#include<iostream>
using namespace std;
int add(int &number);
int main ()
{
int number;
int result;
number=5;
cout << "The value of the variable number before calling the function : " << number << endl;
result=add(&number);
cout << "The value of the variable number after the function is returned : " << number << endl;
cout << "The value of result : " << result << endl;
return(0);
}
int add(int &p)
{
*p=*p+100;
return(*p);
}
This is invalid code on several counts. Running it through g++ gives:
crap.cpp: In function ‘int main()’:
crap.cpp:11: error: invalid initialization of non-const reference of type ‘int&’ from a temporary of type ‘int*’
crap.cpp:3: error: in passing argument 1 of ‘int add(int&)’
crap.cpp: In function ‘int add(int&)’:
crap.cpp:19: error: invalid type argument of ‘unary *’
crap.cpp:19: error: invalid type argument of ‘unary *’
crap.cpp:20: error: invalid type argument of ‘unary *’
A valid version of the code reads:
#include<iostream>
using namespace std;
int add(int &number);
int main ()
{
int number;
int result;
number=5;
cout << "The value of the variable number before calling the function : " << number << endl;
result=add(number);
cout << "The value of the variable number after the function is returned : " << number << endl;
cout << "The value of result : " << result << endl;
return(0);
}
int add(int &p)
{
p=p+100;
return p;
}
What is happening here is that you are passing a variable "as is" to your function. This is roughly equivalent to:
int add(int *p)
{
*p=*p+100;
return *p;
}
However, passing a reference to a function ensures that you cannot do things like pointer arithmetic with the reference. For example:
int add(int &p)
{
*p=*p+100;
return p;
}
is invalid.
If you must use a pointer to a reference, that has to be done explicitly:
int add(int &p)
{
int* i = &p;
i=i+100L;
return *i;
}
Which on a test run gives (as expected) junk output:
The value of the variable number before calling the function : 5
The value of the variable number after the function is returned : 5
The value of result : 1399090792