Problems with pointer functions

[jwC5fSEw]

I've come across an exercise that requires me to make two functions, one of which takes a string& and one that takes a string*. They should both modify an outside string in a unique way. I'm not entirely clear on this whole thing, and this is what I have so far:

#include <iostream>
#include <string>
using namespace std;

string stringref(string& a){
    string* p = &a;
    *p = a + " plus this!";
    cout << a;
}

string stringderef(string* a);

int main(){
    string test = "Hello world";
    stringref(test);
}

As you can see, I have the first function defined and the second prototyped. The first function actually prints the correct output; however, it then crashes and returns a value of -1073741819. I'm sure I did something sloppy (my knowledge of pointers, especially the & operator, is still very shaky). Does anybody have any advice?

[kevinchkin]

I don't understand, if you are passing value by reference, then why is the return value string? Also, you are not returning anything from it.
May be change it to this:

string stringref(string a){
    string* p = &a;
    *p = a + " plus this!";
    cout << a;
    return a ; 
}

[Duthomhas]

That code, as-is, should not have compiled.

You might want to read the CPlusPlus.com pointers and references tutorials.

#include <iostream>
#include <string>
using namespace std;

void stringref( string& s )  // This is a reference to main::test
  {
  s += " two";  // Modifying s is the same as modifying main::test
  }

void stringptr( string* s )  // This is a pointer to main::test
  {
  *s += " three";  // We must first dereference the pointer to modify main::test
  }

int main()
  {
  string test = "one";
  stringref(  test );
  stringptr( &test );
  cout << test << endl;
  return 0;
  }

Don't get confused by the term "dereference". What it means is to access the thing that a pointer is pointing at.

A "reference" does all this automagically behind the scenes, so you don't have to think about it.

Hope this helps.

[jwC5fSEw]

I'll go reread it, as I'm obviously still not grasping a lot of it. One question: for stringref(), would using string s as the argument be any different from using string& s?

[Duthomhas]

Yes, absolutely!

string s is a copy of whatever you pass as argument to the function.

string & s is a reference to (i.e. the same thing as) whatever you pass as argument to the function.

Good luck!

[jwC5fSEw]

Ohh, so if I have a function that adds 2 to an int, the inputted int won't be changed outside of the function. However, if I have use int& in the argument list, it'll actually modify the outside variable. Is that right?