C++ Interview questions Part 11

C++ interview question:What is slicing?
Answer:Slicing means that the data added by a subclass are discarded when an object of the subclass is passed or returned by value or from a function expecting a base class object. 
Explanation:
Consider the following class declaration:
class base
{
...
base& operator =(const base&);
base (const base&);
}
void fun( )
{
base e=m;
e=m;
As base copy functions don't know anything about the derived only the base part of the derived is copied. This is commonly referred to as slicing. One reason to pass objects of classes in a hierarchy is to avoid slicing. Other reasons are to preserve polymorphic behavior and to gain efficiency. 

C++ interview question:What is name mangling?
Answer:Name mangling is the process through which your c++ compilers give each function in your program a unique name. In C++, all programs have at-least a few functions with the same name. Name mangling is a concession to the fact that linker always insists on all function names being unique.
Example:
In general, member names are made unique by concatenating the name of the member with that of the class e.g. given the declaration:
class Bar
{
public: 
int ival;
...
};
ival becomes something like:
// a possible member name mangling
ival__3Bar
Consider this derivation:
class Foo : public Bar 
public:
int ival;
...
}
The internal representation of a Foo object is the concatenation of its base and derived class members.
// Pseudo C++ code
// Internal representation of Foo
class Foo
{
public:
int ival__3Bar;
int ival__3Foo;
...
};
Unambiguous access of either ival members is achieved through name mangling. Member functions, because they can be overloaded, require an extensive mangling to provide each with a unique name. Here the compiler generates the same name for the two overloaded instances(Their argument lists make their instances unique). 



C++ interview question:What are proxy objects?
Answer:Objects that points to other objects are called proxy objects or surrogates. Its an object that provides the same interface as its server object but does not have any functionality. During a method invocation, it routes data to the true server object and sends back the return value to the object. 

C++ interview question:Differentiate between declaration and definition in C++.
Answer:A declaration introduces a name into the program; a definition provides a unique description of an entity (e.g. type, instance, and function). Declarations can be repeated in a given scope, it introduces a name in a given scope. There must be exactly one definition of every object, function or class used in a C++ program. 
A declaration is a definition unless:
Ø it declares a function without specifying its body,
Ø it contains an extern specifier and no initializer or function body,
Ø it is the declaration of a static class data member without a class definition,
Ø it is a class name definition,
Ø it is a typedef declaration.
A definition is a declaration unless:
Ø it defines a static class data member,
Ø it defines a non-inline member function.

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C++ Interview questions Part 10

C++ interview question:When can you tell that a memory leak will occur?
Answer:A memory leak occurs when a program loses the ability to free a block of dynamically allocated memory. 

C++ interview question:What is a parameterized type?
Answer:A template is a parameterized construct or type containing generic code that can use or manipulate any type. It is called parameterized because an actual type is a parameter of the code body. Polymorphism may be achieved through parameterized types. This type of polymorphism is called parameteric polymorphism. Parameteric polymorphism is the mechanism by which the same code is used on different types passed as parameters. 

C++ interview question:Differentiate between a deep copy and a shallow copy? 
Answer:Deep copy involves using the contents of one object to create another instance of the same class. In a deep copy, the two objects may contain ht same information but the target object will have its own buffers and resources. the destruction of either object will not affect the remaining object. The overloaded assignment operator would create a deep copy of objects.
             Shallow copy involves copying the contents of one object into another instance of the same class thus creating a mirror image. Owing to straight copying of references and pointers, the two objects will share the same externally contained contents of the other object to be unpredictable.

Explanation:
                   Using a copy constructor we simply copy the data values member by member. This method of copying is called shallow copy. If the object is a simple class, comprised of built in types and no pointers this would be acceptable. This function would use the values and the objects and its behavior would not be altered with a shallow copy, only the addresses of pointers that are members are copied and not the value the address is pointing to. The data values of the object would then be inadvertently altered by the function. When the function goes out of scope, the copy of the object with all its data is popped off the stack. 
                    If the object has any pointers a deep copy needs to be executed. With the deep copy of an object, memory is allocated for the object in free store and the elements pointed to are copied. A deep copy is used for objects that are returned from a function. 

C++ interview question:What is an opaque pointer?
Answer:A pointer is said to be opaque if the definition of the type to which it points to is not included in the current translation unit. A translation unit is the result of merging an implementation file with all its headers and header files. 

C++ interview question:What is a smart pointer?
Answer:A smart pointer is an object that acts, looks and feels like a normal pointer but offers more functionality. In C++, smart pointers are implemented as template classes that encapsulate a pointer and override standard pointer operators. They have a number of advantages over regular pointers. They are guaranteed to be initialized as either null pointers or pointers to a heap object. Indirection through a null pointer is checked. No delete is ever necessary. Objects are automatically freed when the last pointer to them has gone away. One significant problem with these smart pointers is that unlike regular pointers, they don't respect inheritance. Smart pointers are unattractive for polymorphic code. Given below is an example for the implementation of smart pointers.

Example: 
template 
class smart_pointer
{
public:
smart_pointer(); // makes a null pointer
smart_pointer(const X& x) // makes pointer to copy of x
X& operator *( );
const X& operator*( ) const;
X* operator->() const;
smart_pointer(const smart_pointer &);
const smart_pointer & operator =(const smart_pointer&);
~smart_pointer():
private:
//...
};
This class implement a smart pointer to an object of type X. The object itself is located on the heap. Here is how to use it:
smart_pointer p= employee("Harris",1333);
Like other overloaded operators, p will behave like a regular pointer,
cout<<*p;
p->raise_salary(0.5);

C++ interview question:What is reflexive association?
Answer:The 'is-a' is called a reflexive association because the reflexive association permits classes to bear the is-a association not only with their super-classes but also with themselves. It differs from a 'specializes-from' as 'specializes-from' is usually used to describe the association between a super-class and a sub-class. For example:
Printer is-a printer.

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C++ Interview questions Part 9

C++ interview question: What is a node class?
Answer:A node class is a class that,
Ø relies on the base class for services and implementation,
Ø provides a wider interface to te users than its base class,
Ø relies primarily on virtual functions in its public interface
Ø depends on all its direct and indirect base class
Ø can be understood only in the context of the base class
Ø can be used as base for further derivation
Ø can be used to create objects.
A node class is a class that has added new services or functionality beyond the services inherited from its base class. 

C++ interview question: What is an orthogonal base class?
Answer:If two base classes have no overlapping methods or data they are said to be independent of, or orthogonal to each other. Orthogonal in the sense means that two classes operate in different dimensions and do not interfere with each other in any way. The same derived class may inherit such classes with no difficulty.

C++ interview question:What is a container class? What are the types of container classes?
Answer:A container class is a class that is used to hold objects in memory or external storage. A container class acts as a generic holder. A container class has a predefined behavior and a well-known interface. A container class is a supporting class whose purpose is to hide the topology used for maintaining the list of objects in memory. When a container class contains a group of mixed objects, the container is called a heterogeneous container; when the container is holding a group of objects that are all the same, the container is called a homogeneous container.

C++ interview question:What is a protocol class?
Answer:An abstract class is a protocol class if:
Ø it neither contains nor inherits from classes that contain member data, non-virtual functions, or private (or protected) members of any kind.
Ø it has a non-inline virtual destructor defined with an empty implementation,
Ø all member functions other than the destructor including inherited functions, are declared pure virtual functions and left undefined.

C++ interview question:What is a mixin class?
Answer:A class that provides some but not all of the implementation for a virtual base class is often called mixin. Derivation done just for the purpose of redefining the virtual functions in the base classes is often called mixin inheritance. Mixin classes typically don't share common bases.

C++ interview question:What is a concrete class?
Answer:A concrete class is used to define a useful object that can be instantiated as an automatic variable on the program stack. The implementation of a concrete class is defined. The concrete class is not intended to be a base class and no attempt to minimize dependency on other classes in the implementation or behavior of the class.

C++ interview question:What is the handle class?
Answer:A handle is a class that maintains a pointer to an object that is programmatically accessible through the public interface of the handle class.

Explanation:
           In case of abstract classes, unless one manipulates the objects of these classes through pointers and references, the benefits of the virtual functions are lost. User code may become dependent on details of implementation classes because an abstract type cannot be allocated statistically or on the stack without its size being known. Using pointers or references implies that the burden of memory management falls on the user. Another limitation of abstract class object is of fixed size. Classes however are used to represent concepts that require varying amounts of storage to implement them.
           A popular technique for dealing with these issues is to separate what is used as a single object in two parts: a handle providing the user interface and a representation holding all or most of the object's state. The connection between the handle and the representation is typically a pointer in the handle. Often, handles have a bit more data than the simple representation pointer, but not much more. Hence the layout of the handle is typically stable, even when the representation changes and also that handles are small enough to move around relatively freely so that the user needn’t use the pointers and the references. 

C++ interview question:What is an action class?
Answer:The simplest and most obvious way to specify an action in C++ is to write a function. However, if the action has to be delayed, has to be transmitted 'elsewhere' before being performed, requires its own data, has to be combined with other actions, etc then it often becomes attractive to provide the action in the form of a class that can execute the desired action and provide other services as well. Manipulators used with iostreams is an obvious example.

Explanation:
A common form of action class is a simple class containing just one virtual function.
class Action
{
public:
virtual int do_it( int )=0;
virtual ~Action( );
}
Given this, we can write code say a member that can store actions for later execution without using pointers to functions, without knowing anything about the objects involved, and without even knowing the name of the operation it invokes. For example:
class write_file : public Action
{
File& f;
public:
int do_it(int)
{
return fwrite( ).suceed( );
}
};
class error_message: public Action
{
response_box db(message.cstr( ),"Continue","Cancel","Retry");
switch (db.getresponse( ))
{
case 0: return 0;
case 1: abort();
case 2: current_operation.redo( );return 1;
}
}; 
A user of the Action class will be completely isolated from any knowledge of derived classes such as write_file and error_message.

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C++ Interview questions Part 8

C++ interview question:What is a Null object?
Answer:It is an object of some class whose purpose is to indicate that a real object of that class does not exist. One common use for a null object is a return value from a member function that is supposed to return an object with some specified properties but cannot find such an object. 

C++ interview question:What is class invariant?
Answer:A class invariant is a condition that defines all valid states for an object. It is a logical condition to ensure the correct working of a class. Class invariants must hold when an object is created, and they must be preserved under all operations of the class. In particular all class invariants are both preconditions and post-conditions for all operations or member functions of the class. 

C++ interview question:What do you mean by Stack unwinding?
Answers:It is a process during exception handling when the destructor is called for all local objects between the place where the exception was thrown and where it is caught. 

C++ interview question:Define precondition and post-condition to a member function.
Answers:
Precondition:
       A precondition is a condition that must be true on entry to a member function. A class is used correctly if preconditions are never false. An operation is not responsible for doing anything sensible if its precondition fails to hold. 
       For example, the interface invariants of stack class say nothing about pushing yet another element on a stack that is already full. We say that isful() is a precondition of the push operation.

Post-condition:
        A post-condition is a condition that must be true on exit from a member function if the precondition was valid on entry to that function. A class is implemented correctly if post-conditions are never false.
        For example, after pushing an element on the stack, we know that isempty() must necessarily hold. This is a post-condition of the push operation. 

C++ interview question:What are the conditions that have to be met for a condition to be an invariant of the class?
Answers:
Ø The condition should hold at the end of every constructor.
Ø The condition should hold at the end of every mutator(non-const) operation. 

C++ interview question:What are proxy objects?
Answer:Objects that stand for other objects are called proxy objects or surrogates.
Example:
template
class Array2D 
{
public:
class Array1D
{
public:
T& operator[] (int index);
const T& operator[] (int index) const;
..
};
Array1D operator[] (int index);
const Array1D operator[] (int index) const;
...
};
The following then becomes legal:
Array2Ddata(10,20);
........
cout<

Here data[3] yields an Array1D object and the operator [] invocation on that object yields the float in position(3,6) of the original two dimensional array. Clients of the Array2D class need not be aware of the presence of the Array1D class. Objects of this latter class stand for one-dimensional array objects that, conceptually, do not exist for clients of Array2D. Such clients program as if they were using real, live, two-dimensional arrays. Each Array1D object stands for a one-dimensional array that is absent from a conceptual model used by the clients of Array2D. In the above example, Array1D is a proxy class. Its instances stand for one-dimensional arrays that, conceptually, do not exist. 

C++ interview question:Name some pure object oriented languages.
Answer:
Ø Smalltalk, 
Ø Java, 
Ø Eiffel, 
Ø Sather. 

C++ interview question:Name the operators that cannot be overloaded.
Answers:sizeof . .* .-> :: ?:

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C++ Interview Questions Part 7

C++ interview question:Define namespace.
Answer:It is a feature in c++ to minimize name collisions in the global name space. This namespace keyword assigns a distinct name to a library that allows other libraries to use the same identifier names without creating any name collisions. Furthermore, the compiler uses the namespace signature for differentiating the definitions. 

C++ interview question:What is the use of ‘using’ declaration.
Answer:A using declaration makes it possible to use a name from a namespace without the scope operator. 

C++ interview question:What is an Iterator class?
Answer:A class that is used to traverse through the objects maintained by a container class. There are five categories of iterators: 
Ø input iterators,
Ø output iterators,
Ø forward iterators,
Ø bidirectional iterators,
Ø random access.

             An iterator is an entity that gives access to the contents of a container object without violating encapsulation constraints. Access to the contents is granted on a one-at-a-time basis in order. The order can be storage order (as in lists and queues) or some arbitrary order (as in array indices) or according to some ordering relation (as in an ordered binary tree). The iterator is a construct, which provides an interface that, when called, yields either the next element in the container, or some value denoting the fact that there are no more elements to examine. Iterators hide the details of access to and update of the elements of a container class.

               The simplest and safest iterators are those that permit read-only access to the contents of a container class. The following code fragment shows how an iterator might appear in code:
cont_iter:=new cont_iterator();
x:=cont_iter.next();
while x/=none do
..
s(x);
...
x:=cont_iter.next();
end;
In this example, cont_iter is the name of the iterator. It is created on the first line by instantiation of cont_iterator class, an iterator class defined to iterate over some container class, cont. Succesive elements from the container are carried to x. The loop terminates when x is bound to some empty value. (Here, none)In the middle of the loop, there is s(x) an operation on x, the current element from the container. The next element of the container is obtained at the bottom of the loop. 

C++ interview question::List out some of the OODBMS available.
Answer:
Ø GEMSTONE/OPAL of Gemstone systems.
Ø ONTOS of Ontos.
Ø Objectivity of Objectivity inc.
Ø Versant of Versant object technology.
Ø Object store of Object Design
Ø ARDENT of ARDENT software.
Ø POET of POET software.

C++ interview question:What is an incomplete type?
Answer:Incomplete types refers to pointers in which there is non availability of the implementation of the referenced location or it points to some location whose value is not available for modification.
Example:
int *i=0x400 // i points to address 400
*i=0; //set the value of memory location pointed by i.
Incomplete types are otherwise called uninitialized pointers.

C++ interview Question:What is a dangling pointer?
Answer:A dangling pointer arises when you use the address of an object after its lifetime is over.This may occur in situations like returning addresses of the automatic variables from a function or using the address of the memory block after it is freed.

C++ interview question:Differentiate between the message and method 
Answer:
Message Method
1.Objects communicate by sending messages Provides response to a message to each other.
2.A message is sent to invoke a method. It is an implementation of an operation.

C++ interview question:What is an adaptor class or Wrapper class?
Answer:A class that has no functionality of its own. Its member functions hide the use of a third party software component or an object with the non-compatible interface or a non- object- oriented implementation.

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C++ Interview Questions Part 6

C++ interview question: What are virtual functions? 
Answer: C++ virtual function is a member function of a class, whose functionality can be over-ridden in its derived classes.
C++ virtual function is, 
* A member function of a class 
* Declared with virtual keyword 
* usually has a different functionality in the derived class 
* A function call is resolved at run-time 

C++ interview question: We can overload assignment operator as a normal function.But we can not overload assignment operator as friend function why?
Answer :If the operation modifies the state of the class object, it operates on, it must be a member function, not a friend fucntionThus all operator such as =, *=, +=, etc are naturally defined as member functions not friend functions Conversely, if the operator does not modify any of its operands, but needs only a representation of the object, it does not have to be a member function and often less confusing. This is the reason why binary operators are often implemented as friend functions such as + , *, -, etc..


C++ interview question: What is the difference between class and structure? 
Answer: 1:By default, the members of structures are public while that for class is private
2: structures doesn't provide something like data hiding which is provided by the classes
3: structures contains only data while class bind both data and member functions 

C++ interview question: What is virtual class and friend class? 
Answer: Friend classes are used when two or more classes are designed to work together and virtual base class aids in multiple inheritance.

C++ interview question: Is there any way to write a class such that no class can be inherited from it. Please include code 
Answer: Simple, make all constructors of the class private.

C++ interview question: Why can’t we overload the sizeof, :?, :: ., .* operators in c++ 
Answer: The restriction is for safety. For example if we overload. Operator then we can’t access member in normal way for that we have to use ->.

C++ interview question :What is importance of const. pointer in copy constructor? 
Answer :Because otherwise you will pass the object to copy as an argument of copy constructor as pass by value which by definition creates a copy and so on... an infinite call chain

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C++ Interview Questions Part 5

C++ interview question: What will happen if I say delete this
Answer: if you say "delete this", you are effectively calling the destructor twice, which could well be a disaster if your class uses heap. The destructor will be called when you say “delete this” and again when that object goes out of scope. Since this is the language behavior, there is no way to prevent the destructor from being called twice. Please refrain from forcibly calling a destructor or using clause like this. 

C++ interview question: What is the output of printf ("%d")
Answer :Usually the output value cannot be predicted. It will not give any error. It will print a garbage value. But if the situation is 
main()
{
int a=1,b=2,c=3;
printf("%d");
}
The output will be the value of the last variable, ie. 3 

C++ interview question: # what is an algorithm (in terms of the STL/C++ standard library)?
Answer: Algorithm in STL consist different searching and sorting algos implementation, which takes start and end iterators of STL container on which algo is going to work. 

C++ interview question: How can you force instantiation of a template? 
Answer: you can instantiate a template in two ways. 1. Implicit instantiation and 2. Explicit Instantiation implicit instatanitioan can be done by the following ways:

template 
class A
{
public: 
A(){}
~A(){}
void x();
void z();
};
void main()
{
A ai;
A af;
}

External Instantion can be done the following way:
int main()
{
template class A;
template class A;

C++ interview question: What is the difference between operator new and the new operator? 
Answer: This is what happens when you create a new object: 1. the memory for the object is allocated using "operator new". 2. the constructor of the class is invoked to properly initialize this memory. As you can see, the new operator does both 1 and 2. The operator new merely allocates memory, it does not initialize it. Where as the new operator also initializes it properly by calling the constructor. 

C++ interview question: What is the Basic nature of "cin" and "cout" and what concept or principle we are using on those two? 
Answer: Basically "cin and cout" are INSTANCES of istream and ostream classes respectively. And the concept which is used on cin and cout is operator overloading. Extraction and Insertion operators are overloaded for input and ouput operations.

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C++ Interview Questions Part 4










C++ Interview Question: What is "strstream”?
Answer: Class that reads and writes to an array in memory 

C++ interview question: Can we generate a C++ source code from the binary file? 
Answer: Technically this is possible, but in my knowledge their no such software available yet. Why this is possible? In program flow we do like this to generate binary file. High level language programming code -low level programming code- hex code- binary code. How we can do reverse can be illustrated with this example. When I type 0 on screen the ASCII equivalent is 65 and so the binary code will be by converting 65 (01010 0101) so I can recognize this and decode this. Same technique can be used. Some secret mission defense org. I heard have this code splitter from binary to assembly language (low level language)/ Converter type devices available, they use them for secret national purpose. 

C++ interview question: Explain "passing by value", "passing by pointer" and "passing by reference" 
Answer: There is major difference between these three are when we want to avoid making the copy of variable and we want to change value of actual argument on calling function. There are we use passing by pointer, passing the reference. We can not perform arithmetic operation on reference. 

C++ interview question: Difference between "vector" and "array"? 
Answer: Vector and Array List are very similar. Both of them represent a 'grow able array', where you access to the elements in it through an index.Array List it's part of the Java Collection Framework, and has been added with version 1.2, while Vector it's an object that is present since the first version of the JDK. Vector, anyway, has been retrofitted to implement the List interface.The main difference is that Vector it's a synchronized object, while Array List it's not.While the iterator that are returned by both classes are fail-fast (they cleanly throw Concurrent Modification Exception when the original object has been modified), the Enumeration returned by Vector are not.Unless you have strong reason to use a Vector, the suggestion is to use the Array List. 

C++ interview question: What are the types of STL containers? 
Answer: deque 
hash_map 
hash_multimap 
hash_multiset 
hash_set 
list
map 
multimap 
multiset 
set 
vector 

C++ interview question:Difference between a "assignment operator" and a "copy constructor" 
Answer :Copy constructor is called every time a copy of an object is made. When you pass an object by value, either into a function or as a function's return value, a temporary copy of that object is made. Assignment operator is called whenever you assign to an object. Assignment operator must check to see if the right-hand side of the assignment operator is the object itself. It executes only the two sides are not equal 

C++ interview question: Can we have "Virtual Constructors"?
Answer: No, we cannot have virtual constructors. But if the need arises, we can simulate the implementation of virtual constructor by calling a Init method from the constructor which, should be a virtual function. 

C++ interview question: Explain the need for "Virtual Destructor".
Answer: In case of inheritance, objects should be destructed exactly the opposite way of their construction. If virtual keyword is not added before base class destructor declaration, then derived class destructor will not at all be called. Hence there will be memory leakage if allocated for derived class members while constructing the object.

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