(C++) Interview in English. - Constructors/Destructors

Constructors/Destructors.

我们都知道，在C++中建立一个类，这个类中肯定会包括构造函数、析构函数、复制构造函数和重载赋值操作；即使在你没有明确定义的情况下，编译器也会给你生成这样的四个函数。例如以下类：

class CTest

{

public:

     CTest();        // 构造函数 Constructor

     ~CTest();              // 析构函数 Destructor

     CTest(const CTest &);       // 拷贝构造函数 Copy Constructor.

     CTest& operator=(const CTest &);   // 操作符重载

};

　　

Q: Is it possible to have Virtual Constructor? If yes, how? If not, Why not possible?

A: There is nothing like Virtual Constructor. The Constructor can’t be virtual as the constructor is a code which is responsible for creating an instance of a class and it can’t be delegated to any other object by virtual keyword means.

Q: What is constructor or ctor?

A: Constructor creates an object and initializes it. It also creates vtable for virtual functions. It is different from other methods in a class.

Q: What about Virtual Destructor?

A: Yes there is a Virtual Destructor. A destructor can be virtual as it is possible as at runtime depending on the type of object caller is calling to, proper destructor will be called.

Q: What is the difference between a copy constructor and an overloaded assignment operator?

A: A copy constructor constructs a new object by using the content of the argument object. An overloaded assignment operator assigns the contents of an existing object to another existing object of the same class.

Q: Can a constructor throws an exception? How to handle the error when the constructor fails?

A:The constructor never throws an error.??? 虽然没有返回值，但是抛出异常即可

Q: What is default constructor?

A: Constructor with no arguments or all the arguments has default values.

Q: What is copy constructor?

A: Constructor which initializes the it's object member variables ( by shallow copying) with another object of the same class. If you don't implement one in your class then compiler implements one for you.

class myclass{  

   int *p;  

   public:  

   myclass(int i);  //构造函数  

   myclass(const myclass &ob); //复制构造函数.  

   ~myclass();  

   int getval(){  

    return *p;  

   }  

};

for example:

(a) Boo Obj1(10); // calling Boo constructor

(b) Boo Obj2(Obj1); // calling boo copy constructor

(c) Boo Obj2 = Obj1;// calling boo copy constructor  注意：复制构造函数只有在初始化对象的时候才被调用，直接的赋值过程不能调用.

Q: When are copy constructors called?

A: Copy constructors are called in following cases:

(a) when a function returns an object of that class by value

(b) when the object of that class is passed by value as an argument to a function

(c) when you construct an object based on another object of the same class

(d) When compiler generates a temporary object

Q: Can a copy constructor accept an object of the same class as parameter, instead of reference of the object?

A: No. It is specified in the definition of the copy constructor itself. It should generate an error if a programmer specifies a copy constructor with a first argument that is an object and not a reference.

Q: What is conversion constructor?

A: constructor with a single argument makes that constructor as conversion ctor and it can be used for type conversion.

for example:

class Boo

{

public:

Boo( int i );

};

Boo BooObject =  ; // assigning int 10 Boo object,  conversion constructor is called here.

Q:What is conversion operator??

A:class can have a public method for specific data type conversions.

for example:

class Boo

{

double value;

public:

Boo(int i )

operator double()

{

return value;

}

};

Boo BooObject;

double i = BooObject; // assigning object to variable i of type double. now conversion operator gets called to assign the value.

Q: How can I handle a constructor that fails?

A: throw an exception. Constructors don't have a return type, so it's not possible to use return codes. The best way to signal constructor failure is therefore to throw an exception.

Q: How can I handle a destructor that fails?

A: Write a message to a log-_le. But do not throw an exception. The C++ rule is that you must never throw an exception from a destructor that is being called during the "stack unwinding" process of another exception. For example, if someone says throw Foo(), the stack will be unwound so all the stack frames between the throw Foo() and the } catch (Foo e) { will get popped. This is called stack unwinding. During stack unwinding, all the local objects in all those stack frames are destructed. If one of those destructors throws an exception (say it throws a Bar object), the C++ runtime system is in a no-win situation: should it ignore the Bar and end up in the } catch (Foo e) { where it was originally headed? Should it ignore the Foo and look for a } catch (Bare) { handler? There is no good answer:either choice loses information. So the C++ language guarantees that it will call terminate() at this point, and terminate() kills the process. Bang you're dead.

Q: What is Virtual Destructor?

A: Using virtual destructors, you can destroy objects without knowing their type - the correct destructor for the object is invoked using the virtual function mechanism. Note that destructors can also be declared as pure virtual functions for abstract classes. if someone will derive from your class, and if someone will say "new Derived", where "Derived" is derived from your class, and if someone will say delete p, where the actual object's type is "Derived" but the pointer p's type is your class.

Q: Can a copy constructor accept an object of the same class as parameter, instead of reference of the object?

A: No. It is specified in the definition of the copy constructor itself. It should generate an error if a programmer specifies a copy constructor with a first argument that is an object and not a reference.

Q: What's the order that local objects are destructed?

A: In reverse order of construction: First constructed, last destructed.

In the following example, b's destructor will be executed first, then a's destructor:

void userCode()

{

Fred a;

Fred b;

...

}

Q: What's the order that objects in an array are destructed?

A: In reverse order of construction: First constructed, last destructed.

In the following example, the order for destructors will be a[9], a[8], ..., a[1], a[0]:

void userCode()

{

Fred a[10];

...

}

Q: Can I overload the destructor for my class?

A: No.  You can have only one destructor for a class Fred. It's always called Fred::~Fred(). It never takes any parameters, and it never returns anything. You can't pass parameters to the destructor anyway, since you never explicitly call a destructor (well, almost never).

Q: Should I explicitly call a destructor on a local variable?

A: No! The destructor will get called again at the close } of the block in which the local was created.  This is a guarantee of the language; it happens automagically; there's no way to stop it from happening. But you can get really bad results from calling a destructor on the same object a second time! Bang! You're dead!

Q: What if I want a local to "die" before the close } of the scope in which it was created? Can I call a destructor on a local if I really want to?

A: No! [For context, please read the previous FAQ]. Suppose the (desirable) side effect of destructing a local File object is to close the File. Now suppose you have an object f of a class File and you want File f to be closed before the end of the scope (i.e., the }) of the scope of object f:

void someCode()

{

File f;

...insert code that should execute when f is still open...

We want the side-effect of f's destructor here!

...insert code that should execute after f is closed...

}

There is a simple solution to this problem. But in the mean time, remember: Do not explicitly call the destructor!

Q: OK, OK already; I won't explicitly call the destructor of a local; but how do I handle the above situation?

A: Simply wrap the extent of the lifetime of the local in an artificial block {...}:

void someCode()

{

{

File f;

...insert code that should execute when f is still open...

} f's destructor will automagically be called here!

...insert code here that should execute after f is closed...}

Q: What if I can't wrap the local in an artificial block?

A: Most of the time, you can limit the lifetime of a local by wrapping the local in an artificial block ({...}). But if for some reason you can't do that, add a member function that has a similar effect as the destructor. But do not call the destructor itself! For example, in the case of class File, you might add a close() method. Typically the destructor will simply call this close() method. Note that the close() method will need to mark the File object so a subsequent call won't re-close an already-closed File. E.g., it might set the fileHandle_ data member to some nonsensical value such as -1, and it might check at the beginning to see if the fileHandle_ is already equal to -1:

class File {

public:

void close();

~File();

...

private:

int fileHandle_; // fileHandle_ >= 0 if/only-if it's open

};

File::~File()

{

close();

}

void File::close()

{

if (fileHandle_ >= ) {

...insert code to call the OS to close the file...

fileHandle_ = -;

}

}

Note that the other File methods may also need to check if the fileHandle_ is -1 (i.e., check if the File is closed). Note also that any constructors that don't actually open a file should set fileHandle_ to -1.

Q: But can I explicitly call a destructor if I've allocated my object with new?

A: Probably not.Unless you used placement new, you should simply delete the object rather than explicitlycalling the destructor.

For example, suppose you allocated the object via a typical new expression:

Fred* p = new Fred();

Then the destructor Fred::~Fred() will automagically get called when you delete it via: delete p; // Automagically calls p->~Fred()

You should not explicitly call the destructor, since doing so won't release the memory that was allocated for the Fred object itself. Remember: delete p does two things: it calls the destructor and it deallocates the memory.

Q: What is "placement new" and why would I use it?

A: There are many uses of placement new. The simplest use is to place an object at a particular location in memory. This is done by supplying the place as a pointer parameter to the new part of a new expression:

#include // Must #include this to use "placement new"

#include "Fred.h" // Declaration of class Fred

void someCode()

{

char memory[sizeof(Fred)]; // Line #1

void* place = memory; // Line #2

Fred* f = new(place) Fred(); // Line #3 (see "DANGER" below)

// The pointers f and place will be equal

...

}

Line #1 creates an array of sizeof(Fred) bytes of memory, which is big enough to hold a Fred object. Line #2 creates a pointer place that points to the first byte of this memory (experienced C programmers will note that this step was unnecessary; it's there only to make the code more obvious). Line #3 essentially just calls the constructor Fred::Fred(). The this pointer in the Fred constructor will be equal to place. The returned pointer f will therefore be equal to place.

ADVICE: Don't use this "placement new" syntax unless you have to. Use it only when you really care that an object is placed at a particular location in memory. For example, when your hardware has a memory-mapped I/O timer device, and you want to place a Clock object at that

memory location.

DANGER: You are taking sole responsibility that the pointer you pass to the "placement new" operator points to a region of memory that is big enough and is properly aligned for the object type that you're creating. Neither the compiler nor the run-time system make any attempt to check whether you did this right. If your Fred class needs to be aligned on a 4 byte boundary but you supplied a location that isn't properly aligned, you can have a serious disaster on your hands (if you don't know what "alignment" means, please don't use the placement new syntax). You have been warned. You are also solely responsible for destructing the placed object. This is done by explicitly calling the destructor:

void someCode()

{

char memory[sizeof(Fred)];

void* p = memory;

Fred* f = new(p) Fred();

...

f->~Fred(); // Explicitly call the destructor for the placed object

}

This is about the only time you ever explicitly call a destructor.

Note: there is a much cleaner but more sophisticated way of handling the destruction / deletion situation.

Q: When I write a destructor, do I need to explicitly call the destructors for my member objects?

A: No. You never need to explicitly call a destructor (except with placement new).

A class's destructor (whether or not you explicitly define one) automagically invokes the destructors for member objects. They are destroyed in the reverse order they appear within the declaration for the class.

class Member {

public:

~Member();

...

};

class Fred {

public:

~Fred();

...

private:

Member x_;

Member y_;

Member z_;

};

Fred::~Fred()

{

// Compiler automagically calls z_.~Member()

// Compiler automagically calls y_.~Member()

// Compiler automagically calls x_.~Member()

}

Q: When I write a derived class's destructor, do I need to explicitly call the destructor for my base class?

A: No. You never need to explicitly call a destructor (except with placement new).

A derived class's destructor (whether or not you explicitly define one) automagically invokes the destructors for base class subobjects. Base classes are destructed after member objects. In the event of multiple inheritance, direct base classes are destructed in the reverse order of their appearance in the inheritance list.

class Member {

public:

~Member();

...

};

class Base {

public:

virtual ~Base(); // A virtual destructor

...

};

class Derived : public Base {

public:

~Derived();

...

private:

Member x_;

};

Derived::~Derived()

{

// Compiler automagically calls x_.~Member()

// Compiler automagically calls Base::~Base()

}

Note: Order dependencies with virtual inheritance are trickier. If you are relying on order dependencies in a virtual inheritance hierarchy, you'll need a lot more information than is in this FAQ.

Q: Is there any difference between List x; and List x();?

A: A big difference!

Suppose that List is the name of some class. Then function f() declares a local List object called x:

void f()

{

List x; // Local object named x (of class List)

...

}

But function g() declares a function called x() that returns a List:

void g()

{

List x(); // Function named x (that returns a List)

...

}

Q: Can one constructor of a class call another constructor of the same class to initialize the this object?

A: Nope.

Let's work an example. Suppose you want your constructor Foo::Foo(char) to call another constructor of the same class, say Foo::Foo(char,int), in order that Foo::Foo(char,int) would help initialize the this object. Unfortunately there's no way to do this in C++.

Some people do it anyway. Unfortunately it doesn't do what they want. For example, the line Foo(x, 0); does not call Foo::Foo(char,int) on the this object. Instead it calls Foo::Foo(char,int) to initialize a temporary, local object (not this), then it immediately destructs that temporary when control flows over the ;.

c

lass Foo {

public:

Foo(char x);

Foo(char x, int y);

...

};

Foo::Foo(char x)

{

...

Foo(x, ); // this line does NOT help initialize the this object!!

...

}

You can sometimes combine two constructors via a default parameter:

class Foo {

public:

Foo(char x, int y=); // this line combines the two constructors

...

};

If that doesn't work, e.g., if there isn't an appropriate default parameter that combines the two

constructors, sometimes you can share their common code in a private init() member function:

class Foo {

public:

Foo(char x);

Foo(char x, int y);

...

private:

void init(char x, int y);

};

Foo::Foo(char x)

{

init(x, int(x) + );

...

}

Foo::Foo(char x, int y)

{

init(x, y);

...

}

void Foo::init(char x, int y)

{

...

}

BTW do NOT try to achieve this via placement new. Some people think they can say new(this) Foo(x, int(x)+7) within the body of Foo::Foo(char). However that is bad, bad, bad. Please don't write me and tell me that it seems to work on your particular version of your particular compiler; it's bad. Constructors do a bunch of little magical things behind the scenes, but that bad technique steps on those partially constructed bits. Just say no.

Q: Is the default constructor for Fred always Fred::Fred()?

A: No. A "default constructor" is a constructor that can be called with no arguments. One example of this is a constructor that takes no parameters:

class Fred {

public:

Fred(); // Default constructor: can be called with no args

...

};

Another example of a "default constructor" is one that can take arguments, provided they are given default values:

class Fred {

public:

Fred(int i=, int j=); // Default constructor: can be called with no args

...

};

Q: Which constructor gets called when I create an array of Fred objects?

A: Fred's default constructor (except as discussed below).

class Fred {

public:

Fred();

...

};

int main()

{

Fred a[]; calls the default constructor  times

Fred* p = new Fred[]; calls the default constructor  times

...

}

If your class doesn't have a default constructor, you'll get a compile-time error when you attempt to create an array using the above simple syntax:

c

lass Fred {

public:

Fred(int i, int j); assume there is no default constructor

...

};

int main()

{

Fred a[]; ERROR: Fred doesn't have a default constructor

Fred* p = new Fred[]; ERROR: Fred doesn't have a default constructor

...

}

However, even if your class already has a default constructor, you should try to use std::vector rather than an array (arrays are evil). std::vector lets you decide to use any constructor, not just the default constructor:

#include

int main()

{

std::vector a(, Fred(,)); the  Fred objects in std::vector a will be initialized with Fred(,)

...

}

Even though you ought to use a std::vector rather than an array, there are times when an array might be the right thing to do, and for those, you might need the "explicit initialization of arrays"

syntax. Here's how:

class Fred {

public:

Fred(int i, int j); assume there is no default constructor

...

};

int main()

{

Fred a[] = {

Fred(,), Fred(,), Fred(,), Fred(,), Fred(,), // The 10 Fred objects are

Fred(,), Fred(,), Fred(,), Fred(,), Fred(,) // initialized using Fred(5,7)

};

...

}

Q: Should my constructors use "initialization lists" or "assignment"?

A: Initialization lists. In fact, constructors should initialize as a rule all member objects in theinitialization list.  （ 省略若干处）

Q: Should you use the this pointer in the constructor?

A: Some people feel you should not use the this pointer in a constructor because the object is not fully formed yet. However you can use this in the constructor (in the {body} and even in the initialization list) if you are careful. (省略若干处）

Q: What is the "Named Constructor Idiom"?

A: A technique that provides more intuitive and/or safer construction operations for users of your class.

参考：  C++ Interview Questions ，Compiled by Dr. Fatih Kocan, Wael Kdouh, and Kathryn Patterson for the Data Structures in C++ course(CSE 3358) Spring 2008