如何为 boost log 和自定义类型定义输出流运算符

Question

我能够为一个简单的结构定义输出流运算符，但是，不能为 std::array 定义。以下代码无法编译。出了什么问题，我该如何解决？

#include <array>
#include <iostream>
#include <boost/log/core.hpp>
#include <boost/log/trivial.hpp>

using hash_t = std::array< unsigned char, 32 >;

std::ostream& operator<< ( std::ostream& os, hash_t const& arr )
{
    os << "ole!";
    return os;
}

int main(int, char*[])
{
    hash_t arr;
    std::cerr << arr << std::endl; // complies cleanly
    BOOST_LOG_TRIVIAL(debug) << arr; // Error
    return 0;
}

错误来了。

GCC（提升 1.55，gcc-4.9.2）：

In file included from /usr/include/boost/log/sources/record_ostream.hpp:31:0,
                 from /usr/include/boost/log/trivial.hpp:23,
                 from trival.cpp:4:
/usr/include/boost/log/utility/formatting_ostream.hpp: In instantiation of ‘boost::log::v2s_mt_posix::basic_formatting_ostream<CharT, TraitsT, AllocatorT>& boost::log::v2s_mt_posix::operator<<(boost::log::v2s_mt_posix::basic_formatting_ostream<CharT, TraitsT, AllocatorT>&, const T&) [with CharT = char; TraitsT = std::char_traits<char>; AllocatorT = std::allocator<char>; T = std::array<unsigned char, 32ul>]’:
trival.cpp:18:30:   required from here
/usr/include/boost/log/utility/formatting_ostream.hpp:710:19: error: cannot bind ‘boost::log::v2s_mt_posix::basic_formatting_ostream<char>::ostream_type {aka std::basic_ostream<char>}’ lvalue to ‘std::basic_ostream<char>&&’
     strm.stream() << value;
                   ^
In file included from /usr/include/c++/4.9/iostream:39:0,
                 from trival.cpp:2:
/usr/include/c++/4.9/ostream:602:5: note: initializing argument 1 of ‘std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Tp = std::array<unsigned char, 32ul>]’
     operator<<(basic_ostream<_CharT, _Traits>&& __os, const _Tp& __x)

Clang 错误（提升 1.64，clang-800.0.42.1）：

In file included from /usr/local/include/boost/log/trivial.hpp:23:
In file included from /usr/local/include/boost/log/sources/record_ostream.hpp:36:
/usr/local/include/boost/log/utility/formatting_ostream.hpp:878:19: error: invalid operands to binary expression ('ostream_type' (aka
      'basic_ostream<char, std::__1::char_traits<char> >') and 'std::__1::array<unsigned char, 32>')
    strm.stream() << value;
    ~~~~~~~~~~~~~ ^  ~~~~~
/usr/local/include/boost/log/sources/record_ostream.hpp:390:51: note: in instantiation of function template specialization
      'boost::log::v2_mt_posix::operator<<<boost::log::v2_mt_posix::basic_formatting_ostream<char, std::__1::char_traits<char>, std::__1::allocator<char> >,
      std::__1::array<unsigned char, 32> >' requested here
    static_cast< formatting_ostream_type& >(strm) << value;
                                                  ^
/Users/adam/GitPen/BoostLog/trival/trival.cpp:18:27: note: in instantiation of function template specialization
      'boost::log::v2_mt_posix::operator<<<boost::log::v2_mt_posix::basic_record_ostream<char>, std::__1::array<unsigned char, 32> >' requested here
        BOOST_LOG_TRIVIAL(debug) << arr;

Answer 1

你可能不喜欢这个答案。

要让它在此刻正常工作，您需要将您的 operator<< 添加到 namespace std：

namespace std{
   std::ostream& operator<< ( std::ostream& os, const hash_t& arr )
   {/*...*/}
}

由于 ADL 的工作原理，它只考虑 operator<< 及其在 namespace std 中的特化

你不会喜欢它，因为将这个函数添加到 namespace std 是不合法的：

[命名空间.std]

如果 C++ 程序将声明或定义添加到命名空间 std 或 除非另有说明，否则命名空间 std 内的命名空间。程序可以添加模板特化 仅当声明依赖于用户定义的类型时，任何标准库模板才能命名空间 std 并且专业化满足原始模板的标准库要求并且没有明确 禁止。

也许最简单的做法是从std 类型继承（而不做其他任何事情）：

struct hash_t : std::array< unsigned char, 32 >{};