如何在 C++ 中覆盖 cout? [英] How to override cout in C++?

问题描述

我有一个需求，我需要使用printf 和cout 将数据显示到console 和file 中.对于 printf 我已经做到了，但是对于 cout 我很挣扎，怎么做?

I have a requirement, I need to use printf and cout to display the data into console and file as well. For printf I have done it but for cout I am struggling, how to do it?

   #ifdef _MSC_VER
     #define GWEN_FNULL "NUL"
     #define va_copy(d,s) ((d) = (s))
         #else
         #define GWEN_FNULL "/dev/null"
        #endif
        #include <iostream>
        #include <fstream>

        using namespace std;
        void printf (FILE *  outfile, const char * format, ...) 
        {

            va_list ap1, ap2;
            int i = 5;
            va_start(ap1, format);
            va_copy(ap2, ap1);
            vprintf(format, ap1);
            vfprintf(outfile, format, ap2);
            va_end(ap2);
            va_end(ap1);
        }
    /*    void COUT(const char* fmt, ...)
        {
            ofstream out("output-file.txt");
            std::cout << "Cout to file";
            out << "Cout to file";
        }*/
        int main (int argc, char *argv[]) {

            FILE *outfile;
            char *mode = "a+";
            char outputFilename[] = "PRINT.log";
            outfile = fopen(outputFilename, mode);

            char bigfoot[] = "Hello 

World!
";
        int howbad = 10;

        printf(outfile, "
--------
");
        //myout();

        /* then i realized that i can't send the arguments to fn:PRINTs */
        printf(outfile, "%s %i",bigfoot, howbad); /* error here! I can't send bigfoot and howbad*/

        system("pause");
        return 0;
    }

我已经在 COUT(大写，上面代码的注释部分)中完成了.但是我想使用普通的std::cout，那么我该如何覆盖它.它应该适用于 sting 和 variables 就像

I have done it in COUT(caps, the commented part for the code above) . But I want to use normal std::cout, so how can I override it. And it should work for both sting and variables like

int i = 5;
cout << "Hello world" << i <<endl;

或者无论如何都可以捕获stdout数据，以便它们也可以轻松写入文件和控制台.

Or are there anyway to capture stdout data, so that they can be easily written into file and console as well.

推荐答案

覆盖 std::cout 的行为是一个非常糟糕的主意，因为其他开发人员将很难理解使用std::cout 表现不正常.

Overriding the behaviour of std::cout is a really bad idea as other developers will have a hard time understanding that the use of std::cout doesn't behave as usual.

用一个简单的类明确你的意图

Make your intention clear with a simple class

#include <fstream>
#include <iostream>

class DualStream
{
   std::ofstream file_stream;
   bool valid_state;
   public:
      DualStream(const char* filename) // the ofstream needs a path
      :
         file_stream(filename),  // open the file stream
         valid_state(file_stream) // set the state of the DualStream according to the state of the ofstream
      {
      }
      explicit operator bool() const
      {
         return valid_state;
      }
      template <typename T>
      DualStream& operator<<(T&& t) // provide a generic operator<<
      {
         if ( !valid_state ) // if it previously was in a bad state, don't try anything
         {
            return *this;
         }
         if ( !(std::cout << t) ) // to console!
         {
            valid_state = false;
            return *this;
         }
         if ( !(file_stream << t) ) // to file!
         {
            valid_state = false;
            return *this;
         }
         return *this;
      }
};
// let's test it:
int main()
{
   DualStream ds("testfile");
   if ( (ds << 1 << "
" << 2 << "
") )
   {
      std::cerr << "all went fine
";
   }
   else
   {
      std::cerr << "bad bad stream
";
   }
}

这提供了一个干净的界面，并为控制台和文件输出相同的内容.您可能希望添加刷新方法或以追加模式打开文件.

This provides a clean interface and outputs the same for both the console and the file. You may want to add a flush method or open the file in append mode.

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