默认是这样的


#include <filesystem>
#include <iostream>
#include <expected>

namespace fs = std::filesystem;

std::expected<void, std::string> copy_directory(const fs::path& src, const fs::path& dst) {
    try {
        // 检查源目录是否存在
        if (!fs::exists(src) || !fs::is_directory(src)) {
            return std::unexpected("Source directory does not exist or is not a directory.");
        }

        // 创建目标目录
        if (fs::exists(dst)) {
            return std::unexpected("Destination directory already exists.");
        }
        fs::create_directories(dst);

        // 递归复制
        for (const auto& entry : fs::recursive_directory_iterator(src)) {
            const auto& path = entry.path();
            auto relative_path = fs::relative(path, src);
            fs::path dst_path = dst / relative_path;

            if (fs::is_directory(path)) {
                fs::create_directories(dst_path);
            } else if (fs::is_regular_file(path)) {
                fs::copy_file(path, dst_path);
            } else {
                return std::unexpected("Unsupported file type encountered.");
            }
        }
    } catch (const fs::filesystem_error& e) {
        return std::unexpected(e.what());
    }

    return {}; // 成功，返回一个空的 std::expected<void, std::string>
}

int main() {
    auto result = copy_directory("source_directory", "destination_directory");

    if (!result.has_value()) {
        std::cerr << "Error: " << result.error() << '\n';
    } else {
        std::cout << "Directory copied successfully.\n";
    }

    return 0;
}

【 在 milksea 的大作中提到: 】
: 用 expected 估计能好点，配合类似 rust 的 ? 语法就更好了
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