Ref Class Assignment Operator Could Not Be Generated

In the C++programming language, a copy constructor is a special constructor for creating a new objectas a copy of an existing object. Copy constructors are the standard way of copying objects in C++, as opposed to cloning, and have C++-specific nuances.

The first argument of such a constructor is a reference to an object of the same type as is being constructed (const or non-const), which might be followed by parameters of any type (all having default values).

Normally the compiler automatically creates a copy constructor for each class (known as an implicit copy constructor) but for special cases the programmer creates the copy constructor, known as a user-defined copy constructor. In such cases, the compiler does not create one. Hence, there is always one copy constructor that is either defined by the user or by the system.

A user-defined copy constructor is generally needed when an object owns pointers or non-shareable references, such as to a file, in which case a destructor and an assignment operator should also be written (see Rule of three).

Definition[edit]

Copying of objects is achieved by the use of a copy constructor and an assignment operator. A copy constructor has as its first parameter a (possibly const or volatile) reference to its own class type. It can have more arguments, but the rest must have default values associated with them.[1] The following would be valid copy constructors for class :

X(constX&copy_from_me);X(X&copy_from_me);X(volatileX&copy_from_me);X(constvolatileX&copy_from_me);X(X&copy_from_me,int=0);X(constX&copy_from_me,double=1.0,int=42);...

The first one should be used unless there is a good reason to use one of the others. One of the differences between the first and the second is that temporaries can be copied with the first. For example:

Xa=X();// valid given X(const X& copy_from_me) but not valid given X(X& copy_from_me)// because the second wants a non-const X&// to create a, the compiler first creates a temporary by invoking the default constructor// of X, then uses the copy constructor to initialize as a copy of that temporary. // Temporary objects created while program execution are always of const type. So, const keyword is required.// For some compilers both versions actually work but this behaviour should not be relied // upon because it's non-standard.

Another difference between them is the obvious:

constXa;Xb=a;// valid given X(const X& copy_from_me) but not valid given X(X& copy_from_me)// because the second wants a non-const X&

The form of the copy constructor is used when it is necessary to modify the copied object. This is very rare but it can be seen used in the standard library's . A reference must be provided:

Xa;Xb=a;// valid if any of the copy constructors are defined// since a reference is being passed.

The following are invalid copy constructors (Reason - is not passed as reference) :

X(Xcopy_from_me);X(constXcopy_from_me);

because the call to those constructors would require a copy as well, which would result in an infinitely recursive call.

The following cases may result in a call to a copy constructor:

  1. When an object is returned by value
  2. When an object is passed (to a function) by value as an argument
  3. When an object is thrown
  4. When an object is caught
  5. When an object is placed in a brace-enclosed initializer list

These cases are collectively called copy-initialization and are equivalent to:[2]

It is however, not guaranteed that a copy constructor will be called in these cases, because the C++ Standard allows the compiler to optimize the copy away in certain cases, one example being the return value optimization (sometimes referred to as RVO).

Operation[edit]

An object can be assigned value using one of the two techniques:

  • Explicit assignment in an expression
  • Initialization

Explicit assignment in an expression[edit]

Objecta;Objectb;a=b;// translates as Object::operator=(const Object&), thus a.operator=(b) is called // (invoke simple copy, not copy constructor!)

Initialization[edit]

An object can be initialized by any one of the following ways.

a. Through declaration

Objectb=a;// translates as Object::Object(const Object&) (invoke copy constructor)

b. Through function arguments

c. Through function return value

The copy constructor is used only for initializations, and does not apply to assignments where the assignment operator is used instead.

The implicit copy constructor of a class calls base copy constructors and copies its members by means appropriate to their type. If it is a class type, the copy constructor is called. If it is a scalar type, the built-in assignment operator is used. Finally, if it is an array, each element is copied in the manner appropriate to its type.[3]

By using a user-defined copy constructor the programmer can define the behavior to be performed when an object is copied.

Examples[edit]

These examples illustrate how copy constructors work and why they are required sometimes.

Implicit copy constructor[edit]

Let us consider the following example:

#include<iostream.h>classPerson{public:intage;explicitPerson(inta):age(a){}};intmain(){Persontimmy(10);Personsally(15);Persontimmy_clone=timmy;std::cout<<timmy.age<<" "<<sally.age<<" "<<timmy_clone.age<<std::endl;timmy.age=23;std::cout<<timmy.age<<" "<<sally.age<<" "<<timmy_clone.age<<std::endl;}

Output

10 15 10 23 15 10

As expected, timmy has been copied to the new object, timmy_clone. While timmy's age was changed, timmy_clone's age remained the same. This is because they are totally different objects.

The compiler has generated a copy constructor for us, and it could be written like this:

Person(constPerson&other):age(other.age)// calls the copy constructor of the age{}

So, when do we really need a user-defined copy constructor? The next section will explore that question.

User-defined copy constructor[edit]

Now, consider a very simple dynamic array class like the following:

#include<iostream>classArray{public:intsize;int*data;explicitArray(intsz):size(sz),data(newint[size]){}~Array(){if(data!=NULL)delete[]this->data;}};intmain(){Arrayfirst(20);first.data[0]=25;{Arraycopy=first;std::cout<<first.data[0]<<" "<<copy.data[0]<<std::endl;}// (1)first.data[0]=10;// (2)}

Output

25 25 Segmentation fault

Since we did not specify a copy constructor, the compiler generated one for us. The generated constructor would look something like:

Array(constArray&other):size(other.size),data(other.data){}

The problem with this constructor is that it performs a shallow copy of the data pointer. It only copies the address of the original data member; this means they both share a pointer to the same chunk of memory, which is not what we want. When the program reaches line (1), copy's destructor gets called (because objects on the stack are destroyed automatically when their scope ends). Array's destructor deletes the data array of the original, therefore when it deleted copy's data, because they share the same pointer, it also deleted first's data. Line (2) now accesses invalid data and writes to it! This produces the infamous segmentation fault.

If we write our own copy constructor that performs a deep copy then this problem goes away.

// for std::copy#include<algorithm>Array(constArray&other):size(other.size),data(newint[other.size]){std::copy(other.data,other.data+other.size,data);}

Here, we are creating a new int array and copying the contents to it. Now, other's destructor deletes only its data, and not first's data. Line (2) will not produce a segmentation fault anymore.

Instead of doing a deep copy right away, there are some optimization strategies that can be used. These allow you to safely share the same data between several objects, thus saving space. The copy-on-write strategy makes a copy of the data only when it is written to. Reference counting keeps the count of how many objects are referencing the data, and will delete it only when this count reaches zero (e.g. boost::shared_ptr).

Copy constructors and templates[edit]

Contrary to expectations, a template copy constructor is not a user-defined copy constructor. Thus it is not enough to just have:

template<typenameA>Array::Array(Aconst&other):size(other.size()),data(newint[other.size()]){std::copy(other.begin(),other.end(),data);}

(Note that the type of can be .) A user-defined, non-template copy constructor must also be provided for construction of Array from Array.

Bitwise copy constructor[edit]

There is no such thing as "bitwise copy constructor" in C++. However, the default generated copy constructor copies by invoking copy constructors on members, and for a raw pointer member this will copy the raw pointer (i.e. not a deep copy).

Logical copy constructor[edit]

A logical copy constructor makes a true copy of the structure as well as its dynamic structures. Logical copy constructors come into the picture mainly when there are pointers or complex objects within the object being copied.

Explicit copy constructor[edit]

An explicit copy constructor is one that is declared explicit by using the explicit keyword. For example:

explicitX(constX&copy_from_me);

It is used to prevent copying of objects at function calls or with the copy-initialization syntax.

See also[edit]

References[edit]

External links[edit]

It can be seen that in a logical copy constructor, a new dynamic member variable is created for the pointer along with copying the values. [4]

What does this warning mean, and how do you fix it?

warning C4512: '<some type>' : assignment operator could not be generated

The compiler will auto-generate some class members for you

  • default constructor (if no other constructor is explicitly declared)
  • destructor
  • copy constructor (if no move constructor or move assignment operator is explicitly declared)
  • copy assignment operator (if no move constructor or move assignment operator is explicitly declared)

C++ 11 added two new auto-generated class members (and it added “if destructor then copy constructor and copy assignment operator generation is deprecated”):

  • move constructor (if no copy constructor, move assignment operator or destructor is explicitly declared)
  • move assignment operator (if no copy constructor, copy assignment operator or destructor is explicitly declared)

Compiler-generated functions are public and non-virtual. As a reminder, here are the signatures of all of these functions:

class Object { Object(); // default constructor Object(const Object& other); // copy constructor Object(Object&& other); // move constructor Object& operator=(const Object& other); // copy assignment operator Object& operator=(Object&& other); // move assignment operator ~Object(); // destructor };

So, what if you can’t actually create a meaningful copy assignment operator? For example, if you have const data, you can’t assign to it. Remember that the auto-generated copy assignment operator just generates assignment operator code for each member of the class, recursively, and you can’t assign to const int, you can only construct it.

struct ConstantOne { ConstantOne() : value(1) {} const int value; }; int main(int /*argc*/, char ** /*argv*/) { ConstantOne b; return 0; }

This will give you a warning when you compile, because the auto-generated assignment operator is technically illegal, and so the compiler won’t generate it. It’s a warning, because your code probably doesn’t need an assignment operator. For Visual C++, you’ll see something like this:

warning C4512: 'ConstantOne' : assignment operator could not be generated

You have several options. The easiest is just to declare an assignment operator without a body. As long as you never actually try to use the assignment operator, you’ll be fine. And, the contract for this object says that assignment would be illegal anyway, so you’ll get a valid-to-you compile error if you accidentally try to use it.

struct ConstantOne { ConstantOne() : value(1) {} const int value; private: ConstantOne& operator=(const ConstantOne&); }; int main(int /*argc*/, char ** /*argv*/) { ConstantOne b; ConstantOne c; c = b; return 0; }

The standard is to make these private, to reinforce that they are not meant to be used. If you compile code with an assignment operator, you’ll get a compile-time error.

error C2248: 'ConstantOne::operator =' : cannot access private member declared in class 'ConstantOne'

And in C++11, there’s even a keyword to add here to declare that it indeed should not be allowed:

struct ConstantOne { ConstantOne() : value(1) {} const int value; ConstantOne& operator=(const ConstantOne&) = delete; };

Note that you don’t need the trickery of making it private, and you get a nicer compile-time error if you try to use the assignment operator.

This happens in big real-world projects quite often. In fact, it happens enough that the delete keyword was added in C++11. Visual Studio 2013 and up, GCC 4.7 and up, and Clang 2.9 and up support the delete and default keywords.

Now, there is another approach to the assignment operator when you have const data – generate an assignment operator that can write to const data with const_cast. You’re almost always doing the wrong thing if you do this, but sometimes it has to be done anyway. It looks horrible, and it is horrible. But maybe it’s necessary.

struct ConstantOne { ConstantOne() : value(1) {} const int value; ConstantOne& operator=(const ConstantOne& that) { if (this != &that) { *const_cast(&this->value) = that.value; } return *this; } }; int main(int /*argc*/, char ** /*argv*/) { ConstantOne b; ConstantOne c; c = b; return 0; }

The reason this is horrible is that you are violating the contract – you’re altering a constant value in the LHS of the equation. Depending on your circumstance, that can still be a correct thing to do.

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