Redirecting stdout from another program in C++

问题:

I'm writing a unit test and therefore, cannot change the code within the file that I'm testing. The code that I'm testing has messages in cout that I am trying to redirect into a file to check to make sure that the program is outputting the right messages. Does anyone have a way to redirect stdout in another program that won't cause a lag? I have tried freopen() and that causes my program to hang for some reason.

回答1:

You could create a filebuf then replace cout's streambuf with it:

{
  std::filebuf f;
  f.open("output.txt", std::ios::out);
  std::streambuf* o = std::cout.rdbuf(&f);
  std::cout << "hello" << std::endl;  // endl will flush the stream
  std::cout.rdbuf(o);
}

You need to restore cout's original streambuf again (or set it to a null pointer) or it will probably crash when the global streams are flushed and destroyed, because the filebuf will already have gone out of scope.

回答2:

You can use 'open()' and 'dup2()'. You can use the helpers provided below. An example of how to use them:

void
code_to_test ()
{
    std::cout << "Here we go" << std::endl;
    std::cerr << "Danger danger" << std::endl;
}

run_test(code_to_test);

The run_test helper invokes redirection, and runs the test code.

template <typename TEST> void
run_test (TEST t, bool append = false) {
    flush_output();
    Redirect o(1, "/tmp/test_stdout", append);
    Redirect e(2, "/tmp/test_stderr", append);
    t();
    flush_output();
}

The flush_output helper flushes the streams.

void flush_output () {
    fflush(stdout);
    fflush(stderr);
    std::cout.flush();
    std::cerr.flush();
}

The Redirect class effects the redirection in the constructor. It restores the original descriptors back in the destructor.

class Redirect
{
    int m_what;
    int m_old_what;
public:
    Redirect (int what, std::string where, bool append = false)
        : m_what(what), m_old_what(dup(what)) {
        int flags = O_CREAT|O_WRONLY;
        if (append) flags |= O_APPEND;
        int f = open(where.c_str(), flags, 0660);
        dup2(f, m_what);
        close(f);
    }
    ~Redirect () {
        dup2(m_old_what, m_what);
        close(m_old_what);
    }
};