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Quick Q: Most concise way to disable copy and move semantics

By Adrien Hamelin | Feb 20, 2018 08:40 AM | Tags: c++11 advanced

Quick A: Delete the move assignment.

Recently on SO:

Most concise way to disable copy and move semantics According to this chart (by Howard Hinnant): The most concise way is to =delete move assignment operator (or move constructor, but it can cause problems mentioned in comments). Though, in my opinion the most readable way is to =delete both copy constructor and copy assignment operator.

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14.14 — Introduction to the copy constructor

Consider the following program:

You might be surprised to find that this program compiles just fine, and produces the result:

Let’s take a closer look at how this program works.

The initialization of variable f is just a standard brace initialization that calls the Fraction(int, int) constructor.

But what about the next line? The initialization of variable fCopy is also clearly an initialization, and you know that constructor functions are used to initialize classes. So what constructor is this line calling?

The answer is: the copy constructor.

The copy constructor

A copy constructor is a constructor that is used to initialize an object with an existing object of the same type. After the copy constructor executes, the newly created object should be a copy of the object passed in as the initializer.

An implicit copy constructor

If you do not provide a copy constructor for your classes, C++ will create a public implicit copy constructor for you. In the above example, the statement Fraction fCopy { f }; is invoking the implicit copy constructor to initialize fCopy with f .

By default, the implicit copy constructor will do memberwise initialization. This means each member will be initialized using the corresponding member of the class passed in as the initializer. In the example above, fCopy.m_numerator is initialized using f.m_numerator (which has value 5 ), and fCopy.m_denominator is initialized using f.m_denominator (which has value 3 ).

After the copy constructor has executed, the members of f and fCopy have the same values, so fCopy is a copy of f . Thus calling print() on either has the same result.

Defining your own copy constructor

We can also explicitly define our own copy constructor. In this lesson, we’ll make our copy constructor print a message, so we can show you that it is indeed executing when copies are made.

The copy constructor looks just like you’d expect it to:

When this program is run, you get:

The copy constructor we defined above is functionally equivalent to the one we’d get by default, except we’ve added an output statement to prove the copy constructor is actually being called. This copy constructor is invoked when fCopy is initialized with f .

Access controls work on a per-class basis (not a per-object basis). This means the member functions of a class can access the private members of any class object of the same type (not just the implicit object).

We use that to our advantage in the Fraction copy constructor above in order to directly access the private members of the fraction parameter. Otherwise, we would have no way to access those members directly (without adding access functions, which we might not want to do).

A copy constructor should not do anything other than copy an object. This is because the compiler may optimize the copy constructor out in certain cases. If you are relying on the copy constructor for some behavior other than just copying, that behavior may or may not occur. We discuss this further in lesson 14.15 -- Class initialization and copy elision .

Best practice

Copy constructors should have no side effects beyond copying.

Prefer the implicit copy constructor

Unlike the implicit default constructor, which does nothing (and thus is rarely what we want), the memberwise initialization performed by the implicit copy constructor is usually exactly what we want. Therefore, in most cases, using the implicit copy constructor is perfectly fine.

Prefer the implicit copy constructor, unless you have a specific reason to create your own.

We’ll see cases where the copy constructor needs to be overwritten when we discuss dynamic memory allocation ( 21.13 -- Shallow vs. deep copying ).

The copy constructor’s parameter must be a reference

It is a requirement that the parameter of a copy constructor be an lvalue reference or const lvalue reference. Because the copy constructor should not be modifying the parameter, using a const lvalue reference is preferred.

If you write your own copy constructor, the parameter should be a const lvalue reference.

Pass by value (and return by value) and the copy constructor

When an object is passed by value, the argument is copied into the parameter. When the argument and parameter are the same class type, the copy is made by implicitly invoking the copy constructor. Similarly, when an object is returned back to the caller by value, the copy constructor is implicitly invoked to make the copy.

We see both happen in the following example:

In the above example, the call to printFraction(f) is passing f by value. The copy constructor is invoked to copy f from main into the f parameter of function printFraction .

When generateFraction returns a Fraction back to main , the copy constructor is implicitly called again. And when f2 is passed to printFraction , the copy constructor is called a third time.

On the author’s machine, this example prints:

If you compile and execute the above example, you may find that only two calls to the copy constructor occur. This is a compiler optimization known as copy elision . We discuss copy elision further in lesson 14.15 -- Class initialization and copy elision .

Using = default to generate a default copy constructor

If a class has no copy constructor, the compiler will implicitly generate one for us. If we prefer, we can explicitly request the compiler create a default copy constructor for us using the = default syntax:

Using = delete to prevent copies

Occasionally we run into cases where we do not want objects of a certain class to be copyable. We can prevent this by marking the copy constructor function as deleted, using the = delete syntax:

In the example, when the compiler goes to find a constructor to initialize fCopy with f , it will see that the copy constructor has been deleted. This will cause it to emit a compile error.

As an aside…

You can also prevent the public from making copies of class object by making the copy constructor private (as private functions can’t be called by the public). However, a private copy constructor can still be called from other members of the class, so this solution is not advised unless that is desired.

For advanced readers

The rule of three is a well known C++ principle that states that if a class requires a user-defined copy constructor, destructor, or copy assignment operator, then it probably requires all three. In C++11, this was expanded to the rule of five , which adds the move constructor and move assignment operator to the list.

Not following the rule of three/rule of five is likely to lead to malfunctioning code. We’ll revisit the rule of three and rule of five when we cover dynamic memory allocation.

We discuss destructors in lesson 15.4 -- Introduction to destructors and 19.3 -- Destructors , and copy assignment in lesson 21.12 -- Overloading the assignment operator .

Question #1

In the lesson above, we noted that the parameter for a copy constructor must be a (const) reference. Why aren’t we allowed to use pass by value?

Show Solution

When we pass a class type argument by value, the copy constructor is implicitly invoked to copy the argument into the parameter.

If the copy constructor used pass by value, the copy constructor would need to call itself to copy the initializer argument into the copy constructor parameter. But that call to the copy constructor would also be pass by value, so the copy constructor would be invoked again to copy the argument into the function parameter. This would lead to an infinite chain of calls to the copy constructor.

cppreference.com

Copy constructors

A copy constructor of class T is a non-template constructor whose first parameter is T & , const T & , volatile T & , or const volatile T & , and either there are no other parameters, or the rest of the parameters all have default values. A type with a public copy constructor is CopyConstructible .

[ edit ] Syntax

[ edit ] explanation.

• Typical declaration of a copy constructor
• Forcing a copy constructor to be generated by the compiler
• Avoiding implicit default constructor

The copy constructor is called whenever an object is initialized from another object of the same type, which includes

• initialization, T a = b ; or T a ( b ) ; , where b is of type T
• function argument passing: f ( a ) ; , where a is of type T and f is void f ( T t )
• function return: return a ; inside a function such as T f ( ) , where a is of type T , which has no move constructor.

[ edit ] Implicitly-declared copy constructor

If no user-defined copy constructors are provided for a class type ( struct , class , or union ), the compiler will always declare a copy constructor as an inline public member of its class. This implicitly-declared copy constructor has the form T::T(const T&) if all of the following is true:

• all direct and virtual bases of T have copy constructors with references to const or to const volatile as their first parameters
• all non-static members of T have copy constructors with references to const or to const volatile as their first parameters

Otherwise, the implicitly-declared copy constructor is T :: T ( T & ) . (Note that due to these rules, the implicitly-declared copy constructor cannot bind to a volatile lvalue argument)

A class can have multiple copy constructors, e.g. both T :: T ( const T & ) and T :: T ( T & ) . If some user-defined copy constructors are present, the user may still force the generation of the implicitly declared copy constructor with the keyword default (since C++11) .

[ edit ] Deleted implicitly-declared copy constructor

The implicitly-declared or defaulted copy constructor for class T is undefined (until C++11) / defined as deleted (since C++11) in any of the following is true:

• T has non-static data members that cannot be copied (have deleted, inaccessible, or ambiguous copy constructors)
• T has direct or virtual base class that cannot be copied (has deleted, inaccessible, or ambiguous copy constructors)
• T has direct or virtual base class with a deleted or inaccessible destructor
• T has a user-defined move constructor or move assignment operator (since C++11)
• T is a union and has a variant member with non-trivial copy constructor (since C++11)
• T has a data member of rvalue reference type (since C++11)

[ edit ] Trivial copy constructor

The implicitly-declared copy constructor for class T is trivial if all of the following is true:

• T has no virtual member functions
• T has no virtual base classes
• The copy constructor selected for every direct base of T is trivial
• The copy constructor selected for every non-static class type (or array of class type) memeber of T is trivial

A trivial copy constructor is a constructor that creates a bytewise copy of the object representation of the argument, and performs no other action. Objects with trivial copy constructors can be copied by copying their object representations manually, e.g. with std:: memmove . All data types compatible with the C language (POD types) are trivially copyable.

[ edit ] Implicitly-defined copy constructor

If the implicitly-declared copy constructor is not deleted or trivial, it is defined (that is, a function body is generated and compiled) by the compiler. For union types, the implicitly-defined copy constructor copies the object representation (as by std:: memmove ). For non-union class types ( class and struct ), the implicitly performs full member-wise copy of the object's bases and non-static members, in their initialization order, using direct initialization.

[ edit ] Example

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C++ At Work

Copy Constructors, Assignment Operators, and More

Paul DiLascia

Code download available at: CAtWork0509.exe (276 KB) Browse the Code Online

Q I have a simple C++ problem. I want my copy constructor and assignment operator to do the same thing. Can you tell me the best way to accomplish this?

A At first glance this seems like a simple question with a simple answer: just write a copy constructor that calls operator=.

Or, alternatively, write a common copy method and call it from both your copy constructor and operator=, like so:

This code works fine for many classes, but there's more here than meets the eye. In particular, what happens if your class contains instances of other classes as members? To find out, I wrote the test program in Figure 1 . It has a main class, CMainClass, which contains an instance of another class, CMember. Both classes have a copy constructor and assignment operator, with the copy constructor for CMainClass calling operator= as in the first snippet. The code is sprinkled with printf statements to show which methods are called when. To exercise the constructors, cctest first creates an instance of CMainClass using the default ctor, then creates another instance using the copy constructor:

Figure 1 Copy Constructors and Assignment Operators

If you compile and run cctest, you'll see the following printf messages when cctest constructs obj2:

The member object m_obj got initialized twice! First by the default constructor, and again via assignment. Hey, what's going on?

In C++, assignment and copy construction are different because the copy constructor initializes uninitialized memory, whereas assignment starts with an existing initialized object. If your class contains instances of other classes as data members, the copy constructor must first construct these data members before it calls operator=. The result is that these members get initialized twice, as cctest shows. Got it? It's the same thing that happens with the default constructor when you initialize members using assignment instead of initializers. For example:

As opposed to:

Using assignment, m_obj is initialized twice; with the initializer syntax, only once. So, what's the solution to avoid extra initializations during copy construction? While it goes against your instinct to reuse code, this is one situation where it's best to implement your copy constructor and assignment operator separately, even if they do the same thing. Calling operator= from your copy constructor will certainly work, but it's not the most efficient implementation. My observation about initializers suggests a better way:

Now the main copy ctor calls the member object's copy ctor using an initializer, and m_obj is initialized just once by its copy ctor. In general, copy ctors should invoke the copy ctors of their members. Likewise for assignment. And, I may as well add, the same goes for base classes: your derived copy ctor and assignment operators should invoke the corresponding base class methods. Of course, there are always times when you may want to do something different because you know how your code works—but what I've described are the general rules, which are to be broken only when you have a compelling reason. If you have common tasks to perform after the basic objects have been initialized, you can put them in a common initialization method and call it from your constructors and operator=.

Q Can you tell me how to call a Visual C++® class from C#, and what syntax I need to use for this?

Sunil Peddi

Q I have an application that is written in both C# (the GUI) and in classic C++ (some business logic). Now I need to call from a DLL written in C++ a function (or a method) in a DLL written in Visual C++ .NET. This one calls another DLL written in C#. The Visual C++ .NET DLL acts like a proxy. Is this possible? I was able to use LoadLibrary to call a function present in the Visual C++ .NET DLL, and I can receive a return value, but when I try to pass some parameters to the function in the Visual C++ .NET DLL, I get the following error:

How can I resolve this problem?

Giuseppe Dattilo

A I get a lot of questions about interoperability between the Microsoft® .NET Framework and native C++, so I don't mind revisiting this well-covered topic yet again. There are two directions you can go: calling the Framework from C++ or calling C++ from the Framework. I won't go into COM interop here as that's a separate issue best saved for another day.

Let's start with the easiest one first: calling the Framework from C++. The simplest and easiest way to call the Framework from your C++ program is to use the Managed Extensions. These Microsoft-specific C++ language extensions are designed to make calling the Framework as easy as including a couple of files and then using the classes as if they were written in C++. Here's a very simple C++ program that calls the Framework's Console class:

To use the Managed Extensions, all you need to do is import <mscorlib.dll> and whatever .NET assemblies contain the classes you plan to use. Don't forget to compile with /clr:

Your C++ code can use managed classes more or less as if they were ordinary C++ classes. For example, you can create Framework objects with operator new, and access them using C++ pointer syntax, as shown in the following:

Here, the String s is declared as pointer-to-String because String::Format returns a new String object.

The "Hello, world" and date/time programs seem childishly simple—and they are—but just remember that however complex your program is, however many .NET assemblies and classes you use, the basic idea is the same: use <mscorlib.dll> and whatever other assemblies you need, then create managed objects with new, and use pointer syntax to access them.

So much for calling the Framework from C++. What about going the other way, calling C++ from the Framework? Here the road forks into two options, depending on whether you want to call extern C functions or C++ class member functions. Again, I'll take the simpler case first: calling C functions from .NET. The easiest thing to do here is use P/Invoke. With P/Invoke, you declare the external functions as static methods of a class, using the DllImport attribute to specify that the function lives in an external DLL. In C# it looks like this:

This tells the compiler that MessageBox is a function in user32.dll that takes an IntPtr (HWND), two Strings, and an int. You can then call it from your C# program like so:

Of course, you don't need P/Invoke for MessageBox since the .NET Framework already has a MessageBox class, but there are plenty of API functions that aren't supported directly by the Framework, and then you need P/Invoke. And, of course, you can use P/Invoke to call C functions in your own DLLs. I've used C# in the example, but P/Invoke works with any .NET-based language like Visual Basic® .NET or JScript®.NET. The names are the same, only the syntax is different.

Note that I used IntPtr to declare the HWND. I could have got away with int, but you should always use IntPtr for any kind of handle such as HWND, HANDLE, or HDC. IntPtr will default to 32 or 64 bits depending on the platform, so you never have to worry about the size of the handle.

DllImport has various modifiers you can use to specify details about the imported function. In this example, CharSet=CharSet.Auto tells the Framework to pass Strings as Unicode or Ansi, depending on the target operating system. Another little-known modifier you can use is CallingConvention. Recall that in C, there are different calling conventions, which are the rules that specify how the compiler should pass arguments and return values from one function to another across the stack. The default CallingConvention for DllImport is CallingConvention.Winapi. This is actually a pseudo-convention that uses the default convention for the target platform; for example, StdCall (in which the callee cleans the stack) on Windows® platforms and CDecl (in which the caller cleans the stack) on Windows CE .NET. CDecl is also used with varargs functions like printf.

The calling convention is where Giuseppe ran into trouble. C++ uses yet a third calling convention: thiscall. With this convention, the compiler uses the hardware register ECX to pass the "this" pointer to class member functions that don't have variable arguments. Without knowing the exact details of Giuseppe's program, it sounds from the error message that he's trying to call a C++ member function that expects thiscall from a C# program that's using StdCall—oops!

Aside from calling conventions, another interoperability issue when calling C++ methods from the Framework is linkage: C and C++ use different forms of linkage because C++ requires name-mangling to support function overloading. That's why you have to use extern "C" when you declare C functions in C++ programs: so the compiler won't mangle the name. In Windows, the entire windows.h file (now winuser.h) is enclosed in extern "C" brackets.

While there may be a way to call C++ member functions in a DLL directly using P/Invoke and DllImport with the exact mangled names and CallingConvention=ThisCall, it's not something to attempt if you're in your right mind. The proper way to call C++ classes from managed code—option number two—is to wrap your C++ classes in managed wrappers. Wrapping can be tedious if you have lots of classes, but it's really the only way to go. Say you have a C++ class CWidget and you want to wrap it so .NET clients can use it. The basic formula looks something like this:

The pattern is the same for any class. You write a managed (__gc) class that holds a pointer to the native class, you write a constructor and destructor that allocate and destroy the instance, and you write wrapper methods that call the corresponding native C++ member functions. You don't have to wrap all the member functions, only the ones you want to expose to the managed world.

Figure 2 shows a simple but concrete example in full detail. CPerson is a class that holds the name of a person, with member functions GetName and SetName to change the name. Figure 3 shows the managed wrapper for CPerson. In the example, I converted Get/SetName to a property, so .NET-based programmers can use the property syntax. In C#, using it looks like this:

Figure 3 Managed Person Class

Figure 2 Native CPerson Class

Using properties is purely a matter of style; I could equally well have exposed two methods, GetName and SetName, as in the native class. But properties feel more like .NET. The wrapper class is an assembly like any other, but one that links with the native DLL. This is one of the cool benefits of the Managed Extensions: You can link directly with native C/C++ code. If you download and compile the source for my CPerson example, you'll see that the makefile generates two separate DLLs: person.dll implements a normal native DLL and mperson.dll is the managed assembly that wraps it. There are also two test programs: testcpp.exe, a native C++ program that calls the native person.dll and testcs.exe, which is written in C# and calls the managed wrapper mperson.dll (which in turn calls the native person.dll).

Figure 4** Interop Highway **

I've used a very simple example to highlight the fact that there are fundamentally only a few main highways across the border between the managed and native worlds (see Figure 4 ). If your C++ classes are at all complex, the biggest interop problem you'll encounter is converting parameters between native and managed types, a process called marshaling. The Managed Extensions do an admirable job of making this as painless as possible (for example, automatically converting primitive types and Strings), but there are times where you have to know something about what you're doing.

For example, you can't pass the address of a managed object or subobject to a native function without pinning it first. That's because managed objects live in the managed heap, which the garbage collector is free to rearrange. If the garbage collector moves an object, it can update all the managed references to that object—but it knows nothing of raw native pointers that live outside the managed world. That's what __pin is for; it tells the garbage collector: don't move this object. For strings, the Framework has a special function PtrToStringChars that returns a pinned pointer to the native characters. (Incidentally, for those curious-minded souls, PtrToStringChars is the only function as of this date defined in <vcclr.h>. Figure 5 shows the code.) I used PtrToStringChars in MPerson to set the Name (see Figure 3 ).

Figure 5 PtrToStringChars

Pinning isn't the only interop problem you'll encounter. Other problems arise if you have to deal with arrays, references, structs, and callbacks, or access a subobject within an object. This is where some of the more advanced techniques come in, such as StructLayout, boxing, __value types, and so on. You also need special code to handle exceptions (native or managed) and callbacks/delegates. But don't let these interop details obscure the big picture. First decide which way you're calling (from managed to native or the other way around), and if you're calling from managed to native, whether to use P/Invoke or a wrapper.

In Visual Studio® 2005 (which some of you may already have as beta bits), the Managed Extensions have been renamed and upgraded to something called C++/CLI. Think of the C++/CLI as Managed Extensions Version 2, or What the Managed Extensions Should Have Been. The changes are mostly a matter of syntax, though there are some important semantic changes, too. In general C++/CLI is designed to highlight rather than blur the distinction between managed and native objects. Using pointer syntax for managed objects was a clever and elegant idea, but in the end perhaps a little too clever because it obscures important differences between managed and native objects. C++/CLI introduces the key notion of handles for managed objects, so instead of using C pointer syntax for managed objects, the CLI uses ^ (hat):

As you no doubt noticed, there's also a gcnew operator to clarify when you're allocating objects on the managed heap as opposed to the native one. This has the added benefit that gcnew doesn't collide with C++ new, which can be overloaded or even redefined as a macro. C++/CLI has many other cool features designed to make interoperability as straightforward and intuitive as possible.

Send your questions and comments for Paul to   [email protected] .

Paul DiLascia is a freelance software consultant and Web/UI designer-at-large. He is the author of Windows ++: Writing Reusable Windows Code in C ++ (Addison-Wesley, 1992). In his spare time, Paul develops PixieLib, an MFC class library available from his Web site, www.dilascia.com .

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• Pure Virtual Destructor in C++
• Can Static Functions Be Virtual in C++?
• RTTI (Run-Time Type Information) in C++
• Can Virtual Functions be Private in C++?

C++ Exception Handling

• Exception Handling in C++
• Exception Handling using classes in C++
• Stack Unwinding in C++
• User-defined Custom Exception with class in C++

C++ Files and Streams

• File Handling through C++ Classes
• I/O Redirection in C++
• Templates in C++ with Examples
• Template Specialization in C++
• Using Keyword in C++ STL

C++ Standard Template Library (STL)

• The C++ Standard Template Library (STL)
• Containers in C++ STL (Standard Template Library)
• Introduction to Iterators in C++
• Algorithm Library | C++ Magicians STL Algorithm

C++ Preprocessors

• C Preprocessors
• C Preprocessor Directives
• #include in C
• Difference between Preprocessor Directives and Function Templates in C++

C++ Namespace

• Namespace in C++ | Set 1 (Introduction)
• namespace in C++ | Set 2 (Extending namespace and Unnamed namespace)
• Namespace in C++ | Set 3 (Accessing, creating header, nesting and aliasing)
• C++ Inline Namespaces and Usage of the "using" Directive Inside Namespaces

Advanced C++

• Multithreading in C++
• Smart Pointers in C++
• auto_ptr vs unique_ptr vs shared_ptr vs weak_ptr in C++
• Type of 'this' Pointer in C++
• "delete this" in C++
• Passing a Function as a Parameter in C++
• Signal Handling in C++
• Generics in C++
• Difference between C++ and Objective C
• Write a C program that won't compile in C++
• Write a program that produces different results in C and C++
• How does 'void*' differ in C and C++?
• Type Difference of Character Literals in C and C++
• Cin-Cout vs Scanf-Printf

C++ vs Java

• Similarities and Difference between Java and C++
• Comparison of Inheritance in C++ and Java
• How Does Default Virtual Behavior Differ in C++ and Java?
• Comparison of Exception Handling in C++ and Java
• Foreach in C++ and Java
• Templates in C++ vs Generics in Java
• Floating Point Operations & Associativity in C, C++ and Java

Competitive Programming in C++

• Competitive Programming - A Complete Guide
• C++ tricks for competitive programming (for C++ 11)
• Writing C/C++ code efficiently in Competitive programming
• Why C++ is best for Competitive Programming?
• Test Case Generation | Set 1 (Random Numbers, Arrays and Matrices)
• Fast I/O for Competitive Programming
• Setting up Sublime Text for C++ Competitive Programming Environment
• How to setup Competitive Programming in Visual Studio Code for C++
• Which C++ libraries are useful for competitive programming?
• Common mistakes to be avoided in Competitive Programming in C++ | Beginners
• C++ Interview Questions and Answers (2024)
• Top C++ STL Interview Questions and Answers
• 30 OOPs Interview Questions and Answers (2024)
• Top C++ Exception Handling Interview Questions and Answers
• C++ Programming Examples

Pre-requisite: Constructor in C++ 

A copy constructor is a member function that initializes an object using another object of the same class. In simple terms, a constructor which creates an object by initializing it with an object of the same class, which has been created previously is known as a copy constructor .  

Copy constructor is used to initialize the members of a newly created object by copying the members of an already existing object.

Copy constructor takes a reference to an object of the same class as an argument.

The process of initializing members of an object through a copy constructor is known as copy initialization.

It is also called member-wise initialization because the copy constructor initializes one object with the existing object, both belonging to the same class on a member by member copy basis.

The copy constructor can be defined explicitly by the programmer. If the programmer does not define the copy constructor, the compiler does it for us.  

Syntax of Copy Constructor

Characteristics of Copy Constructor

1. The copy constructor is used to initialize the members of a newly created object by copying the members of an already existing object.

2. Copy constructor takes a reference to an object of the same class as an argument. If you pass the object by value in the copy constructor, it would result in a recursive call to the copy constructor itself. This happens because passing by value involves making a copy, and making a copy involves calling the copy constructor, leading to an infinite loop. Using a reference avoids this recursion. So we use reference of Objects to avoid infinite calls.

3. The process of initializing members of an object through a copy constructor is known as copy initialization.

4 . It is also called member-wise initialization because the copy constructor initializes one object with the existing object, both belonging to the same class on a member-by-member copy basis.

5. The copy constructor can be defined explicitly by the programmer. If the programmer does not define the copy constructor, the compiler does it for us.

Types of Copy Constructors

1. default copy constructor.

An implicitly defined copy constructor will copy the bases and members of an object in the same order that a constructor would initialize the bases and members of the object.

2. User Defined Copy Constructor 

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

When is the copy constructor called? 

In C++, a Copy Constructor may be called in the following cases: 

• When an object of the class is returned by value. 
• When an object of the class is passed (to a function) by value as an argument. 
• When an object is constructed based on another object of the same class. 
• When the compiler generates a temporary object.

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

Copy Elision

In copy elision , the compiler prevents the making of extra copies which results in saving space and better the program complexity(both time and space); Hence making the code more optimized.  

Now it is on the compiler to decide what it wants to print, it could either print the above output or it could print case 1 or case 2 below, and this is what Return Value Optimization is. In simple words, RVO is a technique that gives the compiler some additional power to terminate the temporary object created which results in changing the observable behavior/characteristics of the final program.

When is a user-defined copy constructor needed? 

If we don’t define our own copy constructor, the C++ compiler creates a default copy constructor for each class which does a member-wise copy between objects. The compiler-created copy constructor works fine in general. We need to define our own copy constructor only if an object has pointers or any runtime allocation of the resource like a file handle , a network connection, etc.  

The default constructor does only shallow copy.  

Deep copy is possible only with a user-defined copy constructor. In a user-defined copy constructor, we make sure that pointers (or references) of copied objects point to new memory locations.  

Copy constructor vs Assignment Operator 

The main difference between Copy Constructor and Assignment Operator is that the Copy constructor makes a new memory storage every time it is called while the assignment operator does not make new memory storage.

Which of the following two statements calls the copy constructor and which one calls the assignment operator? 

A copy constructor is called when a new object is created from an existing object, as a copy of the existing object. The assignment operator is called when an already initialized object is assigned a new value from another existing object. In the above example (1) calls the copy constructor and (2) calls the assignment operator. See this for more details.

Example – Class Where a Copy Constructor is Required 

Following is a complete C++ program to demonstrate the use of the Copy constructor. In the following String class, we must write a copy constructor. 

What would be the problem if we remove the copy constructor from the above code? 

If we remove the copy constructor from the above program, we don’t get the expected output. The changes made to str2 reflect in str1 as well which is never expected.   

Can we make the copy constructor private?  

Yes, a copy constructor can be made private. When we make a copy constructor private in a class, objects of that class become non-copyable. This is particularly useful when our class has pointers or dynamically allocated resources. In such situations, we can either write our own copy constructor like the above String example or make a private copy constructor so that users get compiler errors rather than surprises at runtime.

Why argument to a copy constructor must be passed as a reference?  

A copy constructor is called when an object is passed by value. Copy constructor itself is a function. So if we pass an argument by value in a copy constructor, a call to the copy constructor would be made to call the copy constructor which becomes a non-terminating chain of calls. Therefore compiler doesn’t allow parameters to be passed by value.

Why argument to a copy constructor should be const?

One reason for passing const reference is, that we should use const in C++ wherever possible so that objects are not accidentally modified. This is one good reason for passing reference as const , but there is more to it than ‘ Why argument to a copy constructor should be const?’

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