Then you could use the expressions rodents and *prats interchangeably with rats and use the expressions &rodents and prats interchangeably with &rats. From this standpoint, a reference looks a lot like a pointer in disguised notation in which the * dereferencing operator is understood implicitly. And, in fact, that’s more or less what a reference is. But there are differences besides those of notation. For one, it is necessary to initialize the reference when you declare it; you can’t declare the reference and then assign it a value later the way you can with a pointer:
int rat;
int & rodent;
rodent = rat; // No, you can't do this.
Note
You should initialize a reference variable when you declare it.
A reference is rather like a const pointer; you have to initialize it when you create it, and when a reference pledges its allegiance to a particular variable, it sticks to its pledge. That is,
int & rodents = rats;
is, in essence, a disguised notation for something like this:
int * const pr = &rats
Here, the reference rodents plays the same role as the expression *pr.
Listing 8.3 shows what happens if you try to make a reference change allegiance from a rats variable to a bunnies variable.
Listing 8.3. secref.cpp
// secref.cpp -- defining and using a reference
#include
int main()
{
using namespace std;
int rats = 101;
int & rodents = rats; // rodents is a reference
cout << "rats = " << rats;
cout << ", rodents = " << rodents << endl;
cout << "rats address = " << &rats
cout << ", rodents address = " << &rodents << endl;
int bunnies = 50;
rodents = bunnies; // can we change the reference?
cout << "bunnies = " << bunnies;
cout << ", rats = " << rats;
cout << ", rodents = " << rodents << endl;
cout << "bunnies address = " << &bunnies
cout << ", rodents address = " << &rodents << endl;
return 0;
}
Here’s the output of the program in Listing 8.3:
rats = 101, rodents = 101
rats address = 0x0065fd44, rodents address = 0x0065fd44
bunnies = 50, rats = 50, rodents = 50
bunnies address = 0x0065fd48, rodents address = 0x0065fd4
Initially, rodents refers to rats, but then the program apparently attempts to make rodents a reference to bunnies:
rodents = bunnies;
For a moment, it looks as if this attempt has succeeded because the value of rodents changes from 101 to 50. But closer inspection reveals that rats also has changed to 50 and that rats and rodents still share the same address, which differs from the bunnies address. Because rodents is an alias for rats, the assignment statement really means the same as the following:
rats = bunnies;
That is, it means “Assign the value of the bunnies variable to the rat variable.” In short, you can set a reference by an initializing declaration, not by assignment.
Suppose you tried the following:
int rats = 101;
int * pt = &rats
int & rodents = *pt;
int bunnies = 50;
pt = &bunnies
Initializing rodents to *pt makes rodents refer to rats. Subsequently altering pt to point to bunnies does not alter the fact that rodents refers to rats.
References as Function Parameters
Most often, references are used as function parameters, making a variable name in a function an alias for a variable in the calling program. This method of passing arguments is called
Figure 8.2. Passing by value and passing by reference.
Let’s compare using references and using pointers in a common computer problem: swapping the values of two variables. A swapping function has to be able to alter values of variables in the calling program. That means the usual approach of passing variables by value won’t work because the function will end up swapping the contents of copies of the original variables instead of the variables themselves. If you pass references, however, the function can work with the original data. Alternatively, you can pass pointers in order to access the original data. Listing 8.4 shows all three methods, including the one that doesn’t work, so that you can compare them.
Listing 8.4. swaps.cpp
// swaps.cpp -- swapping with references and with pointers
#include
void swapr(int & a, int & b); // a, b are aliases for ints
void swapp(int * p, int * q); // p, q are addresses of ints
void swapv(int a, int b); // a, b are new variables
int main()
{
using namespace std;
int wallet1 = 300;
int wallet2 = 350;
cout << "wallet1 = $" << wallet1;
cout << " wallet2 = $" << wallet2 << endl;
cout << "Using references to swap contents:\n";
swapr(wallet1, wallet2); // pass variables
cout << "wallet1 = $" << wallet1;