Obtain a std::ostream either from std::cout or std::ofstream(file)

问题

how do I bind a std::ostream to either std::cout or to an std::ofstream object, depending on a certain program condition? Although this invalid for many reasons, I would like to achieve something that is semantically equivalent to the following:

std::ostream out = condition ? &std::cout : std::ofstream(filename);

I've seen some examples that are not exception-safe, such as one from http://www2.roguewave.com/support/docs/sourcepro/edition9/html/stdlibug/34-2.html:

int main(int argc, char *argv[])
{
  std::ostream* fp;                                           //1
  if (argc > 1)
     fp = new std::ofstream(argv[1]);                         //2
  else
     fp = &std::cout                                          //3

  *fp << "Hello world!" << std::endl;                         //4
  if (fp!=&std::cout) 
     delete fp;
}

Does anyone know a better, exception-safe solution?

回答1:

std::streambuf * buf;
std::ofstream of;

if(!condition) {
    of.open("file.txt");
    buf = of.rdbuf();
} else {
    buf = std::cout.rdbuf();
}

std::ostream out(buf);

That associates the underlying streambuf of either cout or the output file stream to out. After that you can write to "out" and it will end up in the right destination. If you just want that everything going to std::cout goes into a file, you can aswell do

std::ofstream file("file.txt");
std::streambuf * old = std::cout.rdbuf(file.rdbuf());
// do here output to std::cout
std::cout.rdbuf(old); // restore

This second method has the drawback that it's not exception safe. You possibly want to write a class that does this using RAII:

struct opiped {
    opiped(std::streambuf * buf, std::ostream & os)
    :os(os), old_buf(os.rdbuf(buf)) { }
    ~opiped() { os.rdbuf(old_buf); }

    std::ostream& os;
    std::streambuf * old_buf;
};

int main() {
    // or: std::filebuf of; 
    //     of.open("file.txt", std::ios_base::out);
    std::ofstream of("file.txt");
    {
        // or: opiped raii(&of, std::cout);
        opiped raii(of.rdbuf(), std::cout);
        std::cout << "going into file" << std::endl;
    }
    std::cout << "going on screen" << std::endl;
}

Now, whatever happens, std::cout is in clean state.

回答2:

This is exception-safe:

void process(std::ostream &os);

int main(int argc, char *argv[]) {
    std::ostream* fp = &cout;
    std::ofstream fout;
    if (argc > 1) {
        fout.open(argv[1]);
        fp = &fout;
    }
    process(*fp);
}

Edit: Herb Sutter has addressed this in the article Switching Streams (Guru of the Week).

回答3:

std::ofstream of;
std::ostream& out = condition ? std::cout : of.open(filename);

回答4:

Being a novice to C++, I don't know if this is exception-safe, but here's how I usually do it:

std::ostream& output = (condition)?*(new std::ofstream(filename)):std::cout;

回答5:

The following simple code works for me:

int main(int argc, char const *argv[]){   

    std::ofstream outF;
    if (argc > 1)
    {
        outF = std::ofstream(argv[1], std::ofstream::out); 
    }

    std::ostream& os = (argc > 1)? outF : std::cout;
}

来源：https://stackoverflow.com/questions/366955/obtain-a-stdostream-either-from-stdcout-or-stdofstreamfile

标签

c++

exception-handling

iostream