c++ - 编译用于创建 boost.log 格式的代码时出错

Question

我正在尝试使用选项 -std=c++14 在 Ubuntu 18.04 LTS 上使用 gcc 7.3.0 编译以下代码:

auto createLogFormatter()
{
    boost::log::expressions::stream_type stream;
    stream  << boost::log::expressions::format_date_time<boost::posix_time::ptime>("TimeStamp", "%Y-%m-%d %H:%M:%S  ")
            << boost::log::expressions::attr<int>("Severity")
            << " " // ERROR HERE
            << boost::log::expressions::attr<boost::log::attributes::current_thread_id::value_type>("ThreadID")
            << " "
            << boost::log::expressions::smessage;
    return stream;
}

出现奇怪的错误:

Log.cpp:51:13: error: no match for ‘operator<<’ (operand types are ‘boost::log::v2_mt_posix::expressions::aux::make_output_actor<boost::phoenix::actor<boost::log::v2_mt_posix::expressions::aux::attribute_output_terminal<boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<2> >, 0> >, boost::posix_time::ptime, boost::log::v2_mt_posix::fallback_to_none, boost::log::v2_mt_posix::aux::light_function<void(boost::log::v2_mt_posix::basic_formatting_ostream<char>&, const boost::posix_time::ptime&)> > >, boost::log::v2_mt_posix::expressions::attribute_actor<int, boost::log::v2_mt_posix::fallback_to_none, void, boost::phoenix::actor>, int, false>::type {aka boost::phoenix::actor<boost::log::v2_mt_posix::expressions::aux::attribute_output_terminal<boost::phoenix::actor<boost::log::v2_mt_posix::expressions::aux::attribute_output_terminal<boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<2> >, 0> >, boost::posix_time::ptime, boost::log::v2_mt_posix::fallback_to_none, boost::log::v2_mt_posix::aux::light_function<void(boost::log::v2_mt_posix::basic_formatting_ostream<char>&, const boost::posix_time::ptime&)> > >, int, boost::log::v2_mt_posix::fallback_to_none, boost::log::v2_mt_posix::to_log_fun<void> > >}’ and ‘const char [2]’)
     stream  << boost::log::expressions::format_date_time<boost::posix_time::ptime>("TimeStamp", "%Y-%m-%d %H:%M:%S  ")
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
             << boost::log::expressions::attr<int>("Severity")
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
             << " "
             ^~~~~~

根据 Boost 示例 here ，这样的事情应该有效:

    expr::stream
        << expr::attr< unsigned int >("LineID")
        << ": <" << logging::trivial::severity
        << "> " << expr::smessage

但是我收到这样的错误。我做错了什么？

Answer 1

编译器错误可能是由缺少包含引起的。 Lambda 表达式运算符由 Boost.Phoenix 在 boost/phoenix/operator.hpp 中提供。或者您可以只包含 boost/log/expressions.hpp，它提供所有 Boost.Log 表达式节点和运算符。

但除此之外，您的 createLogFormatter 函数不正确，因为它丢弃了整个格式化程序表达式。您不应该创建和返回 stream 对象，它只是启动格式化程序表达式的终端。您需要返回作为 lambda 表达式的结果创建的函数对象。

auto createLogFormatter()
{
    auto fmt = stream  << boost::log::expressions::format_date_time<boost::posix_time::ptime>("TimeStamp", "%Y-%m-%d %H:%M:%S  ")
            << boost::log::expressions::attr<int>("Severity")
            << " " // ERROR HERE
            << boost::log::expressions::attr<boost::log::attributes::current_thread_id::value_type>("ThreadID")
            << " "
            << boost::log::expressions::smessage;
    return fmt;
}

如果你想有一个稳定的返回类型，你可以使用boost::log::formatter来包装表达式。