Passing by reference to a constructor

Question

I decided to see if assigning a reference to a member would make a member a reference. I wrote the following snippet to test it. There's a simple class Wrapper with an std::string as a member variable. I take take a const string& in the constructor and assign it to the public member variable. Later in the main() method I modify the member variable but the string I passed to the constructor remains unchanged, how come? I think in Java the variable would have changed, why not in this code snippet? How exactly do references work in this case?

#include <iostream> #include <string> using namespace std;  class Wrapper { public:    string str;     Wrapper(const string& newStr)    {       str = newStr;    } };  int main (int argc, char * const argv[])  {    string str = "hello";    cout << str << endl;    Wrapper wrapper(str);    wrapper.str[0] = 'j'; // should change 'hello' to 'jello'    cout << str << endl; } 

Answer 1

To assign a reference in a constructor you need to have a reference member

 class A{      std::string& str;  public:      A(std::string& str_)      :    str(str_) {}   }; 

str is now a reference to the value you passed in. Same applies for const refs

 class A{      const std::string& str;  public:      A(const std::string& str_)      :    str(str_) {}   }; 

However don't forget that once a reference has been assigned it can not be changed so if assignment requires a change to str then it will have to be a pointer instead.

Answer 2

Because Wrapper::str is not a reference, it's an independent object. So when you do str = newStr, you're copying the string.