c++ - Boost Log - 为什么不编译？

Question

在过去的三个小时里，我一直被以下编译错误搞糊涂了。谁能告诉我这是怎么回事？

我试图将 log::formatter(在下面标记)定义为它自己的变量，因此它可以在几个地方重新使用。但是，在尝试重新使用它时出现编译错误。

但是，如果我完全摆脱那个变量，而是复制并粘贴代码，它就会编译。有没有搞错？有什么办法可以做我想做的事吗？

boost::shared_ptr<log::core> logger = log::core::get();

logger->set_logging_enabled( enabled );
logger->set_filter(trivial::severity >= level);                                                                                                                           

logger->add_global_attribute(
    "TimeStamp", attr::local_clock());

logger->add_global_attribute(
    "ProcessID", attr::current_process_id());

logger->add_global_attribute(
    "ThreadID", attr::current_thread_id());

// want this to be it's own variable
log::formatter fmt = expr::stream
    << "[" << expr::format_date_time< boost::posix_time::ptime >("TimeStamp", "%d.%m.%Y %H:%M:%S.%f") << "] "
    << "[" << expr::attr< attr::current_process_id::value_type >("ProcessID") << "] "
    << "[" << expr::attr< attr::current_thread_id::value_type >("ThreadID") << "] "
    << "[" << expr::attr< std::string >("Channel") << "] "
    << "[" << expr::attr< severity_level >("Severity") << "]: "
    << expr::smessage
    ;   

// so it can be reused here, but this is a compiler error
log::add_console_log(
    std::clog, keywords::format=fmt);

// and here, too. But this is also a compiler error
log::add_file_log(
    "test.log", keywords::format=fmt);

编译错误(使用 clang++)是:

In file included from ../src/util/logging/Logging.cpp:34:                                                                                                                 
In file included from /opt/boost-1.54.0/include/boost/log/utility/setup/console.hpp:22:
/opt/boost-1.54.0/include/boost/log/detail/sink_init_helpers.hpp:107:21: error: no matching function for call to 'acquire_formatter'
    s.set_formatter(aux::acquire_formatter(args[keywords::format]));
                    ^~~~~~~~~~~~~~~~~~~~~~
/opt/boost-1.54.0/include/boost/log/utility/setup/console.hpp:76:5: note: in instantiation of function template specialization 'boost::log::v2_mt_posix::aux::setup_format
boost::log::v2_mt_posix::sinks::synchronous_sink<boost::log::v2_mt_posix::sinks::basic_text_ostream_backend<char> >, boost::parameter::aux::tagged_argument<boost::log::v2
posix::keywords::tag::format, boost::log::v2_mt_posix::basic_formatter<char> > >' requested here
    aux::setup_formatter(*pSink, args,
    ^   
/opt/boost-1.54.0/include/boost/log/utility/setup/console.hpp:136:12: note: in instantiation of function template specialization 'boost::log::v2_mt_posix::aux::add_consol
g<char, boost::parameter::aux::tagged_argument<boost::log::v2_mt_posix::keywords::tag::format, boost::log::v2_mt_posix::basic_formatter<char> > >' requested here
    return aux::add_console_log(strm, arg1);
           ^   
../src/util/logging/Logging.cpp:121:2: note: in instantiation of function template specialization 'boost::log::v2_mt_posix::add_console_log<char, boost::parameter::aux::t
d_argument<boost::log::v2_mt_posix::keywords::tag::format, boost::log::v2_mt_posix::basic_formatter<char> > >' requested here
        log::add_console_log(
        ^   
/opt/boost-1.54.0/include/boost/log/detail/sink_init_helpers.hpp:80:33: note: candidate template ignored: failed template argument deduction
inline basic_formatter< CharT > acquire_formatter(const CharT* formatter)
                                ^   
/opt/boost-1.54.0/include/boost/log/detail/sink_init_helpers.hpp:85:33: note: candidate template ignored: failed template argument deduction
inline basic_formatter< CharT > acquire_formatter(std::basic_string< CharT, TraitsT, AllocatorT > const& formatter)
                                ^   
/opt/boost-1.54.0/include/boost/log/detail/sink_init_helpers.hpp:91:5: note: candidate template ignored: disabled by 'enable_if' [with FormatterT = boost::log::v2_mt_posi
asic_formatter<char>]
    phoenix::is_actor< FormatterT >,
    ^   

但是，如果我改为将其改为这样，它会编译:

boost::shared_ptr<log::core> logger = log::core::get();

logger->set_logging_enabled( enabled );
logger->set_filter(trivial::severity >= level);

logger->add_global_attribute(
    "TimeStamp", attr::local_clock());

logger->add_global_attribute(
    "ProcessID", attr::current_process_id());

logger->add_global_attribute(
    "ThreadID", attr::current_thread_id());

// copy and paste the expression
log::add_console_log(                                                                                                                                                     
    std::clog,
    keywords::format =
    (
        expr::stream
            << "[" << expr::format_date_time< boost::posix_time::ptime >("TimeStamp", "%d.%m.%Y %H:%M:%S.%f") << "] "
            << "[" << expr::attr< attr::current_process_id::value_type >("ProcessID") << "] "
            << "[" << expr::attr< attr::current_thread_id::value_type >("ThreadID") << "] "
            << "[" << expr::attr< std::string >("Channel") << "] "
            << "[" << expr::attr< severity_level >("Severity") << "]: "
            << expr::smessage
    )
);

// copy and paste the expression again
log::add_file_log(
    "test.log",
    keywords::format =
    (
        expr::stream
            << "[" << expr::format_date_time< boost::posix_time::ptime >("TimeStamp", "%d.%m.%Y %H:%M:%S.%f") << "] "
            << "[" << expr::attr< attr::current_process_id::value_type >("ProcessID") << "] "
            << "[" << expr::attr< attr::current_thread_id::value_type >("ThreadID") << "] "
            << "[" << expr::attr< std::string >("Channel") << "] "
            << "[" << expr::attr< severity_level >("Severity") << "]: "
            << expr::smessage
    )
);

Answer 1

log::formatter 类型是实际(相当复杂的)表达式类型的包装器。由于某些原因，它不能与 add_file/console_log 一起使用。

如果您使用的是 C++11，auto 关键字将避免包装类:

auto fmt = expr::stream
  << ...

如果没有 C++11，模板函数参数将起作用:

template <class F>
void add_logs(const F & fmt)
{
  log::add_console_log(std::clog, keywords::format = fmt);
  log::add_file_log("test.log", keywords::format = fmt);
}

add_logs(expr::stream
  << ...
  );