How to execute a command and get return code stdout and stderr of command in C++

Given the following answer (first c++11 answer):

How to execute a command and get output of command within C++ using POSIX?

Here is the implementation for your convinience:

#include <cstdio>
#include <iostream>
#include <memory>
#include <stdexcept>
#include <string>
#include <array>

std::string exec(const char* cmd) {
    std::array<char, 128> buffer;
    std::string result;
    std::shared_ptr<FILE> pipe(popen(cmd, "r"), pclose);
    if (!pipe) throw std::runtime_error("popen() failed!");
    while (!feof(pipe.get())) {
        if (fgets(buffer.data(), 128, pipe.get()) != nullptr)
            result += buffer.data();
    }
    return result;
}

This works really nicely to execute a command (e.g. std::string res = exec("ls");) and get the stdout into a string.

But what it does not do is get the command return code (pass/fail integer) or the stderr. Ideally I would like a way to get all three (return code, stdout, stderr).

I would settle for stdout and stderr. I am thinking that I need to add another pipe, but I can't really see how the first pipe is setup to get stdout so I can't think how I would change it to get both.

Any one got any ideas how to do that, or alternative approaches that may work?

update

See my complete example here