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Copy constructors, assignment operators, and exception safe assignment

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Copy constructors.

A copy constructor is a constructor which can be called with an argument of the same class type and copies the content of the argument without mutating the argument.

[ edit ] Syntax

[ edit ] Explanation

The copy constructor is called whenever an object is initialized (by direct-initialization or copy-initialization ) from another object of the same type (unless overload resolution selects a better match or the call is elided ), 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, the compiler will always declare a copy constructor as a non- explicit inline public member of its class. This implicitly-declared copy constructor has the form T :: T ( const T & ) if all of the following are true:

• each direct and virtual base B of T has a copy constructor whose parameters are of type const B & or const volatile B & ;
• each non-static data member M of T of class type or array of class type has a copy constructor whose parameters are of type const M & or const volatile M & .

Otherwise, the implicitly-declared copy constructor is T :: T ( T & ) .

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 & ) .

The implicitly-declared (or defaulted on its first declaration) copy constructor has an exception specification as described in dynamic exception specification (until C++17) noexcept specification (since C++17) .

[ edit ] Implicitly-defined copy constructor

If the implicitly-declared copy constructor is not deleted, it is defined (that is, a function body is generated and compiled) by the compiler if odr-used or needed for constant evaluation (since C++11) . For union types, the implicitly-defined copy constructor copies the object representation (as by std::memmove ). For non-union class types, the constructor performs full member-wise copy of the object's direct base subobjects and member subobjects, in their initialization order, using direct initialization. For each non-static data member of a reference type, the copy constructor binds the reference to the same object or function to which the source reference is bound.

[ edit ] Deleted copy constructor

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

• T has a potentially constructed subobject of class type M (or possibly multi-dimensional array thereof) such that
• M has a destructor that is deleted or (since C++11) inaccessible from the copy constructor, or
• the overload resolution as applied to find M 's copy constructor
• does not result in a usable candidate, or
• in the case of the subobject being a variant member , selects a non-trivial function.

[ edit ] Trivial copy constructor

The copy constructor for class T is trivial if all of the following are true:

• it is not user-provided (that is, it is implicitly-defined or defaulted);
• 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) member of T is trivial;

A trivial copy constructor for a non-union class effectively copies every scalar subobject (including, recursively, subobject of subobjects and so forth) of the argument and performs no other action. However, padding bytes need not be copied, and even the object representations of the copied subobjects need not be the same as long as their values are identical.

TriviallyCopyable objects 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 ] Eligible copy constructor

Triviality of eligible copy constructors determines whether the class is an implicit-lifetime type , and whether the class is a trivially copyable type .

[ edit ] Notes

In many situations, copy constructors are optimized out even if they would produce observable side-effects, see copy elision .

[ edit ] Example

[ edit ] defect reports.

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[ edit ] See also

• converting constructor
• copy assignment
• copy elision
• default constructor
• aggregate initialization
• constant initialization
• copy initialization
• default initialization
• direct initialization
• initializer list
• list initialization
• reference initialization
• value initialization
• zero initialization
• move assignment
• move constructor
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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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Difference Between Copy Constructor and Assignment Operator in C++

C++ is a General purpose, middle-level, case sensitive, platform independent programming language that supports object oriented programming concept. C++ was created by Bjarne Stroustrup at Bell Labs in 1979. Since C++ is a platform independent programming language, it can be used on a variety of systems, including Windows, Mac OS, and various UNIX versions.

Operators in C++ are used to perform specific operations on values and variables. The following are the list of operators used in C++

Arithmetic operators

Assignment operators

Comparison operators

Relational Operators

Logical operators

Bitwise operators

Unary operators

Ternary/Conditional operators.

From the above groups, Assignment operators are used to assign values to variables.

Before learning about copy constructors, let’s get a brief idea about constructors. A Constructor in C++ is a special method, same name as class name with parenthesis "( )", that is automatically invoked when an object is created. Constructor is used to initialize the variables of the object which is created newly.

A Copy Constructor is a type of constructor that uses another object from the same class which has been created previously, to initialize an object.

Now let’s go through the detailed concept and compare and contrast the various features of copy constructor and assignment operator.

What is Assignment operator

The use of Assignment operator is to assign a value to a variable. The left operand of the assignment operator is variable name and the right operand of the operator is value to that variable. The datatype must be same for both the operands, if not so a compilation error will be raised.

The types of assignment operators are

= operator − It only assigns the value to the variable. For example, "a=10", here the value 10 will be assigned to variable "a".

+= operator −This operator first adds the current value of the variable by the value which is on right side and then assigns the new value to variable.

–= operator − This operator first subtracts the current value of the variable by the value which is on right side and then assigns the new value to variable.

*= operator − This operator first multiplies the current value of the variable by the value which is on right side and then assigns the new value to variable.

/= operator − This operator first divides the current value of the variable by the value which is on right side and then assigns the new value to variable.

Example on Assignment Operator

Let’s see an example of assignment operator. Here we are using the assignment operator to assign values to different variables.

In the above example, we have taken two variables "a" and "b" and at first we have assigned the value of a to 5 through assignment operator "=". And we have assigned the value of a to variable b. The above code will result the output as given below.

What is a Copy Constructor?

This is often required in programming to make a separate copy of an object without impacting the original. In these cases, the copy constructor comes in use. The copy constructor is a constructor that creates an object by initializing it using a previously created object of the same class. There are two types of copy constructor.

Default Copy Constructor − When the copy Constructor is not declared, the C++ compiler creates a default Constructor that copies all member variables as they are.

User-Defined Copy Constructor − The copy constructor defined by the user is called user defined copy constructor.

The syntax for Copy Constructor is −

Copy Constructor - Example

The copy constructor is used to initialize one object from another object of the same class, to copy an object to pass as an argument to a function, and to copy an object to pass as a parameter to a function. To return an object from a function, copy the object.

Let’s see an example to understand how exactly we can use a copy constructor.

In the above example, we have taken the class name as Example, and created a constructor and passed the value 20 and 30 to the constructor. The statement Example (Example &ex) indicates the copy constructor. It copies the value of the data members previously created.

The above code will produce the following output −

In our example, we have created two objects obj1 and obj2 and we are assigning the value of obj1 to obj2.

Comparison between Copy Constructor and Assignment Operator

The main purpose of both the concepts in C++ is to assign the value, but the main difference between both is copy constructor creates a new object and assigns the value but assignment operator does not create a new object, instead it assigns the value to the data member of the same object.

The following table highlights the major differences between copy constructor and assignment operator.

The difference between a copy constructor and an assignment operator is that a copy constructor helps to create a copy of an already existing object without altering the original value of the created object, whereas an assignment operator helps to assign a new value to a data member or an object in the program.

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22.3 — Move constructors and move assignment

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You now have enough context to understand the key insight behind move semantics.

Automatic l-values returned by value may be moved instead of copied

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A Comprehensive Guide to Constructors in C++: Everything You Need to Know

In C++, constructors are special member functions of a class that are automatically called when an object of the class is created. They are used to initialize objects. Constructors have the same name as the class and do not have a return type.

• What is a Constructor?

A constructor is a special member function of a class that initializes objects of that class. Constructors are called automatically when an object is created. They have the same name as the class and no return type, not even void.

Table of Content

Getting Started with Constructors in C++

Syntax and basic rules of constructors in c++, types of constructors, importance of constructors in c++, intermediate topics for contructors in c++, 1. constructor overloading, 2. initializer list, 3. constructor initialization order, advanced aspects of constructors, 1. member initializer lists, 2. delegating constructors, 3. explicit constructors, 4. constructor inheritance, 5. smart pointers and raii, 6. rule of three, rule of five, 7. preventing object copying, 8. singleton pattern and constructors, 9. constructors and exception safety, common mistakes and pitfalls, uninitialized member variables, infinite recursion in constructors, object slicing with copy constructors, memory leaks in constructors, best practices for constructors, hands-on examples and exercises, 1. practice problems on default and parameterized constructors, 2. implementing and using copy constructors, 3. practicing move constructors with real-world scenarios, 4. writing constructors with initializer lists, 5. exercises on constructor overloading, summary and conclusion, key takeaways, recap of important concepts, final words on constructors in c++.

• Constructors must have the same name as the class.
• They cannot have a return type.
• Constructors can be overloaded.
• Constructors are called automatically when an object is created.
• Default Constructor: A default constructor is a constructor that can be called with no arguments. If no constructors are defined, the compiler generates a default constructor.
• Parameterized Constructor: A parameterized constructor takes arguments to initialize an object with specific values.
• Copy Constructor: A copy constructor initializes an object using another object of the same class. It performs a deep copy, creating a new instance with the same values.
• Move Constructor: Introduced in C++11, a move constructor transfers resources from a temporary object to a new object, which can improve performance by eliminating unnecessary copying.
• Destructors: A destructor is a special member function that is executed when an object goes out of scope or is explicitly deleted. Destructors have the same name as the class, preceded by a tilde (~), and are used to release resources allocated by the object.

Constructors are essential because they allow for the initialization of objects, ensuring they start in a valid state. Without constructors, each object’s member variables would have to be set manually after creation, which can lead to errors and increased complexity

Here are some key points highlighting the importance of constructors in C++:

1. Object Initialization:

• Automatic Initialization: Constructors are used to initialize the objects when they are created, ensuring that the objects start in a well-defined state.
• Default Values: They can assign default values to data members, which helps in preventing the use of uninitialized variables.

2. Resource Management:

• Memory Allocation: Constructors can allocate resources such as memory, file handles, or network connections, which the object will manage.
• RAII (Resource Acquisition Is Initialization): This principle ensures that resources are properly acquired and released. The constructor acquires the resources, and the destructor releases them, providing automatic resource management.

3. Overloaded Constructors:

• Multiple Ways to Initialize: A class can have multiple constructors with different parameters, allowing objects to be initialized in various ways depending on the provided arguments.
• Constructor Overloading: This feature enhances flexibility by allowing different methods of object creation.

4. Encapsulation:

• Controlled Initialization: Constructors provide a controlled way to initialize the data members, maintaining encapsulation and data integrity.
• Preventing Invalid States: By using constructors, we can ensure that an object cannot be created in an invalid state, as any necessary validation can be performed within the constructor.

5. Inheritance and Polymorphism:

• Base Class Initialization: In a class hierarchy, constructors are essential for initializing base class parts of derived class objects.
• Virtual Constructors: Though C++ does not support virtual constructors directly, the concept can be achieved through factory methods to create objects of derived classes, facilitating polymorphic behavior.

6. Initialization Lists:

• Efficient Initialization: Constructors can use initialization lists to initialize data members directly, often leading to more efficient code.
• Const Members and References: Initialization lists are necessary to initialize constant data members and references, which cannot be assigned values after the object is created.

Example to show the importance of constructors in C++:

Here’s an example illustrating the importance of constructors in C++:

In this example:

• The default constructor initializes the name to “Unknown” and age to 0.
• The parameterized constructor allows initializing name and age with specific values.
• The copy constructor creates a new object as a copy of an existing object.

These constructors ensure that objects of the Person class are always created in a valid state.

Constructor overloading allows a class to have more than one constructor with different parameters. This is useful for initializing objects in different ways.

An initializer list is a more efficient way to initialize member variables. It is used in the constructor definition.

The order in which constructors initialize member variables is determined by the order of declaration in the class, not the order in the initializer list.

Using member initializer lists is more efficient than assigning values inside the constructor body, especially for complex types or when initializing const or reference members.

In this example, the initializer list ensures id is initialized correctly.

C++11 introduced delegating constructors, allowing one constructor to call another constructor within the same class. This helps to avoid code duplication.

Here, the default constructor delegates to the parameterized constructor.

The explicit keyword prevents implicit conversions, which can lead to unexpected behavior.

Using explicit prevents implicit conversion from int to MyClass .

C++11 introduced constructor inheritance, allowing derived classes to inherit constructors from base classes.

In this example, the Base class constructor is inherited by the Derived class.

Using smart pointers (like std::unique_ptr and std::shared_ptr ) with constructors ensures proper resource management following the RAII (Resource Acquisition Is Initialization) principle.

In this example, unique_ptr ensures that the Resource is properly managed and released.

Follow these rules to manage resource ownership correctly:

• Rule of Three : If a class needs a custom destructor, copy constructor, or copy assignment operator, it likely needs all three.
• Rule of Five : In addition to the Rule of Three, include the move constructor and move assignment operator for modern C++.

Use the = delete syntax to prevent copying of objects.

The Singleton pattern ensures a class has only one instance and provides a global point of access to it.

Ensure that constructors are exception-safe by properly handling exceptions and ensuring resources are correctly released.

Always initialize member variables to avoid undefined behavior.

Be careful with constructor delegation to avoid infinite recursion.

Object slicing occurs when a derived class object is copied to a base class object.

Ensure that allocated resources are properly released to avoid memory leaks.

• Use Initializer Lists : Always prefer initializer lists over assignment in the constructor body for better performance and correctness.
• Minimize Resource Allocation in Constructors : Avoid complex logic or resource allocation directly in constructors. Consider using factory functions or initializing after object creation.
• Use explicit for Single-Argument Constructors : Prevent unintended implicit conversions by marking single-argument constructors with explicit .
• Leverage Smart Pointers : Use smart pointers for managing dynamic resources to ensure automatic cleanup and prevent memory leaks.
• Avoid Code Duplication : Use delegating constructors to avoid code duplication and ensure consistency in initialization logic.

Create a class Rectangle with a default constructor and a parameterized constructor to initialize length and width.

Create a class Array that implements a copy constructor to perform a deep copy of an array.

Create a class Buffer that implements a move constructor to transfer ownership of a dynamically allocated buffer.

Create a class Point that uses an initializer list to initialize its member variables.

Create a class Circle with overloaded constructors to initialize the radius in different ways.

• Constructors initialize objects, ensuring they start in a valid state.
• Multiple types of constructors (default, parameterized, copy, move) serve different initialization purposes.
• Destructors clean up resources when objects go out of scope.
• Constructors and destructors are vital for managing resource allocation and deallocation.
• Initializer lists provide an efficient way to initialize member variables.
• Move constructors improve performance by eliminating unnecessary copying.

Understanding constructors and destructors is essential for writing robust and efficient C++ programs. Proper use of these special member functions ensures that objects are initialized and cleaned up correctly, which is critical for resource management and overall program stability.

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Tech Differences

Know the Technical Differences

Difference Between Copy Constructor and Assignment Operator in C++

Let us study the difference between the copy constructor and assignment operator.

Content: Copy Constructor Vs Assignment Operator

Comparison chart.

• Key Differences

Definition of Copy Constructor

A “copy constructor” is a form of an overloaded constructor . A copy constructor is only called or invoked for initialization purpose. A copy constructor initializes the newly created object by another existing object.

When a copy constructor is used to initialize the newly created target object, then both the target object and the source object shares a different memory location. Changes done to the source object do not reflect in the target object. The general form of the copy constructor is

If the programmer does not create a copy constructor in a C++ program, then the compiler implicitly provides a copy constructor. An implicit copy constructor provided by the compiler does the member-wise copy of the source object. But, sometimes the member-wise copy is not sufficient, as the object may contain a pointer variable.

Copying a pointer variable means, we copy the address stored in the pointer variable, but we do not want to copy address stored in the pointer variable, instead, we want to copy what pointer points to. Hence, there is a need of explicit ‘copy constructor’ in the program to solve this kind of problems.

A copy constructor is invoked in three conditions as follow:

• Copy constructor invokes when a new object is initialized with an existing one.
• The object passed to a function as a non-reference parameter.
• The object is returned from the function.

Let us understand copy constructor with an example.

In the code above, I had explicitly declared a constructor “copy( copy &c )”. This copy constructor is being called when object B is initialized using object A. Second time it is called when object C is being initialized using object A.

When object D is initialized using object A the copy constructor is not called because when D is being initialized it is already in the existence, not the newly created one. Hence, here the assignment operator is invoked.

Definition of Assignment Operator

The assignment operator is an assigning operator of C++.  The “=” operator is used to invoke the assignment operator. It copies the data in one object identically to another object. The assignment operator copies one object to another member-wise. If you do not overload the assignment operator, it performs the bitwise copy. Therefore, you need to overload the assignment operator.

In above code when object A is assigned to object B the assignment operator is being invoked as both the objects are already in existence. Similarly, same is the case when object C is initialized with object A.

When the bitwise assignment is performed both the object shares the same memory location and changes in one object reflect in another object.

Key Differences Between Copy Constructor and Assignment Operator

• A copy constructor is an overloaded constructor whereas an assignment operator is a bitwise operator.
• Using copy constructor you can initialize a new object with an already existing object. On the other hand, an assignment operator copies one object to the other object, both of which are already in existence.
• A copy constructor is initialized whenever a new object is initialized with an already existing object, when an object is passed to a function as a non-reference parameter, or when an object is returned from a function. On the other hand, an assignment operator is invoked only when an object is being assigned to another object.
• When an object is being initialized using copy constructor, the initializing object and the initialized object shares the different memory location. On the other hand, when an object is being initialized using an assignment operator then the initialized and initializing objects share the same memory location.
• If you do not explicitly define a copy constructor then the compiler provides one. On the other hand, if you do not overload an assignment operator then a bitwise copy operation is performed.

The Copy constructor is best for copying one object to another when the object contains raw pointers.

Related Differences:

• Difference Between & and &&
• Difference Between Recursion and Iteration
• Difference Between new and malloc( )
• Difference Between Inheritance and Polymorphism
• Difference Between Constructor and Destructor

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Why not only one? Copy constructor and assignment operator

I understand which is invoked in what situation...

My question is Why these 2 different things exist at all? Why can't only one of the two take care of both situations?

• copy-constructor
• copy-assignment

• Because they are different things. But you can leverage copy in the implementation of assignment (see copy-and swap idiom .) –  juanchopanza Commented Jul 16, 2014 at 10:22
• Consider a class that has no default constructor or whose default constructor has undesirable side effects. You must have a copy constructor. But then how do you do a=b; with just a copy constructor? –  David Schwartz Commented Jul 16, 2014 at 10:31

5 Answers 5

No, they are different.

Copy constructor is used for constructing a new object (from another object). In this case you just need to initialize the members.

Assignment operator is used for an existing object (you may have constructed it by default constructor etc), and then assign it by another object. In this case you need to re-initialize members, sometimes means destroying and initializing them again.

Even so, the functionality of them are so similar, so you can share their implementation usually. Such as: What is the copy-and-swap idiom?

The copy constructor is invoked on creation, that means you don't have to take care of old resources in your object. The assignment operator on the other hand has to free the old resources. Besides they have different semantical meaning.

Suppose the String class:

(live example)

Appearently, copy constructor and copy assignment operator does different things. Look at the String::String(const String &other) .

It initializes its object. Set m_len , m_alloced , and m_str , and strcpy the string.

However, the String &String::operator =(const String &rhs) does -

modify its object. Allocate more memory if required, and re set m_len and re copy the string.

Copy constructor is called when the object is created, and does initialize its object.

But copy assignment operator is called after the object is created, and does modify its object.

For example, look at this code.

(it's also in the live example)

str1 is initialized by String::String(const char *) . As you know, it'll contain "asdf" .

str2 is initialized by copy constructor , String::String(const String &other) . By the copy constructor, str2 will contain the same content.

str3 is initialized by String::String(const char *) , like str1 . However, in line 4, it is modified by copy assignment operator . So, str3 contains "12" at first, but its content will be modified into "asdf" , by copy assignment operator.

Well, you could technically get away with just having a copy-constructor and a destructor, but that would require that every time you wanted to do an assignment you would have to destroy the object and then reconstruct it. This would become HIGHLY inefficient in the vast majority of use cases.

We cannot just have an operator= either, as you have no idea what the left-hand-side of the operator will be if it has not yet been constructed. It's members would probably contain garbage data and "shit would hit the fan".

Thus, you can think of the need for both an operator= and a copy-constructor as an optimization.

That being said there are a few design-patterns that can reduce the amount of code you have to write. For example, assuming you have a copy-constructor, a destructor and a swap-function implemented for your class. You could implement operator= using a copy-swap operation:

Whilst this operator has strong exception safety, it is worth to notice that it may be much less efficient than reusing preallocated resources (which a more advanced operator= might do).

Basically, they are used in different situations, many correct answer, i just wanna add something to clear out where the copy constructor is also used:

For example:

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