Example

//main.cpp:

#include <iostream>
#include <string>
#include "test.h"

int main()
{
    std::string s = "We ";
    test t(s);
    std::cout << s;
}

//test.h:

#include <string>

class test {
public:
    test(std::string& s);
    std::string store;
};

//test.cpp:

#include "test.h"
#include <iostream>

test::test(std::string& s) {
    store = s;
    store += "added this";
    s += "injected this";
    std::cout << store << std::endl;
}