C ++ iostream使用stringstream的损坏

问题描述

我想写一个非常简单的线程安全记录器。理想情况下，我希望它像 std :: cout 工作，其中你可以重载<< 操作员，并有一切神奇地显示在日志中。我在Windows机器上，所以这里是我尝试的方法：

  // Threadsafe logger 
 class Logger 
 {
 public：
 Logger（）
 {
 InitializeCriticalSection（& s）; 
} 
 
〜Logger（）
 {
 DeleteCriticalSection（& s）; 
} 
 
 void Log（std :: ostream const& os）
 {
 EnterCriticalSection（& s）; 
 // std :: cout<< static_cast< std :: stringstream const&>（os）.str（）; 
 std :: cout<< os.rdbuf（）; 
 LeaveCriticalSection（& s）; 
} 
 
 private：
 CRITICAL_SECTION s; 
};注意，我已经尝试了两种方法到 Log（） 
 code>函数。我接受 ostream 的原因是因为这是 stringstream 似乎在< ; 运算符被调用。  Log（）函数的两个变体在运行此代码时都会以同样的方式失败：
  #include< iostream> 
 #include< sstream> 
 #include< Windows.h> 
 
 int main（int argc，char * argv []）
 {
 Logger logger; 
 //logger.Log(std::stringstream(\"Test））; 
 logger.Log（std :: stringstream（Another）<<test）; 
 std :: cin.get（）; 
} 
  
输出第一行（Test Log函数的变体。第二行显示错误的输出：
 
 
 
  testher  
 
 
 
这显然 test 写在另一个。我缺少这些流的工作方式是什么？我尝试了一个 flush 调用希望能修复的东西，但它什么也没有。
 
 
 
如何获得尝试在线程安全的日志记录器正确地工作与流？
解决方案
问题不是与记录器，你使用stringstream。 
当std :: stringstream初始化时，流的位置指示符位于流的开头。
 
 
 
现在当你开始使用' 
 
 
 
要解决这个问题，你可以使用
来初始化stringstream std :: stringstream（Another，stringstream :: in | stringstream :: out | std :: stringstream :: ate）
 
 
 
（根据） p> 
I'm trying to write a really simple thread-safe logger. Ideally, I wanted it to work just like std::cout, wherein you could just overload the << operator and have everything magically show up in the log. I'm on a Windows machine, so here's the approach I tried:
// Threadsafe logger
class Logger
{
public:
  Logger()
  {
    InitializeCriticalSection(&s);
  }

  ~Logger()
  {
    DeleteCriticalSection(&s);
  }

  void Log(std::ostream const& os)
  {
    EnterCriticalSection(&s);
    //std::cout << static_cast<std::stringstream const&>(os).str();
    std::cout << os.rdbuf();
    LeaveCriticalSection(&s);
  }

private:
  CRITICAL_SECTION s;
};
Notice that I've tried two approaches to the Log() function. The reason I accept an ostream is because that's what a stringstream seems to produce after the << operator is called. Both variants of the Log() function fail in the same way when I run this code:
#include <iostream>
#include <sstream>
#include <Windows.h>

int main(int argc, char* argv[])
{
  Logger logger;
  //logger.Log(std::stringstream("Test"));
  logger.Log(std::stringstream("Another ") << "test");
  std::cin.get();
}
Outputting the first line ("Test") works correctly and displays properly using both variants of the Log function. The second line outputs a mangled output:
testher
which is obviously test written over Another. What am I missing about the way these streams work? I tried making a flush call hoping that would fix things, but it did nothing.
How can I get this attempt at a thread-safe logger working correctly with the streams?
 解决方案 
The problem isn't with the logger, rather it's with your use of stringstream.When std::stringstream is initialized, the stream's position indicator is positioned at the beginning of the stream.
Now when you start writing to the string using '<<', you start writing at the position indicator, replacing whatever was there before.
To work around this, you can initialize the stringstream withstd::stringstream("Another ", stringstream::in | stringstream::out | std::stringstream::ate) 
(as per http://www.cplusplus.com/reference/iostream/stringstream/stringstream/ )
                        
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