空流，我必须包括ostream吗？ [英] Null stream, do I have to include ostream?

问题描述

我在写一个记录器。如果禁用，这是定义LOG宏的代码：

I am writing a logger. If disabled, this is the code which defines the LOG macro:

#ifdef NO_LOG

#include <ostream>

struct nullstream : std::ostream {
    nullstream() : std::ios(0), std::ostream(0) {}
};

static nullstream logstream;

#define LOG if(0) logstream

#endif

LOG << "Log message " << 123 << std::endl;

它可以正常工作。编译器应该完全删除与LOG宏相关的代码。

It works correctly. The compiler should completely remove the code related to the LOG macro.

但是，我想避免包含ostream并将logstream对象定义为真正的轻可能为空。

However I would like to avoid the inclusion of ostream and define the logstream object as something really "light", possibly null.

谢谢！

推荐答案

// We still need a forward declaration of 'ostream' in order to
// swallow templated manipulators such as 'endl'.
#include <iosfwd>

struct nullstream {};

// Swallow all types
template <typename T>
nullstream & operator<<(nullstream & s, T const &) {return s;}

// Swallow manipulator templates
nullstream & operator<<(nullstream & s, std::ostream &(std::ostream&)) {return s;}

static nullstream logstream;

#define LOG if(0) logstream

// Example (including "iostream" so we can test the behaviour with "endl").
#include <iostream>

int main()
{
    LOG << "Log message " << 123 << std::endl;
}

这篇关于空流，我必须包括ostream吗？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IT屋！