To disallow copying or assigning a class it's common practice to make the copy constructor
and assignment operator private. Both Google and Qt have macros to make this easy and visible.
These macros are:
Google:
#define DISALLOW_COPY_AND_ASSIGN(TypeName) \
TypeName(const TypeName&); \
void operator=(const TypeName&)
Qt:
#define Q_DISABLE_COPY(Class) \
Class(const Class &); \
Class &operator=(const Class &);
Questions:
Why are the signatures of the two assignment operators different? It seems like the Qt version is correct.
What is the practical difference between the two?
It doesn't matter. The return type is not part of a function's signature, as it does not participate in overload resolution. So when you attempt to perform an assignment, both declarations will match, regardless of whether you use the return type.
And since the entire point in these macros is that the functions will never get called, it doesn't matter that one returns void.
I'd just like to mention that there is an alternative strategy for implementing an abstraction for disallowing copy and assignment of a class. The idea is to use inheritance instead of the preprocessor. I personally prefer this approach as I follow the rule of thumb that it is best to avoid using the preprocessor when at all possible.
boost::noncopyable is an example implementation. It is used as follows:
class A : noncopyable
{
...
};
See Boost.Utility, specifically boost::noncopyable. It's not a macro but a base class with private copy and assignment. It prevents the compiler from generating implicit copy and assignment in derived classes.
edit: Sorry, this was not an answer to the original question. By the way, boost::noncopyable uses a const reference as return type for the assignment operator. I was under the impression that the type of the return value doesn't matter since it's not supposed to be used. Still, making the operator private doesn't prevent usage inside the class or friends in which case a non-usual return type (like void, a const reference, etc) might lead to compilation errors and catch additional bugs.
There's no practical difference. The assignment operator signatures differ just as a matter of style. It's usual to have an assignment operator returning a reference to allow chaining:
a = b = c;
but a version returning void is also legal and will work just fine for cases when the only purpose is to just declare the operator private and therefore prohibited to use.
From the standard, 12.8, clause 9: "A user-declared copy assignment operator X::operator= is a non-static non-template member function of class X with exactly one parameter of type X, X&, const X&, volatile X&, or const volatile X&." It says nothing about the return type, so any return type is permissible.
Clause 10 says "If the class definition does not explicitly declare a copy assignment operator, one is declared implicitly."
Therefore, declaring any X::operator=(const X&) (or any other of the specified assignment types) is sufficient. Neither the body nor the return type is significant if the operator will never be used.
Therefore, it's a stylistic difference, with one macro doing what we'd likely expect and one saving a few characters and doing the job in a way that's likely to surprise some people. I think the Qt macro is better stylistically. Since we're talking macro, we're not talking about the programmer having to type anything extra, and failing to surprise people is a good thing in a language construct.
Others have already answered why it's legal to have different return values for operator=; IMHO jalf said it best.
However, you might wonder why Google uses a different return type, and I suspect it's this:
You don't have to repeat the type name when disabling the assignment operator like this. Usually the type name is the longest part of the declaration.
Of course, this reason is void given that a macro is used but still - old habits die hard. :-)
Both serve the same purpose
Once you write this one:
Class &operator=(const Class &);
you will get the benefits of chain assignments. But in this case you want the assignment operator to be private. so it doesn't matter.
Qt version is backward compatible, while google's is not.
If you develop your library and deprecate the use of assignment before you completely remove it, in Qt it will most likely retain the signature it originally had. In this case older application will continue to run with new version of library (however, they won't compile with the newer version).
Google's macro doesn't have such a property.
As several other answers have mentioned, the return type of the function doesn't participate in the function signature, so both declarations are equivalent as far as making the assignment operator unusable by clients of the class.
Personally I prefer the idiom of having a class privately inherit from an empty non-copyable base class (like boost::noncopyable, but I have my own so I can use it in projects that don't have boost available). The empty base class optimization takes care of making sure there's zero overhead, and it's simple, readable, and doesn't rely on the dreaded preprocessor macro functionality.
It also has the advantage that copy and assignment can't even be used within class implementation code - it'll fail at compile time while these macros will fail at link time (likely with a less informative error message).
Incidentally, if you have access to the Boost libraries (You don't? Why the heck not??), The Utility library has had the noncopyable class for a long time:
class YourNonCopyableClass : boost::noncopyable {
Clearer IMHO.
In practice I would say that both should not be used anymore if you have a C++11 compiler.
You should instead use the delete feature , see here
Meaning of = delete after function declaration
and here
http://www.stroustrup.com/C++11FAQ.html#default
Why : essentially because compiler message is much more clearer. When the compiler need one of the copy or copy assignment operator, it immediately points out to the line where the =delete was coded.
Better and complete explanations can also be found in Item 11: Prefer deleted functions to private undefined ones from Effective Modern C++ book by Scott Meyers
Related
Sometimes, users implement functions with the equivalent functionality as their implicitly defined versions. For example, a copy constructor which simply calls the copy constructor of all its members.
struct A
{
int B;
A(const A& a) : B(a.B) { }
}
This is undesirable, because it causes additional maintenance, for example if the class members are renamed/reordered, etc., and reduces readability. Also, adding these functions also means that functions such as std::is_trivially_copy_constructable claim the type is cannot be trivially copy constructed (but in practice, it actually could be).
I have a code base where this seems to be a common occurrence, which I would like to rectify, by deleting these implementations. However, I am uneasy about removing functionality that seems to be identical to implicit implementation, in case it might not actually be equivalent. Is there a method for determining whether a function is equivalent to its implicit version? (Using any toolset/language variation/etc is acceptable).
My suggestion is to not try to programmatically determine if these functions are the same as the default implementation, because the difference might actually be a mistake (and they were supposed to have the normal default behavior).
Instead I would just suggest to write up a set of unit tests that take care of testing the expected behavior of the various functions, and then make sure they pass on the default implementations. Then not only do you have a test framework for future enhancements you can be confident the functions did what you wanted.
Suppose I have a class like this:
class Foo
{
public:
Foo(int something) {}
};
And I create it using this syntax:
Foo f{10};
Then later I add a new constructor:
class Foo
{
public:
Foo(int something) {}
Foo(std::initializer_list<int>) {}
};
What happens to the construction of f? My understanding is that it will no longer call the first constructor but instead now call the init list constructor. If so, this seems bad. Why are so many people recommending using the {} syntax over () for object construction when adding an initializer_list constructor later may break things silently?
I can imagine a case where I'm constructing an rvalue using {} syntax (to avoid most vexing parse) but then later someone adds an std::initializer_list constructor to that object. Now the code breaks and I can no longer construct it using an rvalue because I'd have to switch back to () syntax and that would cause most vexing parse. How would one handle this situation?
What happens to the construction of f? My understanding is that it will no longer call the first constructor but instead now call the init list constructor. If so, this seems bad. Why are so many people recommending using the {} syntax over () for object construction when adding an initializer_list constructor later may break things silently?
On one hand, it's unusual to have the initializer-list constructor and the other one both be viable. On the other hand, "universal initialization" got a bit too much hype around the C++11 standard release, and it shouldn't be used without question.
Braces work best for like aggregates and containers, so I prefer to use them when surrounding some things which will be owned/contained. On the other hand, parentheses are good for arguments which merely describe how something new will be generated.
I can imagine a case where I'm constructing an rvalue using {} syntax (to avoid most vexing parse) but then later someone adds an std::initializer_list constructor to that object. Now the code breaks and I can no longer construct it using an rvalue because I'd have to switch back to () syntax and that would cause most vexing parse. How would one handle this situation?
The MVP only happens with ambiguity between a declarator and an expression, and that only happens as long as all the constructors you're trying to call are default constructors. An empty list {} always calls the default constructor, not an initializer-list constructor with an empty list. (This means that it can be used at no risk. "Universal" value-initialization is a real thing.)
If there's any subexpression inside the braces/parens, the MVP problem is already solved.
Retrofitting classes with initializer lists in updated code is something that sounds like it will be a common thing to happen. So people start using {} syntax for existing constructors before the class is updated, and we want to automatically catch any old uses, especially those used in templates where they may be overlooked.
If I had a class like vector that took a size, then arguably using {} syntax is "wrong", but for the transition we want to catch that anyway. Constructing C c1 {val} means take some (one, in this case) values for the collection, and C c2 (arg) means use val as a descriptive piece of metadata for the class.
In order to support both uses, when the type of element happens to be compatible with the descriptive argument, code that used C c2 {arg} will change meaning. There seems to be no way around it in that case if we want to support both forms with different meanings.
So what would I do? If the compiler provides some way to issue a warning, I'd make the initializer list with one argument give a warning. That sounds tricky not to mention compiler specific, so I'd make a general template for that, if it's not already in Boost, and promote its use.
Other than containers, what other situations would have initializer list and single argument constructors with different meanings where the single argument isn't something of a very distinct type from what you'd be using with the list? For non-containers, it might suffice to notice that they won't be confused because the types are different or the list will always have multiple elements. But it's good to think about that and take additional steps if they could be confused in this manner.
For a non-container being enhanced with initializer_list features, it might be sufficient to specifically avoid designing a one-argument constructor that can be mistaken. So, the one-arg constructor would be removed in the updated class, or the initializer list would require other (possibly tag) arguments first. That is, don't do that, under penalty of pie-in-face at the code review.
Even for container-like classes, a class that's not a standard library class could impose that the one-arg constructor form is no longer available. E.g. C c3 (size); would have to be written as C c3 (size, C()); or designed to take an enumeration argument also, which is handy to specify initialized to one value vs. reserved size, so you can argue it's a feature and point out code that begins with a separate call to reserve. So again, don't do that if I can reasonably avoid it.
In "The C++ programming language", at page 265, the author makes the following statement:
Because of historical accident, the operators = (assignment), & (address-of), and , (sequencing;
§6.2.2) have predefined meanings when applied to class objects. These predefined meanings can
be made inaccessible to general users by making them private:
Then the following example is given:
class X {
private:
void operator=(const X&);
void operator&();
void operator,(const X&);
// ...
};
void f(X a, X b)
{
a = b; // error: operator= private
&a; // error: operator& private
a,b; // error: operator, private
}
I can't quite understand what do these "error" comments refer to? Does that mean I should not define a function like f, or that all of the =, &, and , operators should be used according to the default way, and it is not necessary to redefine them?
This example simply shows a way to prevent yourself or other developers of the code from using operators, which can be used without having been defined in the class, because they're automatically generated (and have default meanings for the operations they represent).
The author of the example meant, that if you try to assign b to a (in line a = b) it will cause an error, because the assignment operator is private in the class definition.
Similar error occurs in case of address-of in the second line, and the comma operator in the third.
Making default operators/constructors private if you know they're not supposed to be used (or haven't been implemented yet) is good, because one may accidentally use a very frequent operator like assignment or copy-constructor, being unaware that it's default behavior conflicts with the class lifecycle. If such operator or constructor is made private at the very beginning of class design, the compiler will generate a compile-time error instead of performing a potentially dangerous operation without notice if the programmer accidentally uses the method.
Think default assignment operator and member pointer: it will copy the pointer whereas you might want the object to be the owner of data. Then, after someone assigns one object to another without knowing that assignment is not implemented, you will end up with a double free error. Instead of that, if the operator is private, you'll get a nice error and the code will not even compile, and you'll know what's going on.
The author intends to point out here that the operators =, & and , are usually implicitly available for a class.
So if you don't want your objects to be operated on through them then you declare them as private thus disallowing their use.
Since they are declared as private you cannot access them anymore outside the class and the compiler gives you a compilation error. The function is an example showing that.
Providing your own implementation of any operator is basically the same as implementing a class method. Operators and methods are the same in terms of accessibility. What you do is disallowing access to operators from the caller's code.
It's absolutely the same as if you defined a private method and then tried to call it from some code that is not part of your class. Just make the operators public and errors will go away.
It basically prevents any one from making an 'X' object and using the "=", "&", and "," operators on that class. Because the author of the class may implement those objects with a meaning that is quite different to what the consumer of the class might think they do ... so its best to prevent them being used at all in the case of ambiguity.
The function f is an example of a user trying to use the private operators. It shows you what code it is that you're preventing by making them private. The comment // error means that a program that contained that line would fail to compile for the stated reason.
Before discussing the error, a key here is to understand that these operations will be implicitly made available for your class. This is the essence of Scott Meyers' advice "Know what functions C++ silently writes and calls."
C++ will automatically implement the assignment operator for your class, but it may not be done correctly (for example, if your class contains a pointer member variable). By defining the assignment operator explicitly, you are telling the compiler to use your implementation instead of generating one for you. And by making it private, you are essentially disallowing assignment of one class instance to another. Anywhere you try to do this in your code, the compiler will complain, which is a good thing if you really don't want assignment to be done.
In function f the author is showing you that these statements will not compile because of how the operators are defined in the class. It is perfectly acceptable to redefine operators for your class, and sometimes it is definitely required (for example, to implement a deep copy of a pointer member variable in your class). The point of the example is to show that a) you can provide your own implementation of these operators for your class, and b) because of this you have control over whether the operators are supported and implemented correctly for your class.
This might be a silly question, but...
I've been writing a number of classes that utilize non-copyable members. These classes are never initialized via the copy constructor in my source. When I try to compile without supplying my own copy-constructor, g++ throws out many errors about how it can't build a default copy constructor, due to the non-copyable member objects.
Is there a way to tell the compiler to just not give me a copy constructor?
EDIT: Yeah... feels silly... I had a case where I was invoking the copy-constructor by accident in a boost::bind call. Lesson learned.
The usual way to make things noncopyable is to declare but not define a copy constructor, and make it private so nothing can call it.
The next revision of the language will provide an explicit way to suppress these generated functions.
If you don't actually cause the copy-constructor to be called then it is not an error if the compiler would be unable to generate one. It sounds like you are (possibly indirectly) causing the copy-constructor to be used.
You can suppress the compiler generated one by declaring your own copy-constructor (you don't need to define it if you're not using it). You can place it in the private section of your class.
If this changes the error to say that the copy-constructor is inaccessible or you get link errors then you really are causing the copy-construtor to be used and you need to analyze why this is.
Not in the current version of C++. In C++ 0x, there will be an =delete; syntax to tell it that you don't want one of the special member functions the compiler will generate by default if you don't defined one yourself.
Until the new C++ 0x standard is fully supported, the best you can do is to delclare a version of the special member function, but not implement them. Normally they are made private (to help make it clear that they shouldn't be used).
Class foo
{
// ... rest of definition
private:
foo (const foo& rhs); // Do Not Implement
const foo& operator= (const foo& rhs); // Do Not Implement
};
No :)
If you want your class to be non-copyable use something like boost::noncopyable
class MyClass : private boost::noncopyable
{
}
or use a parametrizied macro in your class definition that declares a private copy constructor.
There are a few things that I almost always do when I put a class together in C++.
1) Virtual Destructor
2) Copy constructor and assignment operator (I either implement them in terms of a private function called Copy(), or declare them private and thus explicitly disallow the compiler to auto generate them).
What things do you find are almost always useful?
Oddly, most of the suggestions here are things I specifically don't do.
I don't make dtors virtual unless I am designing it specifically to be inherited. It adds a lot of overhead and prevents automatic inlining, which is bad since most dtors are empty anyways (and few classes benefit from inheritance)
I don't make copy ctor/assignment op unless the defaults won't work -- and if it won't, I may want to reconsider the design. Remember, between string & vector, there's hardly ever a reason to call new anymore. And creating your own copy ctor identical to the default one will almost certainly be less efficient.
I don't add string cast. It causes too many problems where the cast is called silently where you didn't intend it to be. Better to add a ToString() method.
I don't add a friend oper<<, because friends are evil and messy. Better to add a Display(ostream) method. Then the oper<< can call that, and doesn't need to be a friend. In fact, you could make the oper<< a template function calling Display() and never have to worry about it again.
I find turning on the gcc flags -Wall, -Werror, and (this is the fun one) -Weffc++ help catch a lot of potential problems. From the gcc man page:
-Weffc++ (C++ only)
Warn about violations of the following style guidelines from Scott
Meyers’ Effective C++ book:
· Item 11: Define a copy constructor and an assignment operator
for classes with dynamically allocated memory.
· Item 12: Prefer initialization to assignment in constructors.
· Item 14: Make destructors virtual in base classes.
· Item 15: Have "operator=" return a reference to *this.
· Item 23: Don’t try to return a reference when you must return
an object.
and about violations of the following style guidelines from Scott
Meyers’ More Effective C++ book:
· Item 6: Distinguish between prefix and postfix forms of incre-
ment and decrement operators.
· Item 7: Never overload "&&", "││", or ",".
If you use this option, you should be aware that the standard
library headers do not obey all of these guidelines; you can use
grep -v to filter out those warnings.
The first i do when putting a class together is putting some doxygen comment above it about why it exists and what it does.
I once worked on a group project where they said they want to document the stuff at the end of the project. And it was all of a mess to put the comments into the code later on. I don't want to have this happen again.
Adding a semicolon after the class definition. This continuously bites me in the ass every time I forget to do it since gcc's error messages are vague and it generally says something like "can't define a type in the return type of a function" which doesn't mean a whole lot...
Stop and think
Often,
operator string () const;
or
friend ostream& operator << (ostream&, const MyClass&);
In header files, do
#ifndef __SOMEDEFINE__
#define __SOMEDEFINE__
#endif
In VS, I add
#pragma warning(disable: 4786)
Oh, I also use
#include <inttypes.h>
cuz I <3 C99 types.
I usually include an enum of return codes, so the class can tell its callers what happened in its member functions. Most often, this will be the only type returned by all the members of the class. All results are passed back by reference.
I start by calling the development environment macro that sets up the include guards (#ifdefs and/or #pragma once).
Next I stub out the class name and any namespace it will be in, without adding any functionality at all.
Then I create the unit test file for that class, and add the first test (usually a parameter constructor test).
I just work from there, writing and refactoring the class as I think about what I really need from it. Things I tend to test specifically: const-correctness, implicit and explicit conversions, types of exceptions thrown, etc.
The first thing I do with a totally new class file is to write several paragraphs of comments on what the class does, why it exists, what classes it uses and how to use it. It should be enough that someone who randomly opens up a file in a module knows enough from that comment to find the file they are actually looking for.
I agree with James - I am very careful not to add functionality to a class that does not need it, most classes do not need a virtual destructor (or a destructor at all). If they do, I question why they don't just use smart pointers and other automatic memory management. Obviously there are many classes (i.e smart scoped locks) that DO need a destructor but it is not just a matter of course to make on.