空流，我必须包括ostream吗？

Question

我正在写一个记录器。如果禁用，这是定义LOG宏的代码：

#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; 

它工作正常。编译器应该完全删除与LOG宏有关的代码。

但是我想避免包含ostream并将logstream对象定义为真的“轻”，可能为null。

谢谢！

Answer 1

// 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; 
} 

Answer 2

为什么不从头实现整个事情：

struct nullstream { }; 

template <typename T> 
nullstream & operator<<(nullstream & o, T const & x) { return o; } 

static nullstream logstream;