c++ - Is "std::cout" usable in Android-ndk

Question

In Android-ndk, we could use "__android_log_write", "__android_log_print", ... etc to output messages to the "LogCat" window. How about if I use "std::cout" to output some strings ? E.g.

std::cout << "some strings" << std::endl;

Where would the strings be sent.

It seems that Android does not have Console Applications and the above strings may not be sent. Could I redirect the "stdout" to a file so that sending strings to "std::cout" is equivalent to logging messages ?

Answer 1

You can create a class derived from std::streambuf which uses the Android specific functions to send the produced sequence of characters. I don't know where the default implementation of std::cout sends characters on Android, however. Basically, this would look something like this:

class androidbuf : public std::streambuf {
public:
    enum { bufsize = 128 }; // ... or some other suitable buffer size
    androidbuf() { this->setp(buffer, buffer + bufsize - 1); }

private:
    int overflow(int c)
    {
        if (c == traits_type::eof()) {
            *this->pptr() = traits_type::to_char_type(c);
            this->sbumpc();
        }
        return this->sync()? traits_type::eof(): traits_type::not_eof(c);
    }

    int sync()
    {
        int rc = 0;
        if (this->pbase() != this->pptr()) {
            char writebuf[bufsize+1];
            memcpy(writebuf, this->pbase(), this->pptr() - this->pbase());
            writebuf[this->pptr() - this->pbase()] = '';

            rc = __android_log_write(ANDROID_LOG_INFO, "std", writebuf) > 0;
            this->setp(buffer, buffer + bufsize - 1);
        }
        return rc;
    }

    char buffer[bufsize];
};

To actually set up std::cout to write to this stream buffer, you would do something like this in your main() function:

int main() {
    std::cout.rdbuf(new androidbuf);
    ...
}

This create a memory leak for the one androidbuf stream which is, however, somewhat intentional: the stream may be written to after main() is exited and it is flushed when when std::cout gets destroyed. If you don't want this, you could either restore std::cout's original stream buffer or set it to null and delete the return from rdbuf():

   // avoid a one-time resource leak but don't get output afterwards:
   delete std::cout.rdbuf(0);