实现无操作 std::ostream

问题描述

我正在考虑制作一个日志类，其中包含 Info、Error 等成员，可以配置输出到控制台、文件或无处.

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.

为了效率，我想避免格式化将被丢弃的消息(即不在详细模式下运行时的信息消息)的开销.如果我实现了一个无处输出的自定义 std::streambuf，我想 std::ostream 层仍将执行所有格式设置.任何人都可以提出一种拥有真正空"的 std::ostream 的方法，避免对使用 << 传递给它的参数做任何工作?

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 <<?

推荐答案

为了防止 operator<<() 调用进行格式化，您应该在编译时知道流类型.这可以通过宏或模板来完成.

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.

我的模板解决方案如下.

My template solution follows.

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

由于您必须在编译时执行此操作，因此当然非常不灵活.

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

例如，您无法在运行时从配置文件中决定日志级别.

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

这篇关于实现无操作 std::ostream的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！