("data.bin", "alice"));
            return 0;
        },

        // A handler runs only if the error carries every type it
        // requests; handlers below with fewer parameters match the
        // same errors when richer contexts were never registered.
        [](write_error const&,
           error_tag const&,
           file_context const& f,
           user_context const& u,
           chunk_context const& c) {
            std::cerr << "write failed: "
                      << "file=" << f.path
                      << ", user=" << u.name
                      << ", chunk=" << c.chunk
                      << ", size=" << c.size
                      << ", time=" << c.timestamp
                      << '\n';
            return 1;
        },

        [](open_error const&,
           error_tag const&,
           file_context const& f,
           user_context const& u) {
            std::cerr << "open failed: "
                      << "file=" << f.path
                      << ", user=" << u.name
                      << '\n';
            return 2;
        },

        [](error_tag const&,
           file_context const& f,
           user_context const& u,
           chunk_context const& c) {
            std::cerr << "unknown error: "
                      << "file=" << f.path
                      << ", user=" << u.name
                      << ", chunk=" << c.chunk
                      << '\n';
            return 3;
        },

        [](error_tag const&,
           file_context const& f,
           user_context const& u) {
            std::cerr << "unknown error: "
                      << "file=" << f.path
                      << ", user=" << u.name
                      << '\n';
            return 4;
        },

        [] {
            std::cerr << "unknown error before file context\n";
            return 5;
        }
    );
}