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I am doing a large project for the first time. I have lots of classes and some of them have public variables, some have private variables with setter and getter methods and same have both types.
I decided to rewrite this code to use primarily only one type. But I don't know which I should use (variables which are used only for methods in the same object are always private and are not subject of this question).
I know the theory what public and private means, but what is used in the real world and why?
private data members are generally considered good because they provide encapsulation.
Providing getters and setters for them breaks that encapsulation, but it's still better than public data members because there's only once access point to that data.
You'll notice this during debugging. If it's private, you know you can only modify the variable inside the class. If it's public, you'll have to search the whole code-base for where it might be modified.
As much as possible, ban getters/setters and make properties private. This follows the principle of information hiding - you shouldn't care about what properties a class has. It should be self-contained. Of course, in practice this isn't feasible, and if it is, a design that follows this will be more cluttered and harder to maintain than one that doesn't.
This is of course a rule of thumb - for example, I'd just use a struct (equivalent with a class with public access) for, say, a simple point class:
struct Point2D
{
double x;
double y;
};
Since you say that you know the theory, and other answers have dug into the meaning of public/private, getters and setters, I'd like to focus myself on the why of using accessors instead of creating public attributes (member data in C++).
Imagine that you have a class Truck in a logistic project:
class Truck {
public:
double capacity;
// lots of more things...
};
Provided you are northamerican, you'll probably use gallons in order to represent the capacity of your trucks. Imagine that your project is finished, it works perfectly, though many direct uses of Truck::capacity are done. Actually, your project becomes a success, so some european firm asks you to adapt your project to them; unfortunately, the project should use the metric system now, so litres instead of gallons should be employed for capacity.
Now, this could be a mess. Of course, one possibility would be to prepare a codebase only for North America, and a codebase only for Europe. But this means that bug fixes should be applied in two different code sources, and that is decided to be unfeasible.
The solution is to create a configuration possibility in your project. The user should be able to set gallons or litres, instead of that being a fixed, hardwired choice of gallons.
With the approach seen above, this will mean a lot of work, you will have to track down all uses of Truck::capacity, and decide what to do with them. This will probably mean to modify files along the whole codebase. Let's suppose, as an alternative, that you decided a more theoretic approach.
class Truck {
public:
double getCapacity() const
{ return capacity; }
// lots of more things...
private:
double capacity;
};
A possible, alternative change involves no modification to the interface of the class:
class Truck {
public:
double getCapacity() const
{ if ( Configuration::Measure == Gallons ) {
return capacity;
} else {
return ( capacity * 3.78 );
}
}
// lots of more things...
private:
double capacity;
};
(Please take int account that there are lots of ways for doing this, that one is only one possibility, and this is only an example)
You'll have to create the global utility class configuration (but you had to do it anyway), and add an include in truck.h for configuration.h, but these are all local changes, the remaining of your codebase stays unchanged, thus avoiding potential bugs.
Finally, you also state that you are working now in a big project, which I think it is the kind of field in which these reasons actually make more sense. Remember that the objective to keep in mind while working in large projects is to create maintainable code, i.e., code that you can correct and extend with new functionalities. You can forget about getters and setters in personal, small projects, though I'd try to make myself used to them.
Hope this helps.
There is no hard rule as to what should be private/public or protected.
It depends on the role of your class and what it offers.
All the methods and members that constitute the internal workings of
the class should be made private.
Everything that a class offers to the outside world should be public.
Members and methods that may have to be extended in a specialization of this class,
could be declared as protected.
From an OOP point of view getters/setters help with encapsulation and should therefore always be used. When you call a getter/setter the class can do whatever it wants behind the scenes and the internals of the class are not exposed to the outside.
On the other hand, from a C++ point of view, it can also be a disadvantage if the class does lots of unexpected things when you just want to get/set a value. People like to know if some access results in huge overhead or is simple and efficient. When you access a public variable you know exactly what you get, when you use a getter/setter you have no idea.
Especially if you only do a small project, spending your time writing getters/setters and adjusting them all accordingly when you decide to change your variable name/type/... produces lots of busywork for little gain. You'd better spend that time writing code that does something useful.
C++ code commonly doesn't use getters/setters when they don't provide real gain. If you design a 1,000,000-line project with lots of modules that have to be as independent as possible it might make sense, but for most normal-sized code you write day to day they are overkill.
There are some data types whose sole purpose is to hold well-specified data. These can typically be written as structs with public data members. Aside from that, a class should define an abstraction. Public variables or trivial setters and getters suggest that the design hasn't been thought through sufficiently, resulting in an agglomeration of weak abstractions that don't abstract much of anything. Instead of thinking about data, think about behavior: this class should do X, Y, and Z. From there, decide what internal data is needed to support the desired behavior. That's not easy at first, but keep reminding yourself that it's behavior that matters, not data.
Private member variables are preferred over public member variables, mainly for the reasons stated above (encapsulation, well-specified data, etc..). They also provide some data protection as well, since it guarantees that no outside entity can alter the member variable without going through the proper channel of a setter if need be.
Another benefit of getters and setters is that if you are using an IDE (like Eclipse or Netbeans), you can use the IDE's functionality to search for every place in the codebase where the function is called. They provide visibility as to where a piece of data in that particular class is being used or modified. Also, you can easily make the access to the member variables thread safe by having an internal mutex. The getter/setter functions would grab this mutex before accessing or modifying the variable.
I'm a proponent of abstraction to the point where it is still useful. Abstraction for the sake of abstraction usually results in a cluttered mess that is more complicated than its worth.
I've worked with complex rpgies and many games and i started to follow this rule of thumb.
Everything is public until a modification from outside can break something inside, then it should be encapsulated.(corner count in a triangle class for example)
I know info hiding principles etc but really don't follow that.
Public variables are usually discouraged, and the better form is to make all variables private and access them with getters and setters:
private int var;
public int getVar() {
return var;
}
public void setVar(int _var) {
var = _var;
}
Modern IDEs like Eclipse and others help you doing this by providing features like "Implement Getters and Setters" and "Encapsulate Field" (which replaces all direct acccesses of variables with the corresponding getter and setter calls).
Okay, just about everywhere I read, I read that getters/setters are "evil".
Now, as a programmer who uses getters/setters often in PHP / C#, I do not see how they are alive. I have read that they break encapsulation, etc etc, however, here is a simple example.
class Armor{
int armorValue;
public:
Armor();
Armor(int); //int here represents armor value
int GetArmorValue();
void SetArmorValue(int);
};
Now, lets say getters and setters are "evil".
How are you supposed to change a member variable after initialization.
Example:
Armor arm=Armor(128); //armor with 128 armor value
//for some reason I would like to change this armor value
arm.SetArmorValue(55); //if i do not use getters / setters how is this possible?
Lets say the above is not okay, for whatever reason.
What if my game restricts armor values from 1 to 500. (No armor can have a piece that has more than 500 armor or less than 1 armor).
Now my implementation becomes
void Armor::SetArmor(int tArmValue){
if (tArmValue>=1 && tArmValue<=500)
armorValue=tArmValue;
else
armorValue=1;
}
So, how else would I impose this restriction without using getters/setters?
How else would I modify a property without using getters/setters?
Should armorValue just be a public member variable in case 1, and the getters/setters used in case 2?
Curious. THanks guys
You have misunderstood something. Not using getters/setters breaks encapsulation and exposes implementation details, and can be considered "evil" for some definition of evil.
I guess they can be considered evil in the sense, that without proper IDE/editor support, they are somewhat tediois to write in C++...
One pitfall of C++ is to create non-const reference getter, which allows also modification. That's same as returning a pointer to internal data, and will lock that part of internal implementation, and is really no better than making field public.
Edit: based on comments and other answers, what you heard probably refers to always creating non-private getter and setter for every field. But I would not call that evil either, just stupid ;-)
Being slightly contrarian: yes, getters and setters (aka accessors and mutators) are mostly evil.
The evil here is not, IMO, so much from "breaking encapsulation", as from simply defining a variable to be of one type (e.g., int) when it's really not that type at all. Looking at your example, you're calling Armor an int, but it's really not. While it's undoubtedly an integer, it's certainly not an int, which (among other things) defines a range. While your type is an integer, it's never intended to support the same range as an int at all. If you want Armor to be of a type integer from 1 to 500, define a type to represent that directly, and define Armor as an instance of that type. In this case, since the invariant you want to enforce is defined as part of the type itself, you don't need to tack a setter onto it to try to enforce it.
template <class T, class less=std::less<T> >
class bounded {
const T lower_, upper_;
T val_;
bool check(T const &value) {
return less()(value, lower_) || less()(upper_, value);
}
void assign(T const &value) {
if (check(value))
throw std::domain_error("Out of Range");
val_ = value;
}
public:
bounded(T const &lower, T const &upper)
: lower_(lower), upper_(upper) {}
bounded(bounded const &init)
: lower_(init.lower), upper_(init.upper), val_(init.val_)
{ }
bounded &operator=(T const &v) { assign(v); return *this; }
operator T() const { return val_; }
friend std::istream &operator>>(std::istream &is, bounded &b) {
T temp;
is >> temp;
if (b.check(temp))
is.setstate(std::ios::failbit);
else
b.val_ = temp;
return is;
}
};
With this in place, defining some armor with a range of 1..500 becomes utterly trivial:
bounded<int> armor(1, 500);
Depending on the situation, you might prefer to define (for example) a saturating type where attempting to assign an out of range value is fine, but the value that actually is assigned will simply be the nearest value that is within range.
saturating<int> armor(1, 500);
armor = 1000;
std::cout << armor; // prints "500"
Of course, what I've given above is also a bit bare-bones. For your armor type, it would probably be convenient to support -= (and possibly +=) so an attack would end up something like x.armor -= 10;.
Bottom line: the (or at least "one") major problem with getters and setters is that they usually point to your having defined a variable as being of one type when you really want some other type that happened to be sort of similar in a few ways.
Now, it's true that some languages (e.g., Java) fail to provide the programmer with the tools necessary to write code like that. Here I'm trusting your use of the C++ tag to indicate that you really do want to write C++ though. C++ does provide you with the necessary tools, and (at least IMO) your code will be better off for your making good use of the tools it provides so your type enforces the required semantic constraints while still using clean, natural, readable syntax.
In short: they aren't evil.
It's nothing wrong with them as long as they don't leak out the internal representation. I see no problems here.
A common criticism of get/set functions is that they can be abused by client code to perform operations that logically should be encapsulated in the class. For example, say a client wants to "polish" their armour, and decides the effect is to increase "value" by 20, so they do their little get and set thing and are happy. Then someone other client code elsewhere decides rusty armour should drop the value by 30, and they do their bit. Meanwhile, a dozen other places in client code are also allowing polishing and rusting effects on armour - as well as say "reinforcing" and "cracking", and implementing them directly. There's no central control of this... the maintainer of the armour class has no ability to do things like:
have the rust, polish, reinforce and crack effects apply at most once per piece of armour
tune the number added to or subtract from value for specific logical effects
decide that the new "leather" armour type can't rust, and ignore client attempts to make it do so
On the other hand, if the first client that wanted to make armour rusty couldn't do so through the interface, they'd go to the maintainer of the armour class and say "hey, give me a function to do this", then other people could start using the logical-level "rust" operation, and if it became useful later to do the kinds of things I describe above they could be implemented easily and centrally in the armour class (e.g. by having a separate boolean to say if the armour was rusty, or a separate variable recording the rust effect).
So, the thing with get/set functions is they frustrate the natural evolution of an API of logical functionality, instead distributing logic throughout client code, leading in extremis to an unmaintainable mess.
Your getter/setter looks ok.
The alternative to getter/setters is to make member variables public. To be more precise, group variables into structure without member functions. And operate on this structure within your class
Giving access to members reduces encapsulation, but sometimes it's necessary. And the best way to do it is by means of getters and setters. Some people implement them when no such access is necessary, just because they can and it's a habit.
Getters are evil whenever:
They access directly data members of the class
When you have to add new getter every time you add data to the class
The data behaviour is different in each getter
Good getters would thus do the following:
They forward the request to some other object or collect the data from several places
You can fetch large amounts of data using just one getter
All the data you fetch is handled the same way
Setters on the other hand are evil always.
how else would I impose this restriction without using getters/setters? How else would I modify a property without using getters/setters?
You can check what you read from the variable and if its value is out of range use a predefined value instead (if possible).
You can also resort to dirty hacks such as protecting the memory underneath the variable from writing, catching write attempts and disallowing/ignoring the ones with invalid values. This is going to be cumbersome to implement and expensive to execute. It may be useful for debugging, though.
I have about 15~20 member variables which needs to be accessed, I was wondering
if it would be good just to let them be public instead of giving every one of them
get/set functions.
The code would be something like
class A { // a singleton class
public:
static A* get();
B x, y, z;
// ... a lot of other object that should only have one copy
// and doesn't change often
private:
A();
virtual ~A();
static A* a;
};
I have also thought about putting the variables into an array, but I don't
know the best way to do a lookup table, would it be better to put them in an array?
EDIT:
Is there a better way than Singleton class to put them in a collection
The C++ world isn't quite as hung up on "everything must be hidden behind accessors/mutators/whatever-they-decide-to-call-them-todays" as some OO-supporting languages.
With that said, it's a bit hard to say what the best approach is, given your limited description.
If your class is simply a 'bag of data' for some other process, than using a struct instead of a class (the only difference is that all members default to public) can be appropriate.
If the class actually does something, however, you might find it more appropriate to group your get/set routines together by function/aspect or interface.
As I mentioned, it's a bit hard to tell without more information.
EDIT: Singleton classes are not smelly code in and of themselves, but you do need to be a bit careful with them. If a singleton is taking care of preference data or something similar, it only makes sense to make individual accessors for each data element.
If, on the other hand, you're storing generic input data in a singleton, it might be time to rethink the design.
You could place them in a POD structure and provide access to an object of that type :
struct VariablesHolder
{
int a;
float b;
char c[20];
};
class A
{
public:
A() : vh()
{
}
VariablesHolder& Access()
{
return vh;
}
const VariablesHolder& Get() const
{
return vh;
}
private:
VariablesHolder vh;
};
No that wouldn't be good. Image you want to change the way they are accessed in the future. For example remove one member variable and let the get/set functions compute its value.
It really depends on why you want to give access to them, how likely they are to change, how much code uses them, how problematic having to rewrite or recompile that code is, how fast access needs to be, whether you need/want virtual access, what's more convenient and intuitive in the using code etc.. Wanting to give access to so many things may be a sign of poor design, or it may be 100% appropriate. Using get/set functions has much more potential benefit for volatile (unstable / possibly subject to frequent tweaks) low-level code that could be used by a large number of client apps.
Given your edit, an array makes sense if your client is likely to want to access the values in a loop, or a numeric index is inherently meaningful. For example, if they're chronologically ordered data samples, an index sounds good. Summarily, arrays make it easier to provide algorithms to work with any or all of the indices - you have to consider whether that's useful to your clients; if not, try to avoid it as it may make it easier to mistakenly access the wrong values, particularly if say two people branch some code, add an extra value at the end, then try to merge their changes. Sometimes it makes sense to provide arrays and named access, or an enum with meaningful names for indices.
This is a horrible design choice, as it allows any component to modify any of these variables. Furthermore, since access to these variables is done directly, you have no way to impose any invariant on the values, and if suddenly you decide to multithread your program, you won't have a single set of functions that need to be mutex-protected, but rather you will have to go off and find every single use of every single data member and individually lock those usages. In general, one should:
Not use singletons or global variables; they introduce subtle, implicit dependencies between components that allow seemingly independent components to interfere with each other.
Make variables const wherever possible and provide setters only where absolutely required.
Never make variables public (unless you are creating a POD struct, and even then, it is best to create POD structs only as an internal implementation detail and not expose them in the API).
Also, you mentioned that you need to use an array. You can use vector<B> or vector<B*> to create a dynamically-sized array of objects of type B or type B*. Rather than using A::getA() to access your singleton instance; it would be better to have functions that need type A to take a parameter of type const A&. This will make the dependency explicit, and it will also limit which functions can modify the members of that class (pass A* or A& to functions that need to mutate it).
As a convention, if you want a data structure to hold several public fields (plain old data), I would suggest using a struct (and use in tandem with other classes -- builder, flyweight, memento, and other design patterns).
Classes generally mean that you're defining an encapsulated data type, so the OOP rule is to hide data members.
In terms of efficiency, modern compilers optimize away calls to accessors/mutators, so the impact on performance would be non-existent.
In terms of extensibility, methods are definitely a win because derived classes would be able to override these (if virtual). Another benefit is that logic to check/observe/notify data can be added if data is accessed via member functions.
Public members in a base class is generally a difficult to keep track of.
I personally find it weird/ugly when a class uses a getter to access its own member data. I know the performance impact is none but I just don't like to see all those method calls.
Are there any strong arguments either way, or is it just one of those things that's personal preference and should be left to each coder, or arbitrarily controlled in a coding standard?
Update: I'm meaning simple getters, specifically for a class' non-public members.
The reason you might want to use a getter/setter is because it conceals the implementation. You won't have to rewrite all of your code if you are using getters/setters in case the implementation does change, because those members can continue to work.
EDIT based on the many clever comments:
As for a class using setters and getters on itself, that may depend on the particulars. After all, the implementation of a particular class is available to the class itself. In the cases where a class is normally instantiated, the class should use the member values directly for its own members (private or otherwise) and its parent classes (if they are protected) and only use getters/setters in the case that those members are private to the parent class.
In the case of an abstract type, which will usually not contain any implementation at all, it should provide pure virtual getters and setters and use only those in the methods it does implement.
Willingness to use getters/setters within class member implementation is the canary in the mine telling that your class is growing unreasonably. It tells that your class is trying to do too many different things, that it serves several purposes where it should serve one instead.
In fact, this is usually encountered when you are using one part of your class to store or access your data, and another part to make operations on it. Maybe you should consider using a standalone class to store and give access to your data, and another one to provide a higher view, with more complex operations with your data.
THE OBVIOUS
getters and setters for protected members makes as much sense as for public... derived classes are just another form of client code, and encapsulating implementation details from them can still be useful. I'm not saying always do it, just to weight pros and cons along the normal lines.
getters and setters for private members is rarely a net benefit, though:
it does provide the same kind of encapsulation benefits
single place for breakpoints/logging of get/set + invariant checks during dev (if used consistently)
virtual potential
etc...
but only to the presumably relatively small implementation of the same struct/class. In enterprise environments, and for public/protected member data, those benefits can be substantial enough to justify get/set methods: a logging function may end up having millions of lines of code depedent on it, and hundreds or thousands of libraries and apps for which a change to a header may trigger recompilation. Generally a single class implementation shouldn't be more than a few hundred (or at worst thousand) lines - not big or complex enough to justify encapsulating internal private data like this... it could be said to constitute a "code smell".
THE NOT-SO OBVIOUS
get/set methods can very occasionally be more readable than direct variable access (though more often less readable)
get/set methods may be able to provide a more uniform and convenient interface for code-generated member or friend methods (whether from macros or external tools/scripts)
less work required to transition between being a member or friend to a freestanding helper function should that become possible
implementation may be rendered more understandable (and hence maintainable) to people who're normally only users of the class (as more operations are expressed via, or in the style of, the public interface)
It's a bit out of scope for the question, but it's worth noting that classes should generally provide action-oriented commands, event-triggered callbacks etc. rather than encouraging a get/set usage pattern.
It seems most people didn't read your question properly, the question is concerning whether or not class methods accessing its own class' members should use getters and setters; not about an external entity accessing the class' members.
I wouldn't bother using getter and setter for accessing a class' own members.
However, I also keep my classes small (typically about 200-500 lines), such that if I do need to change the fields or change its implementations or how they are calculated, search and replace wouldn't be too much work (indeed, I often change variable/class/function names in the early development period, I'm picky name chooser).
I only use getter and setters for accessing my own class members when I am expecting to change the implementation in the near future (e.g. if I'm writing a suboptimal code that can be written quickly, but plans to optimize it in the future) that might involve radically changing the data structure used. Conversely, I don't use getter and setter before I already have the plan; in particular, I don't use getter and setter in expectation of changing things I'm very likely never going to change anyway.
For external interface though, I strictly adhere to the public interface; all variables are private, and I avoid friend except for operator overloads; I use protected members conservatively and they are considered a public interface. However, even for public interface, I usually still avoid having direct getters and setters methods, as they are often indicative of bad OO design (every OO programmers in any language should read: Why getter and setter methods are Evil). Instead, I have methods that does something useful, instead of just fetching the values. For example:
class Rectangle {
private:
int x, y, width, height;
public:
// avoid getX, setX, getY, setY, getWidth, setWidth, getHeight, setHeight
void move(int new_x, int new_y);
void resize(int new_width, int new_height);
int area();
}
The only advantage is that it allows changing internal representation without changing external interface, permitting lazy evaluation, or why not access counting.
In my experience, the number of times I did this is very, very low. And it seems you do, I also prefer to avoid the uglyness and weightyness of getter/setters. It is not that difficult to change it afterwards if I really need it.
As you speak about a class using its own getter/setters in its own implementation functions, then you should consider writing non-friend non-member functions where possible. They improve encapsulation as explained here.
An argument in favor of using getters is that you might decide one day to change how the member field is calculated. You may decide that you need it to be qualified with some other member, for instance. If you used a getter, all you have to do is change that one getter function. If you didn't you have to change each and every place where that field is used currently and in the future.
Just a crude example. Does this help?
struct myclass{
int buf[10];
int getAt(int i){
if(i >= 0 && i < sizeof(buf)){
return buf[i];
}
}
void g(){
int index = 0;
// some logic
// Is it worth repeating the check here (what getAt does) to ensure
// index is within limits
int val = buf[index];
}
};
int main(){}
EDIT:
I would say that it depends. In case the getters do some kind of validation, it is better to go through the validation even if it means the class members being subjected to that validation. Another case where going through a common entry point could be helpful is when the access needs to be essentially in a sequential and synchronized manner e.g. in a multithreaded scenario.
Protecting a member variable by wrapping its access with get/set functions has its advantages. One day you may wish to make your class thread-safe - and in that instance, you'll thank yourself for using those get/set functions
this is actually for supporting the object oriented-ness of the class by abstracting the way to get(getter). and just providing its easier access.
Simple answer. If you are writing a one shoot program, that will never change, you can leave the getters at peace and do without any.
However if you write a program that could change or been written over time, or others might use that code, use getters.
If you use getters it helps change the code faster later on, like putting a guard on the property to verify correctness of value, or counting access to the property(debugging).
Getters to me are about easy possibilities(free lunch). The programmer who write the code does not need getters, he wants them.
hope that help.
My thoughts are as follows.
Everything should be static, constant, and private if possible.
As you need a variable to be instanced meaning more than one unique
copy you remove static.
As you need a variable to be modifiable you remove the const.
As you need a class/variable to be accessed by other classes you remove
the private.
The Usage of Setters/Getters - General Purpose.
Getter's are okay if the value is to ONLY be changed by the class and
we want to protect it. This way we can retrieve the current state of
this value without the chance of it's value getting changed.
Getter's should not be used if you are planning to provide a Setter
with it. At this point you should simply convert the value to public
and just modify it directly. Since this is the intent with a Get/Set.
A Setter is plain useless if you are planning to do more then simply
"this.value = value". Then you shouldn't be calling it "SetValue"
rather describe what it is actually doing.
If let's say you want to make modifications to a value before you
"GET" it's value. Then DO NOT call it "GetValue". This is ambiguous
to your intent and although YOU might know what's happening. Someone
else wouldn't unless they viewed the source code of that function.
If let's say you are indeed only Getting/Setting a value, but you are
doing some form of security. I.e. Size check, Null Check, etc.. this
is an alternative scenario. However you should still clarify that in
the name E.g. "SafeSetValue" , "SafeGetValue" or like in the "printf"
there is "printf_s".
Alternatives to the Get/Set situations
An example that I personally have. Which you can see how I handle a
Get/Set scenario. Is I have a GameTime class which stores all kinds
of values and every game tick these values get changed.
https://github.com/JeremyDX/DX_B/blob/master/DX_B/GameTime.cpp
As you will see in the above my "GETS" are not actually "GETS" of
values except in small cases where modification wasn't needed. Rather
they are descriptions of values I am trying to retrieve out of this
GameTime class. Every value is "Static Private". I cannot do Const
given the information is obtained until runtime and I keep this
static as there is no purpose to have multiple instances of Timing.
As you will also see I don't have any way of performing a "SET" on any of this data, but there are two functions "Begin()" and "Tick()" which both change the values. This is how ALL "setters" should be handled. Basically the "Begin()" function resets all the data and loads in our constants which we CANT set as constants since this is data we retrieve at runtime. Then TICK() updates specific values as time passes in this case so we have fresh up to date information.
If you look far into the code you'll find the values "ResetWindowFrameTime()" and "ElapsedFrameTicks()". Typically I wouldn't do something like this and would have just set the value to public. Since as you'll see I'm retrieving the value and setting the value. This is another form of Set/Get, but it still uses naming that fits the scenario and it uses data from private variables so it didn't make sense to pull another private variable and then multiply it by this rather do the work here and pull the result. There is also NO need to edit the value other then to reset it to the current frame index and then retrieve the elapsed frames. It is used when I open a new window onto my screen so I can know how long I've been viewing this window for and proceed accordingly.
I have a simple container class with a copy constructor.
Do you recommend using getters and setters, or accessing the member variables directly?
public Container
{
public:
Container() {}
Container(const Container& cont) //option 1
{
SetMyString(cont.GetMyString());
}
//OR
Container(const Container& cont) //option 2
{
m_str1 = cont.m_str1;
}
public string GetMyString() { return m_str1;}
public void SetMyString(string str) { m_str1 = str;}
private:
string m_str1;
}
In the example, all code is inline, but in our real code there is no inline code.
Update (29 Sept 09):
Some of these answers are well written however they seem to get missing the point of this question:
this is simple contrived example to discuss using getters/setters vs variables
initializer lists or private validator functions are not really part of this question. I'm wondering if either design will make the code easier to maintain and expand.
Some ppl are focusing on the string in this example however it is just an example, imagine it is a different object instead.
I'm not concerned about performance. we're not programming on the PDP-11
EDIT: Answering the edited question :)
this is simple contrived example to
discuss using getters/setters vs
variables
If you have a simple collection of variables, that don't need any kind of validation, nor additional processing then you might consider using a POD instead. From Stroustrup's FAQ:
A well-designed class presents a clean
and simple interface to its users,
hiding its representation and saving
its users from having to know about
that representation. If the
representation shouldn't be hidden -
say, because users should be able to
change any data member any way they
like - you can think of that class as
"just a plain old data structure"
In short, this is not JAVA. you shouldn't write plain getters/setters because they are as bad as exposing the variables them selves.
initializer lists or private validator functions are not really
part of this question. I'm wondering
if either design will make the code
easier to maintain and expand.
If you are copying another object's variables, then the source object should be in a valid state. How did the ill formed source object got constructed in the first place?! Shouldn't constructors do the job of validation? aren't the modifying member functions responsible of maintaining the class invariant by validating input? Why would you validate a "valid" object in a copy constructor?
I'm not concerned about performance. we're not programming on the PDP-11
This is about the most elegant style, though in C++ the most elegant code has the best performance characteristics usually.
You should use an initializer list. In your code, m_str1 is default constructed then assigned a new value. Your code could be something like this:
class Container
{
public:
Container() {}
Container(const Container& cont) : m_str1(cont.m_str1)
{ }
string GetMyString() { return m_str1;}
void SetMyString(string str) { m_str1 = str;}
private:
string m_str1;
};
#cbrulak You shouldn't IMO validate cont.m_str1 in the copy constructor. What I do, is to validate things in constructors. Validation in copy constructor means you you are copying an ill formed object in the first place, for example:
Container(const string& str) : m_str1(str)
{
if(!valid(m_str1)) // valid() is a function to check your input
{
// throw an exception!
}
}
You should use an initializer list, and then the question becomes meaningless, as in:
Container(const Container& rhs)
: m_str1(rhs.m_str1)
{}
There's a great section in Matthew Wilson's Imperfect C++ that explains all about Member Initializer Lists, and about how you can use them in combination with const and/or references to make your code safer.
Edit: an example showing validation and const:
class Container
{
public:
Container(const string& str)
: m_str1(validate_string(str))
{}
private:
static const string& validate_string(const string& str)
{
if(str.empty())
{
throw runtime_error("invalid argument");
}
return str;
}
private:
const string m_str1;
};
As it's written right now (with no qualification of the input or output) your getter and setter (accessor and mutator, if you prefer) are accomplishing absolutely nothing, so you might as well just make the string public and be done with it.
If the real code really does qualify the string, then chances are pretty good that what you're dealing with isn't properly a string at all -- instead, it's just something that looks a lot like a string. What you're really doing in this case is abusing the type system, sort of exposing a string, when the real type is only something a bit like a string. You're then providing the setter to try to enforce whatever restrictions the real type has compared to a real string.
When you look at it from that direction, the answer becomes fairly obvious: rather than a string, with a setter to make the string act like some other (more restricted) type, what you should be doing instead is defining an actual class for the type you really want. Having defined that class correctly, you make an instance of it public. If (as seems to be the case here) it's reasonable to assign it a value that starts out as a string, then that class should contain an assignment operator that takes a string as an argument. If (as also seems to be the case here) it's reasonable to convert that type to a string under some circumstances, it can also include cast operator that produces a string as the result.
This gives a real improvement over using a setter and getter in a surrounding class. First and foremost, when you put those in a surrounding class, it's easy for code inside that class to bypass the getter/setter, losing enforcement of whatever the setter was supposed to enforce. Second, it maintains a normal-looking notation. Using a getter and a setter forces you to write code that's just plain ugly and hard to read.
One of the major strengths of a string class in C++ is using operator overloading so you can replace something like:
strcpy(strcat(filename, ".ext"));
with:
filename += ".ext";
to improve readability. But look what happens if that string is part of a class that forces us to go through a getter and setter:
some_object.setfilename(some_object.getfilename()+".ext");
If anything, the C code is actually more readable than this mess. On the other hand, consider what happens if we do the job right, with a public object of a class that defines an operator string and operator=:
some_object.filename += ".ext";
Nice, simple and readable, just like it should be. Better still, if we need to enforce something about the string, we can inspect only that small class, we really only have to look one or two specific, well-known places (operator=, possibly a ctor or two for that class) to know that it's always enforced -- a totally different story from when we're using a setter to try to do the job.
Do you anticipate how the string is returned, eg. white space trimmed, null checked, etc.? Same with SetMyString(), if the answer is yes, you are better off with access methods since you don't have to change your code in zillion places but just modify those getter and setter methods.
Ask yourself what the costs and benefits are.
Cost: higher runtime overhead. Calling virtual functions in ctors is a bad idea, but setters and getters are unlikely to be virtual.
Benefits: if the setter/getter does something complicated, you're not repeating code; if it does something unintuitive, you're not forgetting to do that.
The cost/benefit ratio will differ for different classes. Once you're ascertained that ratio, use your judgment. For immutable classes, of course, you don't have setters, and you don't need getters (as const members and references can be public as no one can change/reseat them).
There's no silver bullet as how to write the copy constructor.
If your class only has members which provide a copy constructor that creates
instances which do not share state (or at least do not appear to do so) using an initializer list is a good way.
Otherwise you'll have to actually think.
struct alpha {
beta* m_beta;
alpha() : m_beta(new beta()) {}
~alpha() { delete m_beta; }
alpha(const alpha& a) {
// need to copy? or do you have a shared state? copy on write?
m_beta = new beta(*a.m_beta);
// wrong
m_beta = a.m_beta;
}
Note that you can get around the potential segfault by using smart_ptr - but you can have a lot of fun debugging the resulting bugs.
Of course it can get even funnier.
Members which are created on demand.
new beta(a.beta) is wrong in case you somehow introduce polymorphism.
... a screw the otherwise - please always think when writing a copy constructor.
Why do you need getters and setters at all?
Simple :) - They preserve invariants - i.e. guarantees your class makes, such as "MyString always has an even number of characters".
If implemented as intended, your object is always in a valid state - so a memberwise copy can very well copy the members directly without fear of breaking any guarantee. There is no advantage of passing already validated state through another round of state validation.
As AraK said, the best would be using an initializer list.
Not so simple (1):
Another reason to use getters/setters is not relying on implementation details. That's a strange idea for a copy CTor, when changing such implementation details you almost always need to adjust CDA anyway.
Not so simple (2):
To prove me wrong, you can construct invariants that are dependent on the instance itself, or another external factor. One (very contrieved) example: "if the number of instances is even, the string length is even, otherwise it's odd." In that case, the copy CTor would have to throw, or adjust the string. In such a case it might help to use setters/getters - but that's not the general cas. You shouldn't derive general rules from oddities.
I prefer using an interface for outer classes to access the data, in case you want to change the way it's retrieved. However, when you're within the scope of the class and want to replicate the internal state of the copied value, I'd go with data members directly.
Not to mention that you'll probably save a few function calls if the getter are not inlined.
If your getters are (inline and) not virtual, there's no pluses nor minuses in using them wrt direct member access -- it just looks goofy to me in terms of style, but, no big deal either way.
If your getters are virtual, then there is overhead... but nevertheless that's exactly when you DO want to call them, just in case they're overridden in a subclass!-)
There is a simple test that works for many design questions, this one included: add side-effects and see what breaks.
Suppose setter not only assigns a value, but also writes audit record, logs a message or raises an event. Do you want this happen for every property when copying object? Probably not - so calling setters in constructor is logically wrong (even if setters are in fact just assignments).
Although I agree with other posters that there are many entry-level C++ "no-no's" in your sample, putting that to the side and answering your question directly:
In practice, I tend to make many but not all of my member fields* public to start with, and then move them to get/set when needed.
Now, I will be the first to say that this is not necessarily a recommended practice, and many practitioners will abhor this and say that every field should have setters/getters.
Maybe. But I find that in practice this isn't always necessary. Granted, it causes pain later when I change a field from public to a getter, and sometimes when I know what usage a class will have, I give it set/get and make the field protected or private from the start.
YMMV
RF
you call fields "variables" - I encourage you to use that term only for local variables within a function/method