Question

在Boost.Log documentation中，据说

注意

库使用basic_formatting_ostream流类型进行记录格式化，因此在自定义属性值格式时   operator<<必须使用basic_formatting_ostream代替的规则   std::ostream。

但是，在整个文档中，我看到的只是operator <<上的std::ostream而不是示例代码中的basic_formatting_ostream。例如，请参阅自定义类型severity_level here的重载。

根据我的测试，std::ostream和basic_formatting_ostream上的重载都运行正常。所以，我想知道在一个而不是另一个上重载的优点是什么。

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()返回std::ostream&。如果您使用第一个arg operator <<重载formatting_ostream，那么只能使用boost::log，如果您为std::ostream&重载，那么这可以用于boost::log和另一个输出。

从头文件引用：

 * 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basic_formatting_ostream。如果只重载operator<<(basic_formatting_ostream)，它只适用于输出到该类型的流。

但是，您可能希望重载，例如，如果要向日志提供不同或更多信息（例如对象的地址）。

正确地重载运算符＆lt;＆lt;在Boost.Log中

2 个答案: