I recently read, Rule of three and am wondering if I am violating it?
In my GUI application, classes like MainFrame, Interface, Circuit, Breadboard etc. (class name are indicative) have a single instance of each of them. In their constructors, I have allocated some resources (memory), which I safely release in their destructors.
So I have defined only destructor, but not copy constructor and assignment operator.
I am sure I don't need them, but I am curious if I am violating the rule, and what can/should I do to follow it?
The rule of three is about dealing with all the Big Three, but that does not necessarily mean you'll have to define them if you don't want to. Either you provide them or you forbid them. What you shouldn't do is ignore them.
So I have defined only destructor, but not copy constructor and copy operator.
Am I violating Rule of three?
Yes, you are in violation of the rule. The compiler will generate a copy constructor and copy assignment operator, and since you allocate memory in the constructor and release in the destructor, these copies will have wrong semantics: they'll copy the pointers, and you will have two classes aliasing the same memory. The assignment won't even release the old memory, and simply overwrite the pointer.
Is this a problem?
If, like you imply, you don't make copies or assign to instances of those classes, nothing will go wrong. However, it's better to be on the safe side and declare (and don't even bother defining) the copy constructor and copy assignment operator private, so you don't invoke them accidentally.
In C++11 you can use the = delete syntax instead:
T(T const&) = delete; // no copy constructor
T& operator=(T const&) = delete; // no copy assignment
It depends a lot on you application logic and how the you have documented your interface classes to the users.
Normally, a good c++ programmer must be aware of rule of three (and a half if you know the "copy and swap idiom") and 5 and a 1/2 in case of c++11 (Move semantics).
If you class manages resource and if the same class is copyable (i.e copy ctor and assigment operator not defined as private) then its very important to do deep copying by writing your own copy ctor and assignment operator.
But If you are always toying your class by passing them as REFERENCE then better define a default copy ctor and assignment operator as private so that even if you pass by valy or copy by mistake, the compiler would warn you.
You should declare (but not implement) a private copy constructor and assignment operator. Make sure you don't implement the functions. This will prevent any kind of copying of classes not supposed to be copied.
Yes, it does violate the rule of three as per that definition.
It is, however, a "rule of thumb". A general guideline. If you don't need copy construction or assignment operations, don't implement them. Others have suggested declaring them as private and defining them as empty. I'd go one step further and say don't even define them.
If you define them, then you could potentially still invoke the empty methods. Instead, leave them undefined and, if you ever try to invoke those methods, you will receive a linker error because the method definitions could not be found. Favor build-time errors over run-time errors/undesired behavior.
If you don't need it, don't follow it. The motivation behind the rule of three is that, when you need a destructor, that is usually because you need to do some dynamic deallocations.
If you do deallocations as well, you're going to need the copy constructor and assignment operators as well. Imagine you have a class that has a pointer to something:
struct Foo
{
Foo() { ptr_ = new int; }
~Foo() { delete ptr_; }
int* ptr_;
};
Because you don't define a copy constructor and an assignment operator, whenever you make a copy of a Foo, both the original and the copy will use a pointer to the same int; when either the original or the copy gets destroyed, the pointer is freed, leaving the other with unusable data.
Foo(cont Foo& other) {
other.ptr_ = new int(*ptr_);
}
// Same for operator=
If you don't do any dynamic allocations in your constructor/ destructor, there's a good chance you don't actually need a copy constructor or an assignment operator (but not necessarily).
Related
I know about the rule of five which states that if you implement a destructor, you should most likely also implement a copy constructor, copy assignment operator, a move constructor and a move assignment operator.
However if I implement a move operator, do I absolutely have to implement a copy counterpart, or is it just best practice?
The "Rule of Zero" may be applicable to your situation. Why are you providing a move constructor or move assignment operator in the first place? Is it because your class represents unique ownership of some resource? If so, it may be better to encapsulate ownership of that resource in a unique_ptr member or something similar. Then, you don't need to write the move constructor and move assignment operator anymore. The compiler will generate them automatically, which will move the unique_ptr member and thus transfer ownership. The compiler will also ensure that the class is not copyable.
OK, but let's say that for some reason the Rule of Zero is not appropriate to your class. What will happen if you declare only a move constructor? Then, the compiler will implicitly disable copying for your class, which will mean that objects of your class type can only be initialized from rvalues of the same type, and not from lvalues. The Rule of Five tells you that it is better to be explicit about this situation instead of leaving it to the reader to guess what the compiler is doing. So, it is best practice (but not required) to define the copy constructor as = delete;.
I've read that The Rule of Three, What is The Rule of Three? is summarized as follows:
If you need to explicitly declare either the destructor, copy constructor or copy assignment
operator yourself, you probably need to explicitly declare all three of them.
My question is: In a C++ application, I have a class that manages resources (has a destructor that handles deleting pointers). I know that the application uses assignment operator all over the place, but I am absolutely certain that there is no usage in the application of a copy constructor, i.e., usage of the type Class c(..); Class d(c); so under these circumstances, am I still required to implement both an assignment operator and a copy constructor? Or will an assignment operator alone suffice? Is it possible that the assignment operator uses the copy constructor somehow?
Appreciate your thoughts.
If you know that the copy constructor won't be used, you can express that by making it private and unimplemented, thus:
class C
{
private:
C(const C&); // not implemented
};
(in C++11 you can use the new = delete syntax). That said, you should only do that if you're absolutely sure it will never be needed. Otherwise, you might be better off implementing it. It's important not to just leave it as is, as in that case the compiler will provide a default memberwise copy constructor that will do the wrong thing - it's a problem waiting to happen.
To some extent it depends on what the class is going to be used for - if you're writing a class that's part of a library, for instance, it makes much more sense to implement the copy constructor for consistency reasons. You have no idea a priori how your class is going to be used.
I am absolutely certain that there is no usage in the application of a copy constructor, i.e., usage of the type Class c(..); Class d(c)
Are you aware that the following code
Foo c;
Foo b = c;
invokes the copy constructor and NOT the assignment operator? I'd implement the copy constructor just to be safe.
In almost all cases, the compiler will generate these methods for you and you don't need to do anything. But, if implicitly generated copy constructor/assignment operator won't do what you want, and design-wise your class makes sense to be able to be copied, you should explicitly provide a copy ctor and assignment operator whether you use them both or not (as good practice).
If, design-wise, your class makes sense to be noncopyable, you can declare but not define the copy ctor/assignment op.
I've read that The Rule of Three, What is The Rule of Three? is summarized as follows:
If you need to explicitly declare either the destructor, copy constructor or copy assignment
operator yourself, you probably need to explicitly declare all three of them.
My question is: In a C++ application, I have a class that manages resources (has a destructor that handles deleting pointers). I know that the application uses assignment operator all over the place, but I am absolutely certain that there is no usage in the application of a copy constructor, i.e., usage of the type Class c(..); Class d(c); so under these circumstances, am I still required to implement both an assignment operator and a copy constructor? Or will an assignment operator alone suffice? Is it possible that the assignment operator uses the copy constructor somehow?
Appreciate your thoughts.
If you know that the copy constructor won't be used, you can express that by making it private and unimplemented, thus:
class C
{
private:
C(const C&); // not implemented
};
(in C++11 you can use the new = delete syntax). That said, you should only do that if you're absolutely sure it will never be needed. Otherwise, you might be better off implementing it. It's important not to just leave it as is, as in that case the compiler will provide a default memberwise copy constructor that will do the wrong thing - it's a problem waiting to happen.
To some extent it depends on what the class is going to be used for - if you're writing a class that's part of a library, for instance, it makes much more sense to implement the copy constructor for consistency reasons. You have no idea a priori how your class is going to be used.
I am absolutely certain that there is no usage in the application of a copy constructor, i.e., usage of the type Class c(..); Class d(c)
Are you aware that the following code
Foo c;
Foo b = c;
invokes the copy constructor and NOT the assignment operator? I'd implement the copy constructor just to be safe.
In almost all cases, the compiler will generate these methods for you and you don't need to do anything. But, if implicitly generated copy constructor/assignment operator won't do what you want, and design-wise your class makes sense to be able to be copied, you should explicitly provide a copy ctor and assignment operator whether you use them both or not (as good practice).
If, design-wise, your class makes sense to be noncopyable, you can declare but not define the copy ctor/assignment op.
Considering the high quality of today's compilers regarding return value optimization (both RVO and NRVO), I was wondering at what class complexity it's actually meaningful to start adding move constructors and move assignment operators.
For instance, for this really_trivial class, I just assume that move semantics cannot offer anything more than RVO and NRVO already does when copying around instances of it:
class really_trivial
{
int first_;
int second_;
public:
really_trivial();
...
};
While in this semi_complex class, I'd add a move constructor and move assignment operator without hesitation:
class semi_complex
{
std::vector<std::string> strings_;
public:
semi_complex(semi_complex&& other);
semi_complex& operator=(semi_complex&& other);
...
};
So, at what amount and of what kinds of member variables does it start making sense to add move constructors and move assignment operators?
It is already meaningful for purely semantic reasons, even if you keep the optimization-aspects out completely. Just imagine the case where a class does not/cannot implement copying. For example boost::scoped_ptr cannot be copied, but it can be moved!
In addition to the excellent answers already given, I would like to add a few forward-looking details:
There are new rules in the latest C++0x draft for automatically generating a move constructor and move assignment operator. Although this idea is not entirely new, the latest rules have only been in the draft since Oct. 2010, and not yet widely available in compilers.
If your class does not have a user-declared copy constructor, copy assignment operator, move constructor, move assignment operator and destructor, the compiler will provide defaulted move constructor and move assignment operator for you. These default implementations will simply member-wise move everything.
You can also explicitly default your move members:
semi_complex(semi_complex&&) = default;
semi_complex& operator=(semi_complex&&) = default;
Note that if you do so, you will implicitly inhibit copy semantics unless you explicitly supply or default the copy constructor and copy assignment operator.
In a closely related late-breaking change: If your class has an explicit destructor, and implicit copy members, the implicit generation of those members is now deprecated (the committee would like to remove the implicit generation of copy when there is an explicit destructor, but can't because of backwards compatibility).
So in summary, any time you have a destructor declared, you should probably be thinking about explicitly declaring both copy and move members.
As a rule of thumb I would say add a move constructor whenever you have member variables which hold (conditionally) dynamically allocated memory. In that case its often cheaper if you can just use the existing memory and give the move source the minimal allocation it needs so it can still function (meaning be destroyed). The amount of member variables doesn't matter that much, because for types not involving dynamic memory its unlikely to make a difference whether you copy or move them (even the move constructor will have to copy them from one memory location to another somehow).
Therefore move semantices make sense, when
dynamic memory allocation is involved
move makes sense for one or more member variables (which means those involve dynamic allocation somewhere along the line).
Typically, moving makes sense when the class holds some kind of resource, where a copy would involve copying that resource and moving would not. The easiest example is dynamically allocated memory. However, it is worth noting that the compiler will automatically generate move constructors and operators, just like it will for copying.
Irrespective of anything that might be automatically done by the compiler I'd say that:
If any member has meaningful and beneficial move semantics, that the class should have this move semantics too. (-> std::vector members)
If any dynamic allocations are involved when copying, move operations make sense.
Put otherwise, if move can do something more efficiently than copy, it makes sense to add it. In your really_trivial a move could only be as efficient as the copy already is.
This question already has answers here:
What is The Rule of Three?
(8 answers)
Closed 10 years ago.
Say I've got a class where the sole data member is something like std::string or std::vector. Do I need to provide a Copy Constructor, Destructor and Assignment Operator?
In case your class contains only vector/string objects as its data members, you don't need to implement these. The C++ STL classes (like vector, string) have their own copy ctor, overloaded assignment operator and destructor.
But in case if your class allocates memory dynamically in the constructor then a naive shallow copy will lead to trouble. In that case you'll have to implement copy ctor, overloaded assignment operator and destructor.
The usual rule of thumb says: if you need one of them, then you need them all.
Not all classes need them, though. If you class holds no resources (memory, most notably), you'll be fine without them. For example, a class with a single string or vector constituent doesn't really need them - unless you need some special copying behavior (the default will just copy over the members).
The default copy constructor will copy the vector if it is declared by value. Beware if you stored pointers in your vector, in such a case, you need to provide specific behaviour for copy/assignement/destruction to avoid memory leaks or multiple delete.
I can think of a few cases when you need to write your own Big Three. All standard containers know how to copy and destroy themselves, so you don't necessarily need to write them. Here's how to know when you do:
Does my class own any resources?
The default copy semantics for pointers is to copy the value of the pointer, not what it points to. If you need to deep copy something, even if it's stored inside a standard container, you need to write your own copy constructor and assignment operator. You also need to write your own destructor to properly free those resources.
Might someone inherit from my class?
Base classes need a destructor. Herb Sutter recommends making them either public and virtual (most common case) or protected and non-virtual, depending on what you want to do with them. The compiler-generated destructor is public and non-virtual, so you'll have to write your own, even if it doesn't have any code in it. (Note: this doesn't imply you have to write a copy constructor or assignment operator.)
Should I prevent a user from copying objects of my class?
If you don't want the user to copy your objects (maybe that's too expensive), you need to declare the copy constructor and assignment operators either protected or private. You don't have to implement them unless you need them. (Note: this doesn't imply you have to write a destructor.)
Bottom line:
The most important thing is to understand what the compiler-generated copy constructor, assignment operator, and destructor will do. You don't need to be afraid of them, but you need to think about them and decide if their behavior is appropriate for your class.
No but there are a number of reasons why you shouldn't allow the compiler to auto generate these functions.
In my experience it is always best to define them yourself, and to get into the habit of making sure that they are maintained when you change the class. Firstly you may well want to put a breakpoint on when a particular ctor or dtor is called. Also not defining them can result in code bloat as the compiler will generate inline calls to member ctor and dtor (Scott Meyers has a section on this).
Also you sometimes want to disallow the default copy ctors and assignments. For example I have an application that stores and manipulates very large blocks of data. We routinely have the equivalent of an STL vector holding millions of 3D points and it would be a disaster if we allowed those containers to be copy constructed. So the ctor and assignment operators are declared private and not defined. That way if anyone writes
class myClass {
void doSomething(const bigDataContainer data); // not should be passed by reference
}
then they'll get a compiler error. Our experience is that an explicit become() or clone() method is far less error prone.
So all in all there are many reason to avoid auto generated compiler functions.
those container will need a "copy constructible" element, and if you don't supply the copy constructor, it will call default copy constructor of your class by deducing from your class members (shallow copy).
easy explanation about default copy constructor is here : http://www.fredosaurus.com/notes-cpp/oop-condestructors/copyconstructors.html
it is so with destructor, the container need to have access to your destructor or your default class destructor if you don't provides one (ie. it will not work if you declare your destructor as private )
you need to provide them if you need them. or possible users of your classes. destructor is always a must, and copy constructors and assignment operator are automatically created by compiler. (MSVC at least)
When ever you have a class that requires deep copies, you should define them.
Specifically, any class which contains pointers or references should contain them such as:
class foo {
private:
int a,b;
bar *c;
}
Subjectively, I would say always define them, as the default behavior provided by the compiler generated version may not be what you expect / want.
Not for strings or vectors, since the trivial constructors / destructors etc, will do fine.
If your class has pointers to other data and need deep copies, or if your class holds a resource that has to be deallocated or has to be copied in a special way.