I've sat here for 10 minutes trying to think of the title. If anyone can, feel free to change it.
Anyways, I have two classes campus and building as well as two arrays in campus itself. However those arrays are what I have to use to set the value of ID in building anyways.
In campus, I have loadFile and searchList
loadFile reads from a file, and sets values to three member variables in building and the two arrays mentioned earlier.
However I need to use searchList to print out those values. (It's an assignment. If I had it my way I'd probably do it differently cause I'm frankly bad at this).
Here's where it all goes downhill. I'm confused with how to proceed. The first thing I tried to do was call the constructor for building in loadFile which is part of campus using building::building(ID, name, description) which is supposed to set those values in the constructor itself.
Then in searchList, I thought I could go cout << building::getID() << building::getName() << building::getDescription(); (all of these are member functions in building) but obviously through a series of errors I realized this isn't the way to go.
So I tried instead of using the scope resolution operator, I should make a variable of type building somewhere. So I made building a inside loadFile and set a = building(ID, name, description) after defining those values.
Of course, this way I can't access a.getID in searchList. So I tried making it a global variable but obviously it wouldn't be that simple.
If you need more context, here's the assignment details.
Here's my class header
int const maxPerBuildingType = 7;
class campus
{
friend class building;
//friend building::building(int, std::string, std::string);
private:
//int academic[maxPerBuildingType]; removed
//int recreation[maxPerBuildingType]; removed
building recreation[maxPerBuildingType];
building academic[maxPerBuildingType];
int numberOfAcademic;
int numberOfRecreation;
std::string temp;
protected:
public:
campus();
static int const maxPerBuildingType;
void loadFile();
void searchList();
};
I don't know if you need the definitions since it's wrong anyways but it's here (updated for edit) if you need it. I'll edit it into this post if necessary.
Anyways guys, thanks.
EDIT: Of course, the answer was because I was stupid. Arrays had to be of type building so i could allocate the required number of buildings. I haven't completed the assignment but for now I believe everything's fine. That'll probably change but eh. Thank you guys!
AT the moment you are not storing your buildings anywhere. You put them into a local variable called a then throw them away.
I suspect the intention of the exercise is to store the buildings into the arrays academic and recreation. You currently have them as int arrays, but possibly they should be arrays of the specific building classes (I assume the exercise also wants you to have different building classes for the types of buildings, probably as a subclass of Building).
Once your campus has its own lists of buildings, at lot of your questions will answer themselves naturally.
Related
this is a VERY simplified version of the question to make it obvious what i am asking. I cannot seem to find it on Stack Overflow but i am probably searching using the wrong words!
Here is a Template Class with the obvious parts removed.
template <class T, bool clip = true>
class BOUNDED_VAL {
public:
BOUNDED_VAL(T initialMin, T intialMax) :locked(false) {
assert_or_throw(intialMax >= initialMin, HD_ERR_MINMAX_REVERSED);
min = initialMin;
max = intialMax;
value = initialMin;
};etc.
// assert_or_throw is a typedef which asserts during debug builds to prevent programmer mistakes (especially my own) or throws a rich catachable runtime error for user input just in case something gets around user input limit checking during release builds (A hard wall). Belt and braces approach...
Now i know i can set this up as an initialised private class member variable like so:
private:
BOUNDED_VAL<int> testBoundInt = BOUNDED_VAL<int>(0, 10);
BUT
Does this create a new BOUNDED_VAL and then copy it over the member variable (or compiler smooshes this away during optimisation)?
Is there a more succinct way of doing it which i am just not finding? I know the following do not work but for example:
private:
BOUNDED_VAL<int> testBoundInt(0,10);
or
private:
BOUNDED_VAL<int>(0,10) testBoundInt;
I am self taught in C++ so it might be an obvious question...but you never know...
Many Thanks
I know the following do not work but for example:
private:
BOUNDED_VAL<int> testBoundInt(0,10);
But this will work, provided your compiler was written in the last decade:
BOUNDED_VAL<int> testBoundInt{0,10};
It looks like you might be using an outdated textbook to learn C++ that does not cover uniform initialization syntax from the current version of C++. You are strongly encouraged to get updated learning material.
Does this [copy initialization] create a new BOUNDED_VAL and then copy it over the member variable (or compiler smooshes this away during optimisation)?
This actually depends on the C++ version your compiler supports, and is configured to use. Depending on several factors it gets "smooshed" away, or we take a scenic trip to the countryside, where an object gets constructed, then a second object gets copy-constructed, and the first object deleted.
But this is now a secondary topic, since uniform initialization syntax solves the original problem.
I understand the reason why we should avoid using getters/setters, but I don't know how to avoid using them.
For example, I have three classes as follows,
A (private: point_B)
B (private: point_C)
C (private: val_C)
A has a private member point_B which is a pointer that points to B, and B also has a private member point_C which is a pointer that points to C. And C has a private int value val_C.
How can I access val_C in A?
Update:
In this case,
A is a class called state, which has the address point_B.
B is a class called node, which has a pointer call pointer_C.
C is a class called base_file, which has two derived classes called file and directory.
Update 2:
Ty guys for you help. Some of you are really trying to help instead of acting like someone who knows everything. I appreciate it.
Sry I can't post the whole assignment here since its too large even without documents. I'll post professor's answer here if you guys are interested tomorrow.
Update 3:
Please find reference here
The right thing to do is to leave the implementation to specify class.
Update 4:
The answer is to not to access private value in each class, but to implement functions to use them. That explains why making them private at the first place.
Maybe a little clarification is in order -- getters and setters aren't meant to be avoided at all costs; they have their place. The reason people say they should be avoided is because one goal of good object-oriented program design is encapsulation -- that is to say, each class should keep the details of its own implementation as private as possible, so that users of that class don't need to know (or care) about how the class was implemented. This becomes increasingly important as the program gets larger and more complicated, because a human programmer can only keep so many details in his/her head at once, and if the programmer has to remember everything about how class C works while simultaneously writing/debugging class A, that's an additional/unecessary cognitive burden that at some point will cause the programmer's brain to explode.
So, getting back to the main question -- how to avoid getters and setters -- the way to do it is to define your classes' interfaces at a higher level of abstraction than as simple repositories for state variables. (After all, if you wanted a simple collection of state variables, there's no reason to use a C++ class at all, you could simply declare a C-style struct instead)
For example, if your class C was intended to represent, say, a slot machine, a poor interface to class C might include lots of getters and setters, like this:
int getNumCoins() const {return numCoins;}
void setNumCoins(int newCoinCount) {numCounts = newCoinCount;}
void setDisplayedResult(const string & displayStr) {result = displayStr;}
int getDisplayedResult() const {return result;}
... and the poor programmer who was forced to use class C would have to write code like this:
playersWallet--; // take a coin out of the player's wallet
c.setNumCoins(c.getNumCoins()+1); // insert the coin into the machine
string newResult = "10 J A"; // somehow figure out what the machine should display
c.setDisplayedResult(newResult); // and make the machine display it
if (c.getDisplayedResult() == "7 7 7")
{
cout << "YOU ARE WINNER!" << endl;
int numCoinsWon = 5000; // jackpot!
c.setNumCoins(c.getNumCoins()-numCoinsWon); // deduct from machine's balance
playersWallet += numCoinsWon; // add to player's balance
}
[... and so on...]
Note that in the above code, the programmer had to think about all of the internal mechanisms of the slot machine, and write his own code to handle each step of its operation. With good encapsulation, on the other hand, the slot machine's public interface would be much simpler and more opaque, like this:
// returns the number of coins the player won on this round
int pullTheBigLever();
... and the programmer who was using this API might write code like this:
playersWallet += (c.pullTheBigLever() - 1); // -1 for the coin the player put in
Note that there is only one line of code, and that the programmer didn't have to think at all about how the internals of the slot machine worked. This avoids exploding-programmer-brain-syndrome, and just as importantly it means you (or someone else) can go back later and change the private implementation of how the slot machine works without breaking the code that interacts with the slot machine.
So when are getters and setters acceptable? Answer: when there really isn't any higher level of abstraction to be had. If you are writing a class that represents a light switch, then just being able to examine the switch's current position, or specify a new position for it, may be all the functionality you need. But in many (most?) cases you are implementing the functionality of something more complex than that, and the more of that complexity you can hide behind your public interface, the happier users of that class (including you) will be.
Short answers, in OOP, classes should have "properties" as part of their public API. Properties can have have things like getters, setters and change notifications, as appropriate. Wether a getter directly returns a private member variable, that is an implementation detail, and could change as needed. Distinguish the concept of property from the concept of member variable.
When thinking about it like this, the direct answer to your question is, that there's nothing you should try to "avoid", other than having unnecessary readable properties.
Note that often there is no explicit syntax or support for properties in an object oriented language (popular counter-example: C#), so it's easy to think they are same thing as a member variable with a setter and a getter. But the overlap is sort of a coincident, and not something you should care about when using a class. In a way, there is no getter for a member variable, there is only a getter for the property, even if it happens to map 1:1 with a member variable.
How avoid using getters/setters in C++.
To avoid setter/getter, all code that accesses a data attribute of class C, must be part of a class C method.
Alternate wording: bring the code that uses the data attribute inside the class.
update 2016/01/25
Would an example help? I find it trivial to avoid getters and setters (and public data and friends, etc.) I suppose I'm just used to it.
I recently completed yet another implementation of the game-of-life. The whole game is the entertainment value of watching the cells change patterns. Impressively complex behaviour from a small set of rules.
My class Cell_t has ONLY private data, No getters, no setters, and no friends. No other class has access to any cells data.
Here is a snippet of that part of my game illustrating how easy it is to live without getters, setters and friends creating the undesirable coupling and cohesion:
// somewhere in GameOfLife exists
std::vector<Cell_t> m_ptCellVec; // a vector of cell ptrs
GameOfLife::exec(...)
{
// ... preliminary stuff
do {
// ... some preliminary stuff
// NOTE 1
for ( auto it : m_ptCellVec ) it->countNeighbor();
// NOTE 2
for ( auto it : m_ptCellVec ) { it->updateDisplay();}
// .... more stuff
if(timeElapsed > timeLimit) break;
if(m_generation > genLimit) break;
}while(1);
}
NOTE 1 -- The class GameOfLife does not count neigbors ... each cell does its own counting. The next state is computed from these counts.
NOTE 2 -- The class GameOfLife does not update the display ... each cell updates it's own little piece of the screen.
THUS, there is no getter of Cell_t state, or next state, or living-neighbour count, or dead-neighbour count, etc.
With respect to this aspect of these two classes
The cohesion (of Cell_t) is functional, the most desirable.
The coupling (of GameOfLife_t to Cell_t) is 'none', also the most
desirable.
Changing the name or type of a Cell_t private data attribute has no
impact on any other code.
Oh, and a debug routine I often add (for another example):
std::string Cell_t dump() {
std::stringstream ss;
ss << // .... anything you want to 'dump' from this instance
return (ss.str());
}
I use the method name dump() to indicate an intent for a 'deeper' investigation of the activity of a specific Cell_t ... I have sometimes generated tabular data of state changes, with time stamps.
I often have a very similar method called show(), which typically provides a string for the user ...
These two examples, perhaps, illustrate the idea that a getter is simply bypassing an important aspect of the design process - naming what you are doing.
I believe the question stated in Problem could be modified. The question should not be "How can I avoid getters and setters?". This question is also related to other questions like "Should this method be a non-static member, static member, friend or helper?" or "Should this property be private or protected?". A better question to ask yourself is rather, "Who needs to access a particular property".
One way of writing classes which are easy to maintain is to limit the number of functions which have access to a specific property. This does not necessarily mean that no function should ever have access to a private property or that getters/setters should never be used. Take for example the class std::vector. Which can be simplified to something like this (with a lot of reservartions). The actual implementation of vector is normally much more sophisticated and may have different internal implementation but this simplified construction will be used to show a point.
template<class T, class Allocator<T> a = basic_allocator<T>>
class vector {
size_t sz;
size_t cap;
Allocator a;
T* elem;
// ... private methods
public:
// public methods and operators.
}
This class lets the developer access all elements in the internal array, where data is stored. This is done either via the operator [] (unchecked) or via the function at (checked). The developer have full rights to read or write to these elements. Without this access the vector class would be fairly useless and people would revert to use arrays instead. The class also provides getters to sz and cap via methods size() and capacity(). However sz and cap is otherwise seen as internal information and the developer is not allowed to change these directly. Instead the developer can use methods like push_back(), pop_back(), shrink_to_fit(), resize(), ... To add or remove data, manage allocated memory, etc ... The reason is that these operations requires some quite advanced memory handling and modifying these variables would cause leaks and/or crashes. Further, the developer does really not need to bother about these abstractions, since the developer only need the elements in the array.
So to conclude encapsulation is good and need to be considered. However this does not mean that the developer is never allowed to directly modify properties of some classes.
I am a bit new to C++ having come from a very moderate C background so please excuse me if this question seems very elementary.
I have currently been given some C++ source code to read and modify.
However the code seems to me to be very ugly for a newbie but I am not sure whether the code is considered good C++ practice.
Basically there is only one class called STORAGE and all the information is public.
class STORAGE
{
public:
STORAGE();
virtual ~STORAGE();
//DATA
int np,nn;
int istep;
int print_step;
//...and many more variables.
//METHODS
void eos(double rho, double e, double &p, double &cs);
void ThermalEnergy(double rho,double &e,double p);
void allocation();
void initialization();
void var_dt();
// and many more methods which return void,
};
Now when I am reading the algorithm which calls these methods, I see each of them modifying many member variables of STORAGE, with many methods modifying the same set of variables, in a long list of method calls. Many of the methods are quite irritatingly of the type void A ()
With such a style , it seems to be very hard to keep track mentally of the changes to the large number member-variables.
My question: Is this style of programming common to C++ when using classes? Giving a method access to all members of the class seems a bit dangerous, and it seems that a lot of buggy code could arise.
Psychologically for me it looks much more simpler to write code, if I know that the only variables being modified in a function call are the input variables to a function.
"..all the information is public"
Yes, it it bad practice and contradicts the basic notion of encapsulation. Everybody outside the class, would be able see and modify all the members. Ideal is: to make the data members private, and provide public get/set methods (depending on need).
"Is this style of programming common to C++ when using classes?" -- Common but not good.
"Giving a method access to all members of the class seems a bit dangerous"
I think this is common, member functions should have access to member variables. Otherwise, who else will :) ? However, if you still want to prevent them to modify member variables, you can use the declare the function as const. (This approach is already described by Connor above).
This question already has answers here:
Why use getters and setters/accessors?
(37 answers)
Closed 9 years ago.
I've been told not to make my variables public inside a class. I should always make a get and a set function. For example :
class Whatever
{
public:
void setSentence(const std::string &str) { sentence = str; }
void setAnInteger(const int integer) { anInteger = integer; }
std::string getSentence() { return sentence; }
int getAnInteger() { return anInteger; }
private:
std::string sentence;
int anInteger;
};
Why should I do that? Isn't just simply using those variables more convenient? Also, is that a good c++ programming style?
The main reason is to increase encapsulation. If your class exposes those member variables, many functions in your client code will have a dependency towards those variables.
Suppose one day you want want to change the name of those variables, or you want to change the implementation of your class so that the type and number of member variables would be different than the current one: how many functions would be affected by this change? How many functions would you have to re-write (at least in part)?
Right, potentially infinite. You just can't count them all. On the other hand, if you have getters and setters, only those 4 functions will have access to the internal representation of your class. Changing the internal representation won't require any change to the code of your client functions; only those 4 member functions may have to be changed.
In general, encapsulation makes your life easier with respect to future changes. At a certain point in time you may want to log a message every time a certain property is set. You may want to fire an event every time a certain property is set. You may want to compute a certain value on the fly rather than reading it each time from a cache data member, or read it from a database, or whatever.
Having getters and setters allow you to implement any of those changes without requiring to change the client code.
As far as general design philosophy is concerned, there is no "always" or "never" when it comes to implementing accessors versus not implementing accessors that the community as a whole agrees on.
Many will advise you to make all data members private and provide accessors & mutators. Always.
Others will tell you to make data members private if changing them from client code is undesirable, and leave them public otherwise.
Yet others will tell you that classes shouldn't have more than one or so data member at all, and all the data should be encapsulated in yet another object, preferably a struct.
You have to decide for yourself which is right, keeping in mind that this will depend not only on your approach, but also that of the organization for which you work.
If you ask me, my preference is to make everything public until I have a reason not to. Simple. But that's just me.
You write explicit getters and setters as a sane plan for future development. If your class' users are directly accessing its members and you need to change the class in a way that is incompatible with that habit, you have to change every chunk of code that interfaces with you in this way. If you write a getter and setter, the compiler will optimize it to be time-equivalent to direct access (if that is all it does) and you can later change the logic if you need to - without having to change a ton of other code.
When you make get or set method and use it 40 times in your code, you can handle future changes more easily.
Imagine, that you use public variable and use it 40 times in your code. After a month of developing your program, you'll come up with a great idea: What if I divide this variable by 1000 and so I would have better values to calculate with!
Wow, great, but now I have to find every single line, where I use it and change it. If I only had a get method :(
That's the main reason of getters and setters, even if they are very simple, it's better to have it. You will thank yourself once.
Data encapsulation is one of the major principles of OOP. It is the process of combining data and functions into a single unit called class. Using the method of encapsulation, the programmer cannot directly access the data. Data is only accessible through the functions existing inside the class so that the implementation details of a class that are hidden from the user. It's to protect both the caller and the function from accidentally changing the behavior of a method, or from needing to know how a method works.
The textbook-ish answer recalled from me taking the first OOP class was: Get and set methods are used to wrap around private variables. Usually people compare between having get and set or just simply set those variables to be public; in this case, get and set approach is good because it protects those variables from being modified accidentally due to bugs and etc..
People (me when I took that class) might ask "isn't get and set also modify those variables, if so, how is that different than being modified as a public variable".
The rationale is: to have get and set function, you are asking the user or yourself to explicitly specify they want to modify the variable by calling those functions. Without calling those functions, the private variables will be less likely (still possible depends on implementation) modified unwillingly or accidentally.
In short, you should not do that.
In general, I suggest to read Fowler's Refactoring, then you will have a picture what gets hindered by having naked data, and what kind of access aligns well. And importantly whether the whole thing applies to your cases or not.
And as you know pros&cons you can safely ignore "should do/don't" stuff like at start of this answer or others.
I've read a lot of guides that explain why I should use "private" and the answer is always "Because we don't want anyone else setting this as something". So, let me ask the following questions:
Assuming that I want to have a variable that is set-once (perhaps something like a character name in a video game, ask once, then it's set, and then you just use the get variable(edit:function) for the rest of the game) how do I handle that one set? How would I handle the get for this as well?
What is the actual advantage of using a private access modifier in this case? If I never prompt the user to enter the name again, and never store information back to class.name shouldn't the data remain safe (moderately, assuming code works as intended) anyways?
I hope someone will help me out with this as the explanations I've googled and seen on here have not quite put my thoughts to rest.
Thanks!
The access specifiers mainly serve to denote the class interface, not to effectively limit the programmer's access or protect things. They serve to prevent accidental hacking.
If something is set once, then you should try to set it when it is created, and make it const.
If the interface doesn't need to be especially clear (for example, if few people need to learn it) then it doesn't make sense to spend effort engineering it. Moreover changes that don't make much difference in how the interface is used can be applied later. The exposed variable can be changed to a getter/setter using simple search-and-replace.
If it were a published binary interface, then you would want to get it right the first time. But you're just talking about the internals of your program.
And it's fairly unlikely that anyone will reset the player name by accident.
I won't try to justify the private set method as that sounds a bit weird to me. You could handle the one set by using a friend declaration. But then why would you define a setter when the friend could just set the value directly?
I generally avoid setters if I can at all manage it. Instead I prefer provide facility to set member variables via the constructor. I am quite happy to provide getters if they make sense.
class player_character_t {
std::string name_;
public:
player_character_t(std::string const& name)
: name_ (name)
{
}
std::string const& name() const { return name_; }
};
This forces you to delay construction of the object until you have all the information you require. It simplifies the logic of your objects (ie they have a trivial state diagram) and means you never have to check is something is set before reading it (if the object exists, it is set properly).
http://en.wikipedia.org/wiki/State_diagram
Marking things as private helps prevent accidents. So when you make a mistake and it is no longer the case that the "code works as intended" the compiler may help you detect it. Likewise const can be a big help in detecting when you are using objects incorrectly.
It's that last parenthetical that is important: assuming code works as intended.
In my mind it's similar to permissions in Linux systems. You know the root password and you can delete any file, but you don't stay logged in as root so you don't do anything by accident. Similarly, when you have a private variable characterNameString, and someone (or you) later tries to give it a new value, it will fail. That person will have to go look at the code and see that it's marked private. That person will have to ask themselves "why is this private? Should I be modifying it? Should I be doing this another way?" If they decide they want to, then, they can. But it prevents silly mistakes.
Don't confuse the private and the public interfaces of the class. In theory these are completely different interfaces, and this is just a design feature of C++ that they're located physically in the same class declaration.
It's perfectly ok to have a public getter/setter when the object property should be exposed via the public interface, so there is no rule such as 'setter is always private'.
More on that topic in the (More) Exceptional C++ books by Herb Sutter. It's an absolutely neccessary reading for someone who wants to understand C++ and be proficient with it.
If you have doubts over deciding whether to use getter/setters over the class variables, there are numerous explanations on the internet why getters/setters are better.
If the variable is 'write once then forever read only' I'd recommend making it a const member that is initialized during construction. There's no value in a private 'setter' function because it won't be used. Also you avoid people using the setter function to set the name when it's never meant to be set.
For example:
class Player
{
private:
const std::string m_name;
public:
Player(const std::string& name) : m_name(name) {}
};
Private getters and setters all make sense when the data in question involves several variables, have additional constraints you want to make sure you adhere to, and these operations are done several times in your class. Or when you plan further modifications to the data model and wish to abstract operations on the data, like using std::vector but planning to make it std::map or similar cases.
For a personal example, I have a smart pointer implementation with a private reset(T*, int*) method that is essentially a setter for the stored object and its reference count. It handles checking validity of objects and reference counts, incrementing and decrementing reference counts, and deleting objects and reference counts. It is called eight times in the class, so it made perfect sense to put it into a method instead of just screwing around with member variables each time, slowing programming, bloating code and risking errors in the process.
I am sure private getters can also make sense if you are abstracting the data from the model and/or you have to implement error checking, for example throwing instructions if the data is NULL instead of returning NULL.