C++ Interview Questions and Answers for 1 Year Experienced

  1. How many storage classes are available in C++?
    Storage class are used to specify the visibility/scope and life time of symbols(functions and variables). That means, storage classes specify where all a variable or function can be accessed and till what time those variables will be available during the execution of program.
    Following storage classes are available in C++
    It’s the default storage class for local variables. They can be accessed only from with in the declaration scope. auto variables are allocated at the beginning of enclosing block and deallocated at the end of enclosing block.
    void changeValue(void)
    auto int i = 1 ;
    printf ( “%d “, i ) ;} int main() { changeValue(); changeValue(); changeValue(); changeValue(); return 0; }Output:- 2 2 2 2 In the above example, every time the method changeValue is invoked, memory is allocated for i and de allocated at the end of the method. So it’s output will be same. register It’s similar to auto variables. Difference is that register variables might be stored on the processor register instead of RAM, that means the maximum size of register variable should be the size of CPU register ( like 16bit, 32bit or 64bit). This is normally used for frequently accessed variables like counters, to improve performance. But note that, declaring a variable as register does not mean that they will be stored in the register. It depends on the hardware and implementation. int main() { register int i; int array[10] = {0,1,2,3,4,5,6,7,8,9}; for (i=0;i<10;i++) { printf(“%d “, array[i]) } return 0; }Output:- 0 1 2 3 4 5 6 7 8 9 The variable i might be stored on the CPU register and due to which the access of i in the loop will be faster static A static variable will be kept in existence till the end of the program unlike creating and destroying each time they move into and out of the scope. This helps to maintain their value even if control goes out of the scope. When static is used with global variables, they will have internal linkage, that means it cannot be accessed by other source files. When static is used in case of a class member, it will be shared by all the objects of a class instead of creating separate copies for each object. void changeValue(void) { static int i = 1 ; i++; printf ( “%d “, i ) ;}int main() { changeValue();changeValue(); changeValue(); changeValue(); return 0; }Output:- 2 3 4 5 Since static variable will be kept in existence till the end of program, variable i will retain it’s value across the method invocations. extern extern is used to tell compiler that the symbol is defined in another translation unit (or in a way, source files) and not in the current one. Which means the symbol is linked externally. extern symbols have static storage duration, that is accessible through out the life of program. Since no storage is allocated for extern variable as part of declaration, they cannot be initialized while declaring int x = 10; int main( ) { extern int y ; printf(“x: %d “, x ); printf(“y: %d”, y); return 0; } int y = 70 ;Output:- x: 10 y: 70 extern variable is like global variable, it’s scope is through out the program. It can be defined anywhere in the c++ program. mutable mutable storage class can be used only on non static non const data a member of a class. Mutable data member of a class can be modified even is it’s part of an object which is declared as const. class Test { public: Test(): x(1), y(1) {}; mutable int x; int y; };int main() { const Test object; object.x = 123; //object.y = 123; /* * The above line if uncommented, will create compilation error. */ cout<< “X:”<< object.x << “, Y:” << object.y; return 0; }Output:- X:123, Y:1 In the above example, we are able to change the value of member variable x though it’s part of an object which is declared as const. This is because the variable x is declared as mutable. But if you try to modify the value of member variable y, compiler will throw error.You can find the summary of c++ storage class specifiers below C++ Storage Specifier Storage Location Scope Of Variable Life Time auto Memory (RAM) Local With in function static Memory (RAM) Local Life time is from when the flow reaches the first declaration to the termination of program. register CPU register Local With in function extern Memory (RAM) Global Till the end of main program
  2. What is ‘Copy Constructor’ and when it is called?
    This is a frequent c++ interview question. Copy constructor is a special constructor of a class which is used to create copy of an object. Compiler will give a default copy constructor if you don’t define one. This implicit constructor will copy all the members of source object to target object.
    Implicit copy constructors are not recommended, because if the source object contains pointers they will be copied to target object, and it may cause heap corruption when both the objects with pointers referring to the same location does an update to the memory location. In this case its better to define a custom copy constructor and do a deep copy of the object.
    class SampleClass{
    int* ptr;
    // Copy constructor declaration
    SampleClass(SampleClass &obj);
    }; SampleClass::SampleClass(){ ptr = new int(); *ptr = 5; } // Copy constructor definition SampleClass::SampleClass(SampleClass &obj){ //create a new object for the pointer ptr = new int(); // Now manually assign the value *ptr = *(obj.ptr); cout<<"Copy constructor...\n"; }
  3. What is realloc() and free()? What is difference between them?
    void* realloc (void* ptr, size_t size)
    This function is used to change the size of memory object pointed by address ptr to the size given by size. If ptr is a null pointer, then realloc will behave like malloc(). If the ptr is an invalid pointer, then defined behaviour may occur depending the implementation. Undefined behaviour may occur if the ptr has previously been deallocated by free(), or dealloc() or ptr do not match a pointer returned by an malloc(), calloc() or realloc().
    void free (void* ptr)
    This function is used to deallocate a block of memory that was allocated using malloc(), calloc() or realloc(). If ptr is null, this function does not doe anything.
  4. What is difference between shallow copy and deep copy? Which is default?
[This question can be expected in any interviews, not just c++ interviews. This is a usual question in most of the java interviews.] When you do a shallow copy, all the fields of the source object is copied to target object as it is. That means, if there is a dynamically created field in the source object, shallow copy will copy the same pointer to target object. So you will have two objects with fields that are pointing to same memory location which is not what you usually want.
In case of deep copy, instead of copying the pointer, the object itself is copied to target. In this case if you modify the target object, it will not affect the source. By default copy constructors and assignment operators do shallow copy. To make it as deep copy, you need to create a custom copy constructor and override assignment operator. 5 .What do you mean by persistent and non persistent objects?

[This question may be asked in many ways during c++ interviews, like how to send an object to a remote computer or how to save the your program state across application restarts. All these are related to serialization.] Persistent objects are the ones which we can be serialized and written to disk, or any other stream. So before stopping your application, you can serialize the object and on restart you can deserialize it. [ Drawing applications usually use serializations.] Objects that can not be serialized are called non persistent objects. [ Usually database objects are not serialized because connection and session will not be existing when you restart the application. ] 6. Is it possible to get the source code back from binary file? Technically it is possible to generate the source code from binary. It is called reverse engineering. There are lot of reverse engineering tools available. But, in actual case most of them will not re generate the exact source code back because many information will be lost due to compiler optimization and other interpretations.

7. What are virtual functions and what is its use?[This is a sure question in not just C++ interviews, but any other OOP language interviews. Virtual functions are the important concepts in any object oriented language, not just from interview perspective. Virtual functions are used to implement run time polymorphism in c++.] Virtual functions are member functions of class which is declared using keyword ‘virtual’. When a base class type reference is initialized using object of sub class type and an overridden method which is declared as virtual is invoked using the base reference, the method in child class object will get invoked.
class Base
int a;
a = 1;
virtual void method()
cout << a;

class Child: public Base
int b;
b = 2;
virtual void method()
cout << b;

int main()
Base *pBase;
Child oChild;
pBase = &oChild;
return 0;
In the above example even though the method in invoked on Base class reference, method of the child will get invoked since its declared as virtual.8.What do you mean by pure virtual functions in C++? Give an example?

Pure virtual function is a function which doesn’t have an implementation and the same needs to be implemented by the the next immediate non-abstract class. (A class will become an abstract class if there is at-least a single pure virtual function and thus pure virtual functions are used to create interfaces in c++).

9. How to create a pure virtual function?
A function is made as pure virtual function by the using a specific signature, ” = 0″ appended to the function declaration as given below,
class SymmetricShape {
// draw() is a pure virtual function.
virtual void draw() = 0;
};9.Why pure virtual functions are used if they don’t have implementation / When does a pure virtual function become useful?

Pure virtual functions are used when it doesn’t make sense to provide definition of a virtual function in the base class or a proper definition does not exists in the context of base class. Consider the above example, class SymmetricShape is used as base class for shapes with symmetric structure(Circle, square, equilateral triangle etc). In this case, there exists no proper definition for function draw() in the base class SymmetricShape instead the child classes of SymmetricShape (Cirlce, Square etc) can implement this method and draw proper shape. 10.What is virtual destructors? Why they are used?[This c++ interview question is in a way related to polymorphism.] Virtual destructors are used for the same purpose as virtual functions. When you remove an object of subclass, which is referenced by a parent class pointer, only destructor of base class will get executed. But if the destructor is defined using virtual keyword, both the destructors [ of parent and sub class ] will get invoked.What you mean by early binding and late binding? How it is related to dynamic binding?[This c++ interview question is related to question about virtual functions ] Binding is the process of linking actual address of functions or identifiers to their reference. This happens mainly two times.
During compilation : This is called early binding
For all the direct function references compiler will replace the reference with actual address of the method.
At runtime : This is called late binding.
In case of virtual function calls using a Base reference, as in shown in the example of question no: 2, compiler does not know which method will get called at run time. In this case compiler will replace the reference with code to get the address of function at runtime. Dynamic binding is another name for late binding.

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