how to pass a string by reference in c 2b 2b

1#include <iostream>
2using namespace std;
3void func (string &s)
4{
5  s+="d";
6}
7int main()
8{
9  string a="abc";
10 	func(a);
11  cout << a << endl;
12  return 0;
13}
14// output will be abcd

1// C++ program to demonstrate differences between pointer 
2// and reference. 
3#include <iostream> 
4using namespace std; 
5  
6struct demo 
7{ 
8    int a; 
9}; 
10  
11int main() 
12{ 
13    int x = 5; 
14    int y = 6; 
15    demo d; 
16      
17    int *p; 
18    p =  &x; 
19    p = &y;                     // 1. Pointer reintialization allowed 
20    int &r = x; 
21    // &r = y;                  // 1. Compile Error 
22    r = y;                      // 1. x value becomes 6 
23      
24    p = NULL;            
25    // &r = NULL;               // 2. Compile Error 
26      
27    p++;                        // 3. Points to next memory location 
28    r++;                        // 3. x values becomes 7 
29      
30    cout << &p << " " << &x << endl;    // 4. Different address 
31    cout << &r << " " << &x << endl;    // 4. Same address 
32      
33    demo *q = &d; 
34    demo &qq = d; 
35      
36    q->a = 8; 
37    // q.a = 8;                 // 5. Compile Error  
38    qq.a = 8; 
39    // qq->a = 8;               // 5. Compile Error 
40      
41    cout << p << endl;        // 6. Prints the address 
42    cout << r << endl;        // 6. Print the value of x     
43  
44    return 0; 
45}