C++: Redirecting STDOUT

Question

In my application, I want to redirect the output that would normally go to the stdout stream to a function I define. I read that you can redirect stdio to a file, so why not to a function?

For example:

void MyHandler( const char* data );  //<<Magical redirection code>>  printf( "test" ); std::cout << "test" << std::endl;  //MyHandler should have been called with "test" twice, at this point 

• How can I achieve this / similar behaviour?

Answer 1

@Konrad Rudolph is right, you can totally do this, easily, at least for cout/cerr/clog. You don't even need your own streambuf implementation, just use an ostringstream.

// Redirect cout. streambuf* oldCoutStreamBuf = cout.rdbuf(); ostringstream strCout; cout.rdbuf( strCout.rdbuf() );  // This goes to the string stream. cout << "Hello, World!" << endl;  // Restore old cout. cout.rdbuf( oldCoutStreamBuf );  // Will output our Hello World! from above. cout << strCout.str(); 

Same thing works for cerr and clog, but in my experience that will NOT work for stdout/stderr in general, so printf won't output there. cout goes to stdout, but redirecting cout will not redirect all stdout. At least, that was my experience.

If the amount of data is expected to be small, the freopen/setbuf thing works fine. I ended up doing the fancier dup/dup2 thing redirecting to a pipe.

Update: I wrote a blog post showing the dup2 method I ended up using, which you can read here. It's written for OS X, but might work in other Unix flavors. I seriously doubt it would work in Windows. Cocoa version of the same thing here.

Answer 2

All the other answers are wrong. You can do this, and you don’t need to resort to freopen, nor to any other C or nonstandard functions.

Instead, you need to create your own std::streambuf implementation, i.e. your own stream buffer.

Once you have that, you can redirect the cout stream by switching the buffer:

your_stream_buffer new_buffer; streambuf* old_buffer = cout.rdbuf(&new_buffer);  cout << "Hello"; // This will be redirected to `new_buffer`.  // Restore original buffer: cout.rdbuf(old_buffer); 

Since I’ve never done that myself I cannot tell you exactly how the implementation of streambuf has to look like. My advice is to have a look at the documentation and do a dummy implementation that prints diagnostics for all implemented functions. This should help figuring out how the class works.

Alternatively, have a look at a C++ reference book of your choice that describes the streambuf class.