Monitoring standard output buffer and redirect to a string C++

Question

I have multiple .cpp files which are printing out and I want to redirect this to a string. The structure of my project is: a main file which calls the functions from another files and the another files. If I have one .cpp file it is easy with stringstream, but if there are multiple files how can I resolve that?

Main.cpp:

#include "Second.h"

int main() {
    std::string buffer = "First line";
    printOut(buffer);
    std::cout << "Hello world" << std::endl;
}

Second.h:

#include <string>

void printOut(std::string buffer);

Second.cpp

#include "Second.h"

void printOut(std::string buffer) {
    std::cout << buffer << std::endl;
}

In this case the string should look like this:

redirectedString = First line\nHello World\n

Answer 1

Output to the standard streams can be intercepted by using a suitable stream buffer. For example:

void do_something_with(std::string const& s) {
    // ...
 }

struct basicbuf: std::streambuf {
    std::string buf;
    int_type overflow(int_type c) {
        if (c != traits_type::eof()) {
             this->buf.push_back(c);
        }
        return traits_type::not_eof(c);
    }
    int sync() {
         do_something_with(buf);
         buf.clear();
         return 0;
    }
};

int main() {
     basicbuf buf;
     std::streambuf* orig = std::cout.rdbuf(&buf);

     std::cout << "hello, world\n" << std::flush;
     std::cout.rdbuf(orig);
}

When using multiple threads you may want to use thread local buffers to avoid having data races. The called function would actively transfer the buffer to wherever it is needed.