实现no-op std :: ostream

问题描述

为了提高效率，我建议使用一个日志类，该类包含Info，Error等可以配置输出到控制台，文件或无处不在的成员。 ，我想避免格式化将被抛弃的消息（即不在详细模式下运行时的信息消息）的开销。如果我实现一个自定义std :: streambuf输出到无处，我想std :: ostream层仍然会做所有的格式化。任何人都可以建议一个真正的nullstd :: ostream，避免做任何工作的参数传递给它与< ？

感谢。

解决方案

为了防止 ;<（）执行格式化的调用，你应该知道编译时的streamtype。这可以通过宏或模板来完成。

我的模板解决方案如下。

 code> class NullStream {
 public：
 void setFile（）{/ * no-op * /} 
 template< typename TPrintable> 
 NullStream& <<<（TPrintable const&）
 {/ * no-op * /} 
} 
 
 template< class TErrorStream> // add TInfoStream etc 
 class Logger {
 public：
 TErrorStream& errorStream（）{
 return m_errorStream; 
} 
 
 private：
 TErrorStream m_errorStream; 
}; 
 
 //用法
 int main（）{
 Logger< std :: ofstream> normal_logger; // does real output 
 normal_logger.errorStream（）。open（out.txt）; 
 normal_logger.errorStream（）<< 我的年龄是< 19; 
 
 Logger< NullStream> null_logger; //零输出零开销
 null_logger.errorStream（）。open（out.txt）; // no-op 
 null_logger.errorStream（）<< 我的年龄是< 19; // no-op 
} 
  

由于在编译时必须这样做，

例如，您无法从配置文件中确定运行时的日志记录级别。

I'm looking at making a logging class which has members like Info, Error etc that can configurably output to console, file, or to nowhere.

For efficiency, I would like to avoid the overhead of formatting messages that are going to be thrown away (ie info messages when not running in a verbose mode). If I implement a custom std::streambuf that outputs to nowhere, I imagine that the std::ostream layer will still do all the formatting. Can anyone suggest a way to have a truly "null" std::ostream that avoids doing any work at all on the parameters passed to it with <<?

Thanks.

解决方案

To prevent the operator<<() invocations from doing formatting, you should know the streamtype at compile-time. This can be done either with macros or with templates.

My template solution follows.

class NullStream {
public:
    void setFile() { /* no-op */ }
    template<typename TPrintable>
    NullStream& operator<<(TPrintable const&)
    { /* no-op */ }
}

template<class TErrorStream> // add TInfoStream etc
class Logger {
public:
    TErrorStream& errorStream() {
        return m_errorStream;
    }

private:
    TErrorStream m_errorStream;
};

//usage
int main() {
    Logger<std::ofstream> normal_logger; // does real output
    normal_logger.errorStream().open("out.txt");
    normal_logger.errorStream() << "My age is " << 19;

    Logger<NullStream> null_logger; // does zero output with zero overhead
    null_logger.errorStream().open("out.txt"); // no-op
    null_logger.errorStream() << "My age is " << 19; // no-op
}

Since you have to do this at compile-time, it is of course quite inflexible.

For example, you cannot decide the logging level at runtime from a configuration file.

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