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I understand the syntax and general semantics of pointers versus references, but how should I decide when it is more-or-less appropriate to use references or pointers in an API?
Naturally some situations need one or the other (operator++ needs a reference argument), but in general I'm finding I prefer to use pointers (and const pointers) as the syntax is clear that the variables are being passed destructively.
E.g. in the following code:
void add_one(int& n) { n += 1; }
void add_one(int* const n) { *n += 1; }
int main() {
int a = 0;
add_one(a); // Not clear that a may be modified
add_one(&a); // 'a' is clearly being passed destructively
}
With the pointer, it's always (more) obvious what's going on, so for APIs and the like where clarity is a big concern are pointers not more appropriate than references? Does that mean references should only be used when necessary (e.g. operator++)? Are there any performance concerns with one or the other?
EDIT (OUTDATED):
Besides allowing NULL values and dealing with raw arrays, it seems the choice comes down to personal preference. I've accepted the answer below that references Google's C++ Style Guide, as they present the view that "References can be confusing, as they have value syntax but pointer semantics.".
Due to the additional work required to sanitise pointer arguments that should not be NULL (e.g. add_one(0) will call the pointer version and break during runtime), it makes sense from a maintainability perspective to use references where an object MUST be present, though it is a shame to lose the syntactic clarity.
Use reference wherever you can, pointers wherever you must.
Avoid pointers until you can't.
The reason is that pointers make things harder to follow/read, less safe and far more dangerous manipulations than any other constructs.
So the rule of thumb is to use pointers only if there is no other choice.
For example, returning a pointer to an object is a valid option when the function can return nullptr in some cases and it is assumed it will. That said, a better option would be to use something similar to std::optional (requires C++17; before that, there's boost::optional).
Another example is to use pointers to raw memory for specific memory manipulations. That should be hidden and localized in very narrow parts of the code, to help limit the dangerous parts of the whole code base.
In your example, there is no point in using a pointer as argument because:
if you provide nullptr as the argument, you're going in undefined-behaviour-land;
the reference attribute version doesn't allow (without easy to spot tricks) the problem with 1.
the reference attribute version is simpler to understand for the user: you have to provide a valid object, not something that could be null.
If the behaviour of the function would have to work with or without a given object, then using a pointer as attribute suggests that you can pass nullptr as the argument and it is fine for the function. That's kind of a contract between the user and the implementation.
The performances are exactly the same, as references are implemented internally as pointers. Thus you do not need to worry about that.
There is no generally accepted convention regarding when to use references and pointers. In a few cases you have to return or accept references (copy constructor, for instance), but other than that you are free to do as you wish. A rather common convention I've encountered is to use references when the parameter must refer an existing object and pointers when a NULL value is ok.
Some coding convention (like Google's) prescribe that one should always use pointers, or const references, because references have a bit of unclear-syntax: they have reference behaviour but value syntax.
From C++ FAQ Lite -
Use references when you can, and pointers when you have to.
References are usually preferred over pointers whenever you don't need
"reseating". This usually means that references are most useful in a
class's public interface. References typically appear on the skin of
an object, and pointers on the inside.
The exception to the above is where a function's parameter or return
value needs a "sentinel" reference — a reference that does not refer
to an object. This is usually best done by returning/taking a pointer,
and giving the NULL pointer this special significance (references must
always alias objects, not a dereferenced NULL pointer).
Note: Old line C programmers sometimes don't like references since
they provide reference semantics that isn't explicit in the caller's
code. After some C++ experience, however, one quickly realizes this is
a form of information hiding, which is an asset rather than a
liability. E.g., programmers should write code in the language of the
problem rather than the language of the machine.
My rule of thumb is:
Use pointers for outgoing or in/out parameters. So it can be seen that the value is going to be changed. (You must use &)
Use pointers if NULL parameter is acceptable value. (Make sure it's const if it's an incoming parameter)
Use references for incoming parameter if it cannot be NULL and is not a primitive type (const T&).
Use pointers or smart pointers when returning a newly created object.
Use pointers or smart pointers as struct or class members instead of references.
Use references for aliasing (eg. int ¤t = someArray[i])
Regardless which one you use, don't forget to document your functions and the meaning of their parameters if they are not obvious.
Disclaimer: other than the fact that references cannot be NULL nor "rebound" (meaning thay can't change the object they're the alias of), it really comes down to a matter of taste, so I'm not going to say "this is better".
That said, I disagree with your last statement in the post, in that I don't think the code loses clarity with references. In your example,
add_one(&a);
might be clearer than
add_one(a);
since you know that most likely the value of a is going to change. On the other hand though, the signature of the function
void add_one(int* const n);
is somewhat not clear either: is n going to be a single integer or an array? Sometimes you only have access to (poorly documentated) headers, and signatures like
foo(int* const a, int b);
are not easy to interpret at first sight.
Imho, references are as good as pointers when no (re)allocation nor rebinding (in the sense explained before) is needed. Moreover, if a developer only uses pointers for arrays, functions signatures are somewhat less ambiguous. Not to mention the fact that operators syntax is way more readable with references.
Like others already answered: Always use references, unless the variable being NULL/nullptr is really a valid state.
John Carmack's viewpoint on the subject is similar:
NULL pointers are the biggest problem in C/C++, at least in our code. The dual use of a single value as both a flag and an address causes an incredible number of fatal issues. C++ references should be favored over pointers whenever possible; while a reference is “really” just a pointer, it has the implicit contract of being not-NULL. Perform NULL checks when pointers are turned into references, then you can ignore the issue thereafter.
http://www.altdevblogaday.com/2011/12/24/static-code-analysis/
Edit 2012-03-13
User Bret Kuhns rightly remarks:
The C++11 standard has been finalized. I think it's time in this thread to mention that most code should do perfectly fine with a combination of references, shared_ptr, and unique_ptr.
True enough, but the question still remains, even when replacing raw pointers with smart pointers.
For example, both std::unique_ptr and std::shared_ptr can be constructed as "empty" pointers through their default constructor:
http://en.cppreference.com/w/cpp/memory/unique_ptr/unique_ptr
http://en.cppreference.com/w/cpp/memory/shared_ptr/shared_ptr
... meaning that using them without verifying they are not empty risks a crash, which is exactly what J. Carmack's discussion is all about.
And then, we have the amusing problem of "how do we pass a smart pointer as a function parameter?"
Jon's answer for the question C++ - passing references to boost::shared_ptr, and the following comments show that even then, passing a smart pointer by copy or by reference is not as clear cut as one would like (I favor myself the "by-reference" by default, but I could be wrong).
It is not a matter of taste. Here are some definitive rules.
If you want to refer to a statically declared variable within the scope in which it was declared then use a C++ reference, and it will be perfectly safe. The same applies to a statically declared smart pointer. Passing parameters by reference is an example of this usage.
If you want to refer to anything from a scope that is wider than the scope in which it is declared then you should use a reference counted smart pointer for it to be perfectly safe.
You can refer to an element of a collection with a reference for syntactic convenience, but it is not safe; the element can be deleted at anytime.
To safely hold a reference to an element of a collection you must use a reference counted smart pointer.
There is problem with "use references wherever possible" rule and it arises if you want to keep reference for further use. To illustrate this with example, imagine you have following classes.
class SimCard
{
public:
explicit SimCard(int id):
m_id(id)
{
}
int getId() const
{
return m_id;
}
private:
int m_id;
};
class RefPhone
{
public:
explicit RefPhone(const SimCard & card):
m_card(card)
{
}
int getSimId()
{
return m_card.getId();
}
private:
const SimCard & m_card;
};
At first it may seem to be a good idea to have parameter in RefPhone(const SimCard & card) constructor passed by a reference, because it prevents passing wrong/null pointers to the constructor. It somehow encourages allocation of variables on stack and taking benefits from RAII.
PtrPhone nullPhone(0); //this will not happen that easily
SimCard * cardPtr = new SimCard(666); //evil pointer
delete cardPtr; //muahaha
PtrPhone uninitPhone(cardPtr); //this will not happen that easily
But then temporaries come to destroy your happy world.
RefPhone tempPhone(SimCard(666)); //evil temporary
//function referring to destroyed object
tempPhone.getSimId(); //this can happen
So if you blindly stick to references you trade off possibility of passing invalid pointers for the possibility of storing references to destroyed objects, which has basically same effect.
edit: Note that I sticked to the rule "Use reference wherever you can, pointers wherever you must. Avoid pointers until you can't." from the most upvoted and accepted answer (other answers also suggest so). Though it should be obvious, example is not to show that references as such are bad. They can be misused however, just like pointers and they can bring their own threats to the code.
There are following differences between pointers and references.
When it comes to passing variables, pass by reference looks like pass by value, but has pointer semantics (acts like pointer).
Reference can not be directly initialized to 0 (null).
Reference (reference, not referenced object) can not be modified (equivalent to "* const" pointer).
const reference can accept temporary parameter.
Local const references prolong the lifetime of temporary objects
Taking those into account my current rules are as follows.
Use references for parameters that will be used locally within a function scope.
Use pointers when 0 (null) is acceptable parameter value or you need to store parameter for further use. If 0 (null) is acceptable I am adding "_n" suffix to parameter, use guarded pointer (like QPointer in Qt) or just document it. You can also use smart pointers. You have to be even more careful with shared pointers than with normal pointers (otherwise you can end up with by design memory leaks and responsibility mess).
Any performance difference would be so small that it wouldn't justify using the approach that's less clear.
First, one case that wasn't mentioned where references are generally superior is const references. For non-simple types, passing a const reference avoids creating a temporary and doesn't cause the confusion you're concerned about (because the value isn't modified). Here, forcing a person to pass a pointer causes the very confusion you're worried about, as seeing the address taken and passed to a function might make you think the value changed.
In any event, I basically agree with you. I don't like functions taking references to modify their value when it's not very obvious that this is what the function is doing. I too prefer to use pointers in that case.
When you need to return a value in a complex type, I tend to prefer references. For example:
bool GetFooArray(array &foo); // my preference
bool GetFooArray(array *foo); // alternative
Here, the function name makes it clear that you're getting information back in an array. So there's no confusion.
The main advantages of references are that they always contain a valid value, are cleaner than pointers, and support polymorphism without needing any extra syntax. If none of these advantages apply, there is no reason to prefer a reference over a pointer.
Copied from wiki-
A consequence of this is that in many implementations, operating on a variable with automatic or static lifetime through a reference, although syntactically similar to accessing it directly, can involve hidden dereference operations that are costly. References are a syntactically controversial feature of C++ because they obscure an identifier's level of indirection; that is, unlike C code where pointers usually stand out syntactically, in a large block of C++ code it may not be immediately obvious if the object being accessed is defined as a local or global variable or whether it is a reference (implicit pointer) to some other location, especially if the code mixes references and pointers. This aspect can make poorly written C++ code harder to read and debug (see Aliasing).
I agree 100% with this, and this is why I believe that you should only use a reference when you a have very good reason for doing so.
Points to keep in mind:
Pointers can be NULL, references cannot be NULL.
References are easier to use, const can be used for a reference when we don't want to change value and just need a reference in a function.
Pointer used with a * while references used with a &.
Use pointers when pointer arithmetic operation are required.
You can have pointers to a void type int a=5; void *p = &a; but cannot have a reference to a void type.
Pointer Vs Reference
void fun(int *a)
{
cout<<a<<'\n'; // address of a = 0x7fff79f83eac
cout<<*a<<'\n'; // value at a = 5
cout<<a+1<<'\n'; // address of a increment by 4 bytes(int) = 0x7fff79f83eb0
cout<<*(a+1)<<'\n'; // value here is by default = 0
}
void fun(int &a)
{
cout<<a<<'\n'; // reference of original a passed a = 5
}
int a=5;
fun(&a);
fun(a);
Verdict when to use what
Pointer: For array, linklist, tree implementations and pointer arithmetic.
Reference: In function parameters and return types.
The following are some guidelines.
A function uses passed data without modifying it:
If the data object is small, such as a built-in data type or a small structure, pass it by value.
If the data object is an array, use a pointer because that’s your only choice. Make the pointer a pointer to const.
If the data object is a good-sized structure, use a const pointer or a const
reference to increase program efficiency.You save the time and space needed to
copy a structure or a class design. Make the pointer or reference const.
If the data object is a class object, use a const reference.The semantics of class design often require using a reference, which is the main reason C++ added
this feature.Thus, the standard way to pass class object arguments is by reference.
A function modifies data in the calling function:
1.If the data object is a built-in data type, use a pointer. If you spot code
like fixit(&x), where x is an int, it’s pretty clear that this function intends to modify x.
2.If the data object is an array, use your only choice: a pointer.
3.If the data object is a structure, use a reference or a pointer.
4.If the data object is a class object, use a reference.
Of course, these are just guidelines, and there might be reasons for making different
choices. For example, cin uses references for basic types so that you can use cin >> n
instead of cin >> &n.
Your properly written example should look like
void add_one(int& n) { n += 1; }
void add_one(int* const n)
{
if (n)
*n += 1;
}
That's why references are preferable if possible
...
References are cleaner and easier to use, and they do a better job of hiding information.
References cannot be reassigned, however.
If you need to point first to one object and then to another, you must use a pointer. References cannot be null, so if any chance exists that the object in question might be null, you must not use a reference. You must use a pointer.
If you want to handle object manipulation on your own i.e if you want to allocate memory space for an object on the Heap rather on the Stack you must use Pointer
int *pInt = new int; // allocates *pInt on the Heap
In my practice I personally settled down with one simple rule - Use references for primitives and values that are copyable/movable and pointers for objects with long life cycle.
For Node example I would definitely use
AddChild(Node* pNode);
Just putting my dime in. I just performed a test. A sneeky one at that. I just let g++ create the assembly files of the same mini-program using pointers compared to using references.
When looking at the output they are exactly the same. Other than the symbolnaming. So looking at performance (in a simple example) there is no issue.
Now on the topic of pointers vs references. IMHO I think clearity stands above all. As soon as I read implicit behaviour my toes start to curl. I agree that it is nice implicit behaviour that a reference cannot be NULL.
Dereferencing a NULL pointer is not the problem. it will crash your application and will be easy to debug. A bigger problem is uninitialized pointers containing invalid values. This will most likely result in memory corruption causing undefined behaviour without a clear origin.
This is where I think references are much safer than pointers. And I agree with a previous statement, that the interface (which should be clearly documented, see design by contract, Bertrand Meyer) defines the result of the parameters to a function. Now taking this all into consideration my preferences go to
using references wherever/whenever possible.
For pointers, you need them to point to something, so pointers cost memory space.
For example a function that takes an integer pointer will not take the integer variable. So you will need to create a pointer for that first to pass on to the function.
As for a reference, it will not cost memory. You have an integer variable, and you can pass it as a reference variable. That's it. You don't need to create a reference variable specially for it.
This question already has answers here:
What are the differences between a pointer variable and a reference variable?
(44 answers)
Closed 9 years ago.
I haven't understand passing by reference in C++ completely. I already read related questions like the following ones.
What are the differences between a pointer variable and a reference variable in C++?
Are there benefits of passing by pointer over passing by reference in C++?
When to use references vs. pointers
But even though I understand that it is different to a pointer, I don't fully understand the concept of reference in C++. How does it work? I seems like reference is a special case of a pointer and might be implemented as such by the compiler. But it behaves different on programming level. Is that correct?
Is the concept of passing by reference a special case of passing a pointer to that value? Many times I have to decide whether to use reference or pointer while programming. Moreover I want to understand the underlying principle. Is only the memory address copied?
A reference is a pointer with some special restrictions. A reference must never be un-initialized. When a reference is created it must point to something. This is useful when wanting a "pointer" that will not be NULL.
Furthermore a reference cannot have the address it holds changed, therefor it is a constant pointer.
It has some nice syntactic sugar to allow it to use the dot operator, making it useful when changing from by-value code to by-reference code.
In practice using a reference in a function as a parameter or return value usually would signify "by address, cannot be NULL and must be a valid object". Using a pointer instead would usually signify "could be NULL, so NULL must be handle". By pointer also allows for pointer arithmetic and manipulation.
What you should use is really what you're most comfortable with, assuming you understand both. I myself prefer pointers.
Edit:
As pointed out in the comments here, you cannot take the address of a reference directly, as it will return the address of what the reference is referring to. This is yet another restriction that does not apply to an average pointer.
yes, it's essentially the same as a pointer but a nicer and safer version as the pointer address cant be changed or set to null. i see it mainly as a means to have nice function interfaces without requiring the user of the interface to worry about pointers - ie function remain simple yet the object is not copied, purely its address. class copy constructor is a prime example of where references are crucial.
I understand the syntax and general semantics of pointers versus references, but how should I decide when it is more-or-less appropriate to use references or pointers in an API?
Naturally some situations need one or the other (operator++ needs a reference argument), but in general I'm finding I prefer to use pointers (and const pointers) as the syntax is clear that the variables are being passed destructively.
E.g. in the following code:
void add_one(int& n) { n += 1; }
void add_one(int* const n) { *n += 1; }
int main() {
int a = 0;
add_one(a); // Not clear that a may be modified
add_one(&a); // 'a' is clearly being passed destructively
}
With the pointer, it's always (more) obvious what's going on, so for APIs and the like where clarity is a big concern are pointers not more appropriate than references? Does that mean references should only be used when necessary (e.g. operator++)? Are there any performance concerns with one or the other?
EDIT (OUTDATED):
Besides allowing NULL values and dealing with raw arrays, it seems the choice comes down to personal preference. I've accepted the answer below that references Google's C++ Style Guide, as they present the view that "References can be confusing, as they have value syntax but pointer semantics.".
Due to the additional work required to sanitise pointer arguments that should not be NULL (e.g. add_one(0) will call the pointer version and break during runtime), it makes sense from a maintainability perspective to use references where an object MUST be present, though it is a shame to lose the syntactic clarity.
Use reference wherever you can, pointers wherever you must.
Avoid pointers until you can't.
The reason is that pointers make things harder to follow/read, less safe and far more dangerous manipulations than any other constructs.
So the rule of thumb is to use pointers only if there is no other choice.
For example, returning a pointer to an object is a valid option when the function can return nullptr in some cases and it is assumed it will. That said, a better option would be to use something similar to std::optional (requires C++17; before that, there's boost::optional).
Another example is to use pointers to raw memory for specific memory manipulations. That should be hidden and localized in very narrow parts of the code, to help limit the dangerous parts of the whole code base.
In your example, there is no point in using a pointer as argument because:
if you provide nullptr as the argument, you're going in undefined-behaviour-land;
the reference attribute version doesn't allow (without easy to spot tricks) the problem with 1.
the reference attribute version is simpler to understand for the user: you have to provide a valid object, not something that could be null.
If the behaviour of the function would have to work with or without a given object, then using a pointer as attribute suggests that you can pass nullptr as the argument and it is fine for the function. That's kind of a contract between the user and the implementation.
The performances are exactly the same, as references are implemented internally as pointers. Thus you do not need to worry about that.
There is no generally accepted convention regarding when to use references and pointers. In a few cases you have to return or accept references (copy constructor, for instance), but other than that you are free to do as you wish. A rather common convention I've encountered is to use references when the parameter must refer an existing object and pointers when a NULL value is ok.
Some coding convention (like Google's) prescribe that one should always use pointers, or const references, because references have a bit of unclear-syntax: they have reference behaviour but value syntax.
From C++ FAQ Lite -
Use references when you can, and pointers when you have to.
References are usually preferred over pointers whenever you don't need
"reseating". This usually means that references are most useful in a
class's public interface. References typically appear on the skin of
an object, and pointers on the inside.
The exception to the above is where a function's parameter or return
value needs a "sentinel" reference — a reference that does not refer
to an object. This is usually best done by returning/taking a pointer,
and giving the NULL pointer this special significance (references must
always alias objects, not a dereferenced NULL pointer).
Note: Old line C programmers sometimes don't like references since
they provide reference semantics that isn't explicit in the caller's
code. After some C++ experience, however, one quickly realizes this is
a form of information hiding, which is an asset rather than a
liability. E.g., programmers should write code in the language of the
problem rather than the language of the machine.
My rule of thumb is:
Use pointers for outgoing or in/out parameters. So it can be seen that the value is going to be changed. (You must use &)
Use pointers if NULL parameter is acceptable value. (Make sure it's const if it's an incoming parameter)
Use references for incoming parameter if it cannot be NULL and is not a primitive type (const T&).
Use pointers or smart pointers when returning a newly created object.
Use pointers or smart pointers as struct or class members instead of references.
Use references for aliasing (eg. int ¤t = someArray[i])
Regardless which one you use, don't forget to document your functions and the meaning of their parameters if they are not obvious.
Disclaimer: other than the fact that references cannot be NULL nor "rebound" (meaning thay can't change the object they're the alias of), it really comes down to a matter of taste, so I'm not going to say "this is better".
That said, I disagree with your last statement in the post, in that I don't think the code loses clarity with references. In your example,
add_one(&a);
might be clearer than
add_one(a);
since you know that most likely the value of a is going to change. On the other hand though, the signature of the function
void add_one(int* const n);
is somewhat not clear either: is n going to be a single integer or an array? Sometimes you only have access to (poorly documentated) headers, and signatures like
foo(int* const a, int b);
are not easy to interpret at first sight.
Imho, references are as good as pointers when no (re)allocation nor rebinding (in the sense explained before) is needed. Moreover, if a developer only uses pointers for arrays, functions signatures are somewhat less ambiguous. Not to mention the fact that operators syntax is way more readable with references.
Like others already answered: Always use references, unless the variable being NULL/nullptr is really a valid state.
John Carmack's viewpoint on the subject is similar:
NULL pointers are the biggest problem in C/C++, at least in our code. The dual use of a single value as both a flag and an address causes an incredible number of fatal issues. C++ references should be favored over pointers whenever possible; while a reference is “really” just a pointer, it has the implicit contract of being not-NULL. Perform NULL checks when pointers are turned into references, then you can ignore the issue thereafter.
http://www.altdevblogaday.com/2011/12/24/static-code-analysis/
Edit 2012-03-13
User Bret Kuhns rightly remarks:
The C++11 standard has been finalized. I think it's time in this thread to mention that most code should do perfectly fine with a combination of references, shared_ptr, and unique_ptr.
True enough, but the question still remains, even when replacing raw pointers with smart pointers.
For example, both std::unique_ptr and std::shared_ptr can be constructed as "empty" pointers through their default constructor:
http://en.cppreference.com/w/cpp/memory/unique_ptr/unique_ptr
http://en.cppreference.com/w/cpp/memory/shared_ptr/shared_ptr
... meaning that using them without verifying they are not empty risks a crash, which is exactly what J. Carmack's discussion is all about.
And then, we have the amusing problem of "how do we pass a smart pointer as a function parameter?"
Jon's answer for the question C++ - passing references to boost::shared_ptr, and the following comments show that even then, passing a smart pointer by copy or by reference is not as clear cut as one would like (I favor myself the "by-reference" by default, but I could be wrong).
It is not a matter of taste. Here are some definitive rules.
If you want to refer to a statically declared variable within the scope in which it was declared then use a C++ reference, and it will be perfectly safe. The same applies to a statically declared smart pointer. Passing parameters by reference is an example of this usage.
If you want to refer to anything from a scope that is wider than the scope in which it is declared then you should use a reference counted smart pointer for it to be perfectly safe.
You can refer to an element of a collection with a reference for syntactic convenience, but it is not safe; the element can be deleted at anytime.
To safely hold a reference to an element of a collection you must use a reference counted smart pointer.
There is problem with "use references wherever possible" rule and it arises if you want to keep reference for further use. To illustrate this with example, imagine you have following classes.
class SimCard
{
public:
explicit SimCard(int id):
m_id(id)
{
}
int getId() const
{
return m_id;
}
private:
int m_id;
};
class RefPhone
{
public:
explicit RefPhone(const SimCard & card):
m_card(card)
{
}
int getSimId()
{
return m_card.getId();
}
private:
const SimCard & m_card;
};
At first it may seem to be a good idea to have parameter in RefPhone(const SimCard & card) constructor passed by a reference, because it prevents passing wrong/null pointers to the constructor. It somehow encourages allocation of variables on stack and taking benefits from RAII.
PtrPhone nullPhone(0); //this will not happen that easily
SimCard * cardPtr = new SimCard(666); //evil pointer
delete cardPtr; //muahaha
PtrPhone uninitPhone(cardPtr); //this will not happen that easily
But then temporaries come to destroy your happy world.
RefPhone tempPhone(SimCard(666)); //evil temporary
//function referring to destroyed object
tempPhone.getSimId(); //this can happen
So if you blindly stick to references you trade off possibility of passing invalid pointers for the possibility of storing references to destroyed objects, which has basically same effect.
edit: Note that I sticked to the rule "Use reference wherever you can, pointers wherever you must. Avoid pointers until you can't." from the most upvoted and accepted answer (other answers also suggest so). Though it should be obvious, example is not to show that references as such are bad. They can be misused however, just like pointers and they can bring their own threats to the code.
There are following differences between pointers and references.
When it comes to passing variables, pass by reference looks like pass by value, but has pointer semantics (acts like pointer).
Reference can not be directly initialized to 0 (null).
Reference (reference, not referenced object) can not be modified (equivalent to "* const" pointer).
const reference can accept temporary parameter.
Local const references prolong the lifetime of temporary objects
Taking those into account my current rules are as follows.
Use references for parameters that will be used locally within a function scope.
Use pointers when 0 (null) is acceptable parameter value or you need to store parameter for further use. If 0 (null) is acceptable I am adding "_n" suffix to parameter, use guarded pointer (like QPointer in Qt) or just document it. You can also use smart pointers. You have to be even more careful with shared pointers than with normal pointers (otherwise you can end up with by design memory leaks and responsibility mess).
Any performance difference would be so small that it wouldn't justify using the approach that's less clear.
First, one case that wasn't mentioned where references are generally superior is const references. For non-simple types, passing a const reference avoids creating a temporary and doesn't cause the confusion you're concerned about (because the value isn't modified). Here, forcing a person to pass a pointer causes the very confusion you're worried about, as seeing the address taken and passed to a function might make you think the value changed.
In any event, I basically agree with you. I don't like functions taking references to modify their value when it's not very obvious that this is what the function is doing. I too prefer to use pointers in that case.
When you need to return a value in a complex type, I tend to prefer references. For example:
bool GetFooArray(array &foo); // my preference
bool GetFooArray(array *foo); // alternative
Here, the function name makes it clear that you're getting information back in an array. So there's no confusion.
The main advantages of references are that they always contain a valid value, are cleaner than pointers, and support polymorphism without needing any extra syntax. If none of these advantages apply, there is no reason to prefer a reference over a pointer.
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A consequence of this is that in many implementations, operating on a variable with automatic or static lifetime through a reference, although syntactically similar to accessing it directly, can involve hidden dereference operations that are costly. References are a syntactically controversial feature of C++ because they obscure an identifier's level of indirection; that is, unlike C code where pointers usually stand out syntactically, in a large block of C++ code it may not be immediately obvious if the object being accessed is defined as a local or global variable or whether it is a reference (implicit pointer) to some other location, especially if the code mixes references and pointers. This aspect can make poorly written C++ code harder to read and debug (see Aliasing).
I agree 100% with this, and this is why I believe that you should only use a reference when you a have very good reason for doing so.
Points to keep in mind:
Pointers can be NULL, references cannot be NULL.
References are easier to use, const can be used for a reference when we don't want to change value and just need a reference in a function.
Pointer used with a * while references used with a &.
Use pointers when pointer arithmetic operation are required.
You can have pointers to a void type int a=5; void *p = &a; but cannot have a reference to a void type.
Pointer Vs Reference
void fun(int *a)
{
cout<<a<<'\n'; // address of a = 0x7fff79f83eac
cout<<*a<<'\n'; // value at a = 5
cout<<a+1<<'\n'; // address of a increment by 4 bytes(int) = 0x7fff79f83eb0
cout<<*(a+1)<<'\n'; // value here is by default = 0
}
void fun(int &a)
{
cout<<a<<'\n'; // reference of original a passed a = 5
}
int a=5;
fun(&a);
fun(a);
Verdict when to use what
Pointer: For array, linklist, tree implementations and pointer arithmetic.
Reference: In function parameters and return types.
The following are some guidelines.
A function uses passed data without modifying it:
If the data object is small, such as a built-in data type or a small structure, pass it by value.
If the data object is an array, use a pointer because that’s your only choice. Make the pointer a pointer to const.
If the data object is a good-sized structure, use a const pointer or a const
reference to increase program efficiency.You save the time and space needed to
copy a structure or a class design. Make the pointer or reference const.
If the data object is a class object, use a const reference.The semantics of class design often require using a reference, which is the main reason C++ added
this feature.Thus, the standard way to pass class object arguments is by reference.
A function modifies data in the calling function:
1.If the data object is a built-in data type, use a pointer. If you spot code
like fixit(&x), where x is an int, it’s pretty clear that this function intends to modify x.
2.If the data object is an array, use your only choice: a pointer.
3.If the data object is a structure, use a reference or a pointer.
4.If the data object is a class object, use a reference.
Of course, these are just guidelines, and there might be reasons for making different
choices. For example, cin uses references for basic types so that you can use cin >> n
instead of cin >> &n.
Your properly written example should look like
void add_one(int& n) { n += 1; }
void add_one(int* const n)
{
if (n)
*n += 1;
}
That's why references are preferable if possible
...
References are cleaner and easier to use, and they do a better job of hiding information.
References cannot be reassigned, however.
If you need to point first to one object and then to another, you must use a pointer. References cannot be null, so if any chance exists that the object in question might be null, you must not use a reference. You must use a pointer.
If you want to handle object manipulation on your own i.e if you want to allocate memory space for an object on the Heap rather on the Stack you must use Pointer
int *pInt = new int; // allocates *pInt on the Heap
In my practice I personally settled down with one simple rule - Use references for primitives and values that are copyable/movable and pointers for objects with long life cycle.
For Node example I would definitely use
AddChild(Node* pNode);
Just putting my dime in. I just performed a test. A sneeky one at that. I just let g++ create the assembly files of the same mini-program using pointers compared to using references.
When looking at the output they are exactly the same. Other than the symbolnaming. So looking at performance (in a simple example) there is no issue.
Now on the topic of pointers vs references. IMHO I think clearity stands above all. As soon as I read implicit behaviour my toes start to curl. I agree that it is nice implicit behaviour that a reference cannot be NULL.
Dereferencing a NULL pointer is not the problem. it will crash your application and will be easy to debug. A bigger problem is uninitialized pointers containing invalid values. This will most likely result in memory corruption causing undefined behaviour without a clear origin.
This is where I think references are much safer than pointers. And I agree with a previous statement, that the interface (which should be clearly documented, see design by contract, Bertrand Meyer) defines the result of the parameters to a function. Now taking this all into consideration my preferences go to
using references wherever/whenever possible.
For pointers, you need them to point to something, so pointers cost memory space.
For example a function that takes an integer pointer will not take the integer variable. So you will need to create a pointer for that first to pass on to the function.
As for a reference, it will not cost memory. You have an integer variable, and you can pass it as a reference variable. That's it. You don't need to create a reference variable specially for it.
I'm moving from Java to C++ and am a bit confused of the language's flexibility. One point is that there are three ways to store objects: A pointer, a reference and a scalar (storing the object itself if I understand it correctly).
I tend to use references where possible, because that is as close to Java as possible. In some cases, e.g. getters for derived attributes, this is not possible:
MyType &MyClass::getSomeAttribute() {
MyType t;
return t;
}
This does not compile, because t exists only within the scope of getSomeAttribute() and if I return a reference to it, it would point nowhere before the client can use it.
Therefore I'm left with two options:
Return a pointer
Return a scalar
Returning a pointer would look like this:
MyType *MyClass::getSomeAttribute() {
MyType *t = new MyType;
return t;
}
This'd work, but the client would have to check this pointer for NULL in order to be really sure, something that's not necessary with references. Another problem is that the caller would have to make sure that t is deallocated, I'd rather not deal with that if I can avoid it.
The alternative would be to return the object itself (scalar):
MyType MyClass::getSomeAttribute() {
MyType t;
return t;
}
That's pretty straightforward and just what I want in this case: It feels like a reference and it can't be null. If the object is out of scope in the client's code, it is deleted. Pretty handy. However, I rarely see anyone doing that, is there a reason for that? Is there some kind of performance problem if I return a scalar instead of a pointer or reference?
What is the most common/elegant approach to handle this problem?
Return by value. The compiler can optimize away the copy, so the end result is what you want. An object is created, and returned to the caller.
I think the reason why you rarely see people do this is because you're looking at the wrong C++ code. ;)
Most people coming from Java feel uncomfortable doing something like this, so they call new all over the place. And then they get memory leaks all over the place, have to check for NULL and all the other problems that can cause. :)
It might also be worth pointing out that C++ references have very little in common with Java references.
A reference in Java is much more similar to a pointer (it can be reseated, or set to NULL).
In fact the only real differences are that a pointer can point to a garbage value as well (if it is uninitialized, or it points to an object that has gone out of scope), and that you can do pointer arithmetics on a pointer into an array.
A C++ references is an alias for an object. A Java reference doesn't behave like that.
Quite simply, avoid using pointers and dynamic allocation by new wherever possible. Use values, references and automatically allocated objects instead. Of course you can't always avoid dynamic allocation, but it should be a last resort, not a first.
Returning by value can introduce performance penalties because this means the object needs to be copied. If it is a large object, like a list, that operation might be very expensive.
But modern compilers are very good about making this not happen. The C++ standards explicitly states that the compiler is allowed to elide copies in certain circumstances. The particular instance that would be relevant in the example code you gave is called the 'return value optimization'.
Personally, I return by (usually const) reference when I'm returning a member variable, and return some sort of smart pointer object of some kind (frequently ::std::auto_ptr) when I need to dynamically allocate something. Otherwise I return by value.
I also very frequently have const reference parameters, and this is very common in C++. This is a way of passing a parameter and saying "the function is not allowed to touch this". Basically a read-only parameter. It should only be used for objects that are more complex than a single integer or pointer though.
I think one big change from Java is that const is important and used very frequently. Learn to understand it and make it your friend.
I also think Neil's answer is correct in stating that avoiding dynamic allocation whenever possible is a good idea. You should not contort your design too much to make that happen, but you should definitely prefer design choices in which it doesn't have to happen.
Returning by value is a common thing practised in C++. However, when you are passing an object, you pass by reference.
Example
main()
{
equity trader;
isTraderAllowed(trader);
....
}
bool isTraderAllowed(const equity& trdobj)
{
... // Perform your function routine here.
}
The above is a simple example of passing an object by reference. In reality, you would have a method called isTraderAllowed for the class equity, but I was showing you a real use of passing by reference.
A point regarding passing by value or reference:
Considering optimizations, assuming a function is inline, if its parameter is declared as "const DataType objectName" that DataType could be anything even primitives, no object copy will be involved; and if its parameter is declared as "const DataType & objectName" or "DataType & objectName" that again DataType could be anything even primitives, no address taking or pointer will be involved. In both previous cases input arguments are used directly in assembly code.
A point regarding references:
A reference is not always a pointer, as instance when you have following code in the body of a function, the reference is not a pointer:
int adad=5;
int & reference=adad;
A point regarding returning by value:
as some people have mentioned, using good compilers with capability of optimizations, returning by value of any type will not cause an extra copy.
A point regarding return by reference:
In case of inline functions and optimizations, returning by reference will not involve address taking or pointer.
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Possible Duplicate:
What is the proper way to return an object from a C++ function ?
Hi there,
i would like to know whats the difference between the following two functions with respect to the return types?
MyClass& func1(void)
MyClass* func2(void)
I always thought this would be the same?
Heinrich
The first one is only capable of returning a reference to a single object and may not be null. Or rather, it should not be null.
The second one may be returning the pointer to a single object, or an array of objects.
In cases where you wish to return a single object that cannot be null, #1 tends to be the preferred form. In cases where a null can be returned #2 has to be used. Some APIs don't return references at all (like QT).
This is strictly a syntactic difference however. These two types are handled exactly the same in the compiled code: a pointer to the object (or array) will be used. That is, strictly speaking, the reference notation & adds no new semantic functionality over a normal pointer.
(This perhaps just summarizes what the other people wrote)
the first one returns reference to an object (or its address). The other one returns pointer, not reference
Major difference: the second one can return NULL