在 Boost.Log 中正确重载运算符 <<

Question

在Boost.Log documentation中表示

Note

The library uses basic_formatting_ostream stream type for record formatting, so when customizing attribute value formatting rules the operator<< must use basic_formatting_ostream instead of std::ostream.

但是，在整个文档中，我所看到的只是在 std::ostream 上重载 operator <<，而不是示例代码中的 basic_formatting_ostream。例如，参见自定义类型 severity_level here.

的重载

根据我的测试，std::ostream 和 basic_formatting_ostream 上的重载都工作正常。所以，我想知道重载一个而不是另一个的优点是什么。

Answer 1

仅 operator << (std::ostream&, ...) 重载没有问题，因为 formatting_ostream 有

template< typename CharT, typename TraitsT, typename AllocatorT, typename T >
inline basic_formatting_ostream< CharT, TraitsT, AllocatorT >&
operator<< (basic_formatting_ostream< CharT, TraitsT, AllocatorT >& strm, T const& value)
{
    strm.stream() << value;
    return strm;
}

其中 stream() returns std::ostream&。如果你用第一个参数 formatting_ostream 重载 operator <<，那么它只能用于 boost::log，如果你重载 std::ostream&，那么它可以用于 boost::log ] 和另一个输出。

引用自头文件：

 * This stream wrapper is used by the library for log record formatting. It implements the standard string stream interface
 * with a few differences:
 *
 * \li It does not derive from standard types <tt>std::basic_ostream</tt>, <tt>std::basic_ios</tt> and <tt>std::ios_base</tt>,
 *     although it tries to implement their interfaces closely. There are a few small differences, mostly regarding <tt>rdbuf</tt>
 *     and <tt>str</tt> signatures, as well as the supported insertion operator overloads. The actual wrapped stream can be accessed
 *     through the <tt>stream</tt> methods.
 * \li By default, \c bool values are formatted using alphabetical representation rather than numeric.
 * \li The stream supports writing strings of character types different from the stream character type. The stream will perform
 *     character code conversion as needed using the imbued locale.
 * \li The stream operates on an external string object rather than on the embedded one. The string can be attached or detached
 *     from the stream dynamically.
 *
 * Although <tt>basic_formatting_ostream</tt> does not derive from <tt>std::basic_ostream</tt>, users are not required to add
 * special overloads of \c operator<< for it since the stream will by default reuse the operators for <tt>std::basic_ostream</tt>.
 * However, one can define special overloads of \c operator<< for <tt>basic_formatting_ostream</tt> if a certain type needs
 * special formatting when output to log.

Answer 2

如果您只重载 operator<<(std::ostream)，它将适用于每个流输出，包括到 basic_formatting_ostream 的输出。如果你只重载 operator<<(basic_formatting_ostream) 它只会输出到那种类型的流。

但是，您可能希望重载两者，例如，如果您希望向日志提供不同的或更多的信息（例如对象的地址）。