试图写std：out和文件在同一时间 [英] trying to write std:out and file at the same time

问题描述

我想通过重载cstream在c ++中同时写入文件和stdout

test.h

  #pragma once 
 
 #include< iostream> 
 
使用std :: ofstream; 
 
 class OutputAndConsole：public ofstream 
 {
 public：
 std :: string fileName; 
 OutputAndConsole（const std :: string& fileName）：ofstream（fileName），fileName（fileName）{
}; 
 template< typename T> 
 OutputAndConsole&运算符<<（T var）; 
}; 
 
 
 template< typename T> 
 OutputAndConsole& OutputAndConsole :: operator<<（T var）
 {
 std :: cout< var; 
 ofstream :: operator<< （var）; 
 return（* this）; 
}; 
  

test.cpp

  OutputAndConsole文件（output.txt）; 
 file<< test; 
  

文件中的输出是

  01400930 
  

，但在控制台中

 测试
  

它看起来像进入

  _Myt& __CLR_OR_THIS_CALL operator< <（const void * _Val）
  

我做错了什么？

解决方案

问题

  ofstream :: operator<< （var）; 
  

这是您使用 ofstream :: < 作为限定的函数调用。你要求函数查找找到 ofstream 的成员函数;成员的最佳匹配是 void * 的一个，而 char * 的打印实际字符串内容是一个自由函数（即不是成员函数）。

如果你这样做， $ c> cout ：

  std :: cout.operator< ; 
  

解决方案

：

  static_cast< ofstream&>（* this）< var; 
  

，因为您仍然使用正常的运算符语法（具有所有的重载分辨率）以 ofstream 作为LHS操作数。

虽然我没有实际测试过。 p>

结论

另外， 运算符< ;

因此：

  struct OutputAndConsole：std :: ofstream 
 {
 OutputAndConsole（const std :: string& fileName）
：std :: ofstream（fileName）
，fileName（fileName）
 {}; 
 
 const std :: string fileName; 
}; 
 
 template< typename T> 
 OutputAndConsole& operator<<<（OutputAndConsole& strm，const T& var）
 {
 std :: cout< var; 
 static_cast< std :: ofstream&>（strm）<< var; 
 return strm; 
}; 
  

我也自由地做一些小的语法调整。

I am trying to write to file and stdout at the same time within c++ by overloading ofstream

test.h

 #pragma once 

#include <iostream>

using  std::ofstream;

class OutputAndConsole:public ofstream
{
public:
    std::string fileName;        
    OutputAndConsole(const std::string& fileName):ofstream(fileName),fileName(fileName){
    };
    template <typename T>
    OutputAndConsole& operator<<(T var);
};


template <typename T>
OutputAndConsole& OutputAndConsole::operator<<(T var)
{
    std::cout << var;
    ofstream::operator << (var);
    return (*this);
};

test.cpp

  OutputAndConsole file("output.txt");
  file << "test" ;

The output in the file is

01400930

but in the console is

test

I debugged the code it looks like it is entering into

_Myt& __CLR_OR_THIS_CALL operator<<(const void *_Val)

What am I doing wrong?

解决方案

The problem

ofstream::operator << (var);

It's your use of ofstream::operator<< as a qualified function call. You're mandating that the function lookup locate a member function of ofstream; the best match that's a member is the one for void*, whereas the specialisation for char* that prints the actual string contents is a free function (i.e. not a member function).

You'll find the same problem if you do this with cout, too:

std::cout.operator<<(var);

The solution

This might do it:

static_cast<ofstream&>(*this) << var;

because you're still using normal operator syntax (with all the overload resolution that this entails), but doing so with an ofstream as the LHS operand.

I haven't actually tested it, though.

Conclusion

As an aside, your operator<< ought to be a free function too, in order to fit in with this convention.

So:

struct OutputAndConsole : std::ofstream
{
    OutputAndConsole(const std::string& fileName)
       : std::ofstream(fileName)
       , fileName(fileName)
    {};

    const std::string fileName;
};

template <typename T>
OutputAndConsole& operator<<(OutputAndConsole& strm, const T& var)
{
    std::cout << var;
    static_cast<std::ofstream&>(strm) << var;
    return strm;
};

I also took the liberty of making some minor syntax adjustments.

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