命名空間的Boost.Log錯誤

Question

使用operator <<覆蓋位於命名空間下的操作數時，我遇到了錯誤。 我什至不完全確定可能導致此問題的原因，請查看源代碼：

#ifndef ENUM_UTILS_H
#define ENUM_UTILS_H

#include <sstream>
#include <unordered_map>
#include <type_traits>
#include <stdexcept>
#include <algorithm>

namespace sj
{

template <typename T>
struct enum_hasher
{
    size_t operator()(T e) const
    {
        return static_cast<typename std::underlying_type<T>::type>(e);
    }
};

template <typename T>
struct enum_strings
{
    typedef const std::unordered_map<T, std::string,
        enum_hasher<T>> maptype;
    static maptype data;
};

template <typename T>
typename std::enable_if<std::is_enum<T>::value, std::string>::type
enum_to_string(T e)
{
    return enum_strings<T>::data.at(e);
}

template <typename T>
typename std::enable_if<std::is_enum<T>::value, T>::type
string_to_enum(const std::string& str)
{
    auto& map = enum_strings<T>::data;
    auto got = std::find_if(map.begin(), map.end(),
    [&](const std::pair<T, std::string>& x) { return x.second == str; });

    if (got != map.end())
    {
        return got->first;
    }
    std::stringstream ss;
    ss << "conversion from " << str << " to enum failed";
    throw std::invalid_argument(ss.str());
}


} // namespace sj

/******* OPERATORS ON GLOBAL NS ********/

template <typename T>
typename std::enable_if<std::is_enum<T>::value, std::ostream&>::type
operator<<(std::ostream& os, T e)
{
    return os << sj::enum_strings<T>::data.at(e);
}


#endif // ENUM_UTILS_H


// logger.h

#ifndef LOGGER_H
#define LOGGER_H

#include <boost/log/common.hpp>


enum severity_level
{
    sev_debug,
    sev_info,
    sev_warning,
    sev_error,
    sev_fatal
};

// this macro returns the logger stream
// example: LOG(debug) << "stuff to log";
#define LOG(lvl) BOOST_LOG_SEV(Logger::getLogger(), lvl)

class Logger
{
public:
    typedef boost::log::sources::severity_logger<severity_level> MyLogger;

    static void initialize();
    static MyLogger& getLogger() { return _logger; }

private:
    static bool _initialized;
    static MyLogger _logger;
};



#endif // LOGGER_H


//// logger.cpp




#include <iostream>

#include "boost/log/common.hpp"
#include "boost/log/expressions.hpp"
#include "boost/log/utility/setup/file.hpp"
#include "boost/log/utility/setup/console.hpp"
#include "boost/log/utility/setup/common_attributes.hpp"
#include "boost/log/attributes/timer.hpp"
#include "boost/log/attributes/named_scope.hpp"
#include "boost/log/sources/logger.hpp"
#include "boost/log/support/date_time.hpp"

bool Logger::_initialized = false;
Logger::MyLogger Logger::_logger = Logger::MyLogger();

template< typename CharT, typename TraitsT >
inline std::basic_ostream< CharT, TraitsT >& operator<< (
    std::basic_ostream< CharT, TraitsT >& strm, severity_level lvl)
{
    static const char* const str[] =
    {
        "debug",
        "info",
        "warning",
        "error",
        "fatal"
    };
    if (static_cast< std::size_t >(lvl) < (sizeof(str) / sizeof(*str)))
        strm << str[lvl];
    else
        strm << static_cast< int >(lvl);
    return strm;
}



#include <string>

enum class TestEnum
{
    ValA,
    ValB
};

template<> sj::enum_strings<TestEnum>::maptype
    sj::enum_strings<TestEnum>::data =
{
    {TestEnum::ValA, "ValA"},
    {TestEnum::ValB, "ValB"}
};

namespace myns
{

enum class TestEnumNS
{
    ValC,
    ValD
};

}

template<> sj::enum_strings<myns::TestEnumNS>::maptype
    sj::enum_strings<myns::TestEnumNS>::data =
{
    {myns::TestEnumNS::ValC, "ValC"},
    {myns::TestEnumNS::ValD, "ValD"}
};


int main()
{
    LOG(sev_debug) << TestEnum::ValB;
    std::cout << TestEnum::ValB << std::endl;

    // this line doesn't compile for some reason!
    LOG(sev_debug) << myns::TestEnumNS::ValC;
    // std::cout works without problems
    std::cout << myns::TestEnumNS::ValC << std::endl;

    return 0;
}

使用MSVC2013編譯時完整的編譯日志（按要求）：

C:\build-env\boost\master\include\boost/log/utility/formatting_ostream.hpp(710) : error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'const myns::TestEnumNS' (or there is no acceptable conversion)
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(498): could be 'std::basic_ostream<char,std::char_traits<char>> &std::basic_ostream<char,std::char_traits<char>>::operator <<(std::basic_streambuf<char,std::char_traits<char>> *)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(478): or       'std::basic_ostream<char,std::char_traits<char>> &std::basic_ostream<char,std::char_traits<char>>::operator <<(const void *)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(458): or       'std::basic_ostream<char,std::char_traits<char>> &std::basic_ostream<char,std::char_traits<char>>::operator <<(long double)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(438): or       'std::basic_ostream<char,std::char_traits<char>> &std::basic_ostream<char,std::char_traits<char>>::operator <<(double)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(418): or       'std::basic_ostream<char,std::char_traits<char>> &std::basic_ostream<char,std::char_traits<char>>::operator <<(float)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(397): or       'std::basic_ostream<char,std::char_traits<char>> &std::basic_ostream<char,std::char_traits<char>>::operator <<(unsigned __int64)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(377): or       'std::basic_ostream<char,std::char_traits<char>> &std::basic_ostream<char,std::char_traits<char>>::operator <<(__int64)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(356): or       'std::basic_ostream<char,std::char_traits<char>> &std::basic_ostream<char,std::char_traits<char>>::operator <<(unsigned long)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(336): or       'std::basic_ostream<char,std::char_traits<char>> &std::basic_ostream<char,std::char_traits<char>>::operator <<(long)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(316): or       'std::basic_ostream<char,std::char_traits<char>> &std::basic_ostream<char,std::char_traits<char>>::operator <<(unsigned int)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(291): or       'std::basic_ostream<char,std::char_traits<char>> &std::basic_ostream<char,std::char_traits<char>>::operator <<(int)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(271): or       'std::basic_ostream<char,std::char_traits<char>> &std::basic_ostream<char,std::char_traits<char>>::operator <<(unsigned short)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(237): or       'std::basic_ostream<char,std::char_traits<char>> &std::basic_ostream<char,std::char_traits<char>>::operator <<(short)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(217): or       'std::basic_ostream<char,std::char_traits<char>> &std::basic_ostream<char,std::char_traits<char>>::operator <<(std::_Bool)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(210): or       'std::basic_ostream<char,std::char_traits<char>> &std::basic_ostream<char,std::char_traits<char>>::operator <<(std::ios_base &(__cdecl *)(std::ios_base &))'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(203): or       'std::basic_ostream<char,std::char_traits<char>> &std::basic_ostream<char,std::char_traits<char>>::operator <<(std::basic_ios<char,std::char_traits<char>> &(__cdecl *)(std::basic_ios<char,std::char_traits<char>> &))'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(197): or       'std::basic_ostream<char,std::char_traits<char>> &std::basic_ostream<char,std::char_traits<char>>::operator <<(std::basic_ostream<char,std::char_traits<char>> &(__cdecl *)(std::basic_ostream<char,std::char_traits<char>> &))'
        C:\build-env\boost\master\include\boost/log/attributes/attribute_name.hpp(175): or       'std::basic_ostream<char,std::char_traits<char>> &boost::log::v2s_mt_nt5::operator <<<char,std::char_traits<char>>(std::basic_ostream<char,std::char_traits<char>> &,const boost::log::v2s_mt_nt5::attribute_name &)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(699): or       'std::basic_ostream<char,std::char_traits<char>> &std::operator <<<char,std::char_traits<char>>(std::basic_ostream<char,std::char_traits<char>> &,const char *)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(746): or       'std::basic_ostream<char,std::char_traits<char>> &std::operator <<<char,std::char_traits<char>>(std::basic_ostream<char,std::char_traits<char>> &,char)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(784): or       'std::basic_ostream<char,std::char_traits<char>> &std::operator <<<std::char_traits<char>>(std::basic_ostream<char,std::char_traits<char>> &,const char *)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(831): or       'std::basic_ostream<char,std::char_traits<char>> &std::operator <<<std::char_traits<char>>(std::basic_ostream<char,std::char_traits<char>> &,char)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(957): or       'std::basic_ostream<char,std::char_traits<char>> &std::operator <<<std::char_traits<char>>(std::basic_ostream<char,std::char_traits<char>> &,const signed char *)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(964): or       'std::basic_ostream<char,std::char_traits<char>> &std::operator <<<std::char_traits<char>>(std::basic_ostream<char,std::char_traits<char>> &,signed char)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(971): or       'std::basic_ostream<char,std::char_traits<char>> &std::operator <<<std::char_traits<char>>(std::basic_ostream<char,std::char_traits<char>> &,const unsigned char *)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(978): or       'std::basic_ostream<char,std::char_traits<char>> &std::operator <<<std::char_traits<char>>(std::basic_ostream<char,std::char_traits<char>> &,unsigned char)'
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(988): or       'std::basic_ostream<char,std::char_traits<char>> &std::operator <<<char,std::char_traits<char>,myns::TestEnumNS>(std::basic_ostream<char,std::char_traits<char>> &&,const _Ty &)'
        with
        [
            _Ty=myns::TestEnumNS
        ]
        C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\INCLUDE\ostream(1026): or       'std::basic_ostream<char,std::char_traits<char>> &std::operator <<<char,std::char_traits<char>>(std::basic_ostream<char,std::char_traits<char>> &,const std::error_code &)'
        while trying to match the argument list '(std::basic_ostream<char,std::char_traits<char>>, const myns::TestEnumNS)'
        ..\BoostLogTest\boostlogtest.cpp(190) : see reference to function template instantiation 'boost::log::v2s_mt_nt5::basic_formatting_ostream<char,std::char_traits<char>,std::allocator<char>> &boost::log::v2s_mt_nt5::operator <<<char,std::char_traits<char>,std::allocator<char>,myns::TestEnumNS>(boost::log::v2s_mt_nt5::basic_formatting_ostream<char,std::char_traits<char>,std::allocator<char>> &,const T &)' being compiled
        with
        [
            T=myns::TestEnumNS
        ]

Boost.Log中是否存在錯誤或我做錯了什么？

Boost版本為1.55。

Answer 1

失敗的操作員查找不直接是此處的操作員：

LOG(sev_debug) << myns::TestEnumNS::ValC;

但是，正如clang ++所說：

/usr/local/include/boost/log/utility/formatting_ostream.hpp:664:19：錯誤：對二進制表達式（'ostream_type'（aka'basic_ostream'）和'myns :: TestEnumNS'無效的操作數

 strm.stream() << value; ~~~~~~~~~~~~~ ^ ~~~~~ 

如果此表達式周圍的名稱空間中的任何其他重載停止了不合格的查找，則該表達式可能無法在全局名稱空間中找到operator<<函數。

仔細查看文件formatting_ostream.hpp ，該表達式出現在boost::log命名空間內。 在boost::log或boost命名空間中名稱為operator<<任何函數都將停止不合格的查找，並防止不合格的查找找到在全局命名空間聲明的operator<< 。

您應該以可以通過ADL找到的方式來提供操作員，例如

namespace myns
{

    enum class TestEnumNS
    {
        ValC,
        ValD
    };

    using ::operator<<;

}

注意：我目前不能完全確定是否足夠，或者是否必須在myns中編寫適當的函數。 clang ++使用此添加功能編譯代碼，但發出大量鏈接器錯誤。