std :: cout，ostream和其他类型的获取输出流

Question

In my project (Unreal Engine 4) I don't have an output stream - instead of this I can communicate via UE_LOG function, which works pretty much similar to printf() . The problem is that I just made a .dll library (without Unreal includes) which I want to communicate through the iostream . My idea is - inside .dll library I use standard cout to write messages into ostream, I use all of it in Unreal Engine functions, where I grab ostream in form of string and output it into UE_LOG function.

Problem is I always treated std::cout as a part of magic, without thinking what is really inside (I am pretty sure most of us did). How I can handle this? Easy ways won't work (like grabbing stringstream and outputing it into UE_LOG).

Answer 1

My idea is - inside .dll library I use standard cout to write messages into ostream

You actually can replace the output buffer used with std::cout with your own implementation. Use the std::ostream::rdbuf() function to do so (example from the reference docs):

#include <iostream>
#include <sstream>

int main()
{
    std::ostringstream local;
    auto cout_buff = std::cout.rdbuf(); // save pointer to std::cout buffer

    std::cout.rdbuf(local.rdbuf()); // substitute internal std::cout buffer with
        // buffer of 'local' object

    // now std::cout work with 'local' buffer
    // you don't see this message
    std::cout << "some message";

    // go back to old buffer
    std::cout.rdbuf(cout_buff);

    // you will see this message
    std::cout << "back to default buffer\n";

    // print 'local' content
    std::cout << "local content: " << local.str() << "\n";
}

Answer 2

(in case my edit won't be positively reviewed)

From OP: Thanks to your hints I finally found how to solve my problem. Suppose I want to get stream from cout and send it to printf (because I think stdio library is superior to iostream). Here how I can do this:

#include <iostream>
#include <sstream>
#include <cstdio>

using namespace std;

class ssbuf : public stringbuf{
protected:
    int sync(){
        printf("My buffer: %s",this->str().c_str());
        str("");
        return this->stringbuf::sync();
    }
};


int main(){
    ssbuf *buf = new ssbuf();
    cout.rdbuf(buf);
    cout<<"This is out stream "<<"and you cant do anything about it"<<endl;
    cout<<"(don't) "<<"Vote Trump"<<endl;
}

Code is very raw, but it does it's job. I made child class of buffer which has method sync() downcasting original virtual method sync(). Except this it works like usual buffer, just grabs all console-out stream - exactly what we wanted. The str("") inside is to clean the buffer - probably not outputted stream doesn't clean itself.

Great thanks for help! Big GRIN for you! :D