c++ - 命名空间的 Boost.Log 错误

Question

我在对位于命名空间下的操作数使用 operator<< 覆盖时遇到错误。我什至不确定是什么导致了这个问题，请查看源代码:

#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 编译时的完整编译日志(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 中是否存在错误或我做错了什么？

Boost 版本为 1.55。

Answer 1

查找失败的运算符不直接是这里的运算符:

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

但是，正如 clang++ 所说:

/usr/local/include/boost/log/utility/formatting_ostream.hpp:664:19: error: invalid operands to binary expression ('ostream_type' (aka 'basic_ostream') and 'myns::TestEnumNS')

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

这个表达式可能找不到operator<<全局命名空间中的函数，如果围绕此表达式的命名空间中的任何其他重载停止不合格查找。

仔细查看文件 formatting_ostream.hpp , 这个表达式出现在 boost::log 里面命名空间。任何名称为 operator<< 的函数在boost::log或 boost命名空间将停止不合格查找并防止不合格查找找到您的 operator<<在全局命名空间中声明。

您应该以可以通过 ADL 找到的方式提供您的运营商，例如

namespace myns
{

    enum class TestEnumNS
    {
        ValC,
        ValD
    };

    using ::operator<<;

}

注意我目前不完全确定这是否足够，或者您是否必须在 myns 中编写适当的函数。 clang++ 使用此添加编译代码，但会发出大量链接器错误。