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Jun 6, 2010 (last update: Aug 20, 2010)

Distinguish between pointers and references in C++

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Pointers and references look different enough (pointers use the * and -> operators, references use .), but they seem to do similar things. Both pointers and references let you refer to other objects indirectly. How, then, do you decide when to use one and not the other?
First, recognize that there is no such thing as a null reference. A reference must always refer to some object. As a result, if you have a variable whose purpose is to refer to another object, but it is possible that there might not be an object to refer to, you should make the variable a pointer, because then you can set it to null. On the other hand, if the variable must always refer to an object, i.e., if your design does not allow for the possibility that the variable is null, you should probably make the variable a reference.
"But wait," you wonder, "what about underhandedness like this?"
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char *pc = 0;          // set pointer to null  
char& rc = *pc;        // make reference refer to                         
                       // dereferenced null pointer  


Well, this is evil, pure and simple. The results are undefined (compilers can generate output to do anything they like), and people who write this kind of code should be shunned until they agree to cease and desist. If you have to worry about things like this in your software, you're probably best off avoiding references entirely. Either that or finding a better class of programmers to work with. We'll henceforth ignore the possibility that a reference can be null
Because a reference must refer to an object, C++ requires that references be initialized:
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string& rs;             // error! References must  
                        // be initialized  
string s("xyzzy");  
string& rs = s;         // okay, rs refers to s   

Pointers are subject to no such restriction:
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string *ps;             // uninitialized pointer:  
                        // valid but risky  

The fact that there is no such thing as a null reference implies that it can be more efficient to use references than to use pointers. That's because there's no need to test the validity of a reference before using it:
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void printDouble(const double& rd)  
{  
    cout << rd;         // no need to test rd; it  
}                       // must refer to a double   

Pointers, on the other hand, should generally be tested against null:
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void printDouble(const double *pd)  
{  
if (pd) {             // check for null pointer  
cout << *pd;  
}  
}  

Another important difference between pointers and references is that pointers may be reassigned to refer to different objects. A reference, however, always refers to the object with which it is initialized:
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string s1("Nancy");  
string s2("Clancy");  
string& rs = s1;         // rs refers to s1  
  
string *ps = &s1;        // ps points to s1<a name="31186"></a>  
  
rs = s2;                 // rs still refers to s1,  
                         // but s1's value is now  
                         // "Clancy"  
  
ps = &s2;                // ps now points to s2;  
                         // s1 is unchanged   

In general, you should use a pointer whenever you need to take into account the possibility that there's nothing to refer to (in which case you can set the pointer to null) or whenever you need to be able to refer to different things at different times (in which case you can change where the pointer points). You should use a reference whenever you know there will always be an object to refer to and you also know that once you're referring to that object, you'll never want to refer to anything else.
There is one other situation in which you should use a reference, and that's when you're implementing certain operators. The most common example is operator[]. This operator typically needs to return something that can be used as the target of an assignment:
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vector<int> v(10);       // create an int vector of size 10;  
                         // vector is a template in the  
                         // standard C++ library  
v[5] = 10;               // the target of this assignment is  
                         // the return value of operator[]   

If operator[] returned a pointer, this last statement would have to be written this way:
*v[5] = 10;
But this makes it look like v is a vector of pointers, which it's not. For this reason, you'll almost always want operator[] to return a reference.
References, then, are the feature of choice when you know you have something to refer to, when you'll never want to refer to anything else, and when implementing operators whose syntactic requirements make the use of pointers undesirable. In all other cases, stick with pointers.