正确重载运算符<<在Boost.Log

在Boost.Log documentation，据说正确重载运算符<<在Boost.Log

注意

库使用basic_formatting_ostream流类型的记录格式，所以定制属性值格式时规则operator<<必须使用basic_formatting_ostream而不是 std::ostream。

然而，在整个文件中，所有我看到的是在示例代码超载对std::ostream而非basic_formatting_ostreamoperator <<。例如，请参阅自定义类型的过载severity_levelhere。

根据我的测试，std::ostream和basic_formatting_ostream的过载都正常工作。所以，我想知道重载的好处是什么，而不是另一个。

2015-10-02 Lingxi

使用'std :: ostream'的好处应该是非常明显的：您也可以将它用于“普通”输出。 :) –

有超载只是operator << (std::ostream&, ...)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()回报std::ostream&。如果将operator <<与第一个参数formatting_ostream重叠，则该参数只能与boost::log一起使用，如果您的参数超载为std::ostream&，则可以将其用于boost::log和另一个输出。从头文件

报价：

* 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.

来源

2015-10-02 08:05:40 ForEveR

但插入'basic_formatting_ostream'似乎做比插入'std :: ostream'更多的工作。例如，查看'basic_formatting_ostream'为'const char *'提供的重载的'operator <<'。 – Lingxi

@Lingxi nope，'basic_formatting_ostream'只是尽量接近'basic_ostream'的接口。 – ForEveR

因此，Boost.Log文档只是过时或什么？ – Lingxi

如果你只重载operator<<(std::ostream)，它会为每一个流输出工作，包括对basic_formatting_ostream。如果您只重载operator<<(basic_formatting_ostream)它只会用于输出到该类型的流。例如，如果要为日志提供不同或更多的信息（如对象地址），则可能需要重载。

来源

2015-10-02 08:24:19 Walter

正确重载运算符<<在Boost.Log

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