Redirecting in C++

问题:

#include <iostream>
#include <fstream>
using namespace std;

void foo(){
  streambuf *psbuf;
  ofstream filestr;
  filestr.open ("test.txt");
  psbuf = filestr.rdbuf(); 
  cout.rdbuf(psbuf);    
}

int main () {
  foo();
  cout << "This is written to the file";
  return 0;
}

Does cout write to the given file?

If not, is there a way to do it without sending the variables to foo, like new?

---

update :

I can't use a solution that uses class or uses global so plz can some give me solution that use new. Also passing the from main to foo

streambuf *psbuf;
ofstream filestr;

should work right?

I am trying to do this but its not working? I pass the stream to foo so it exist in the main so it wont end when foo finish.

 void foo(streambuf *psbuf){

  ofstream filestr;
  filestr.open ("test.txt");
  psbuf = filestr.rdbuf(); 
  cout.rdbuf(psbuf);    
}

int main () {
streambuf *psbuf
  foo(psbuf);
  cout << "This is written to the file";
  return 0;
}

回答1:

I suspect that by now compiled and run your code and found that you get a segmentation fault.

You are getting this because you create and open an ofstream object within foo(), which is then destroyed (and closed) at the end of foo. When you attempt to write to the stream in main(), you attempt to access a buffer which no longer exists.

One workaround to this is to make your filestr object global. There are plenty of better ones!

Edit: Here is a better solution as suggested by @MSalters:

#include <iostream>
#include <fstream>

class scoped_cout_redirector
{
public:
    scoped_cout_redirector(const std::string& filename)
        :backup_(std::cout.rdbuf())
        ,filestr_(filename.c_str())
        ,sbuf_(filestr_.rdbuf())
    {
        std::cout.rdbuf(sbuf_);
    }

    ~scoped_cout_redirector()
    {
        std::cout.rdbuf(backup_);
    }

private:
    scoped_cout_redirector();
    scoped_cout_redirector(const scoped_cout_redirector& copy);
    scoped_cout_redirector& operator =(const scoped_cout_redirector& assign);

    std::streambuf* backup_;
    std::ofstream filestr_;
    std::streambuf* sbuf_;
};


int main()
{
    {
        scoped_cout_redirector file1("file1.txt");
        std::cout << "This is written to the first file." << std::endl;
    }


    std::cout << "This is written to stdout." << std::endl;

    {
        scoped_cout_redirector file2("file2.txt");
        std::cout << "This is written to the second file." << std::endl;
    }

    return 0;
}

回答2:

It seems to me that your code should work but ... Why don't you try yourself ? You will see if everything is written in test.txt or not.