1.What is difference between C and C++ ?
- C++ is Multi-Paradigm( not pure OOP, supports both procedural and object oriented) while C follows procedural style programming.
- In C data security is less, but in C++ you can use modifiers for your class members to make it inaccessible from outside.
- C follows top-down approach ( solution is created in step by step manner, like each step is processed into details as we proceed ) but C++ follows a bottom-up approach ( where base elements are established first and are linked to make complex solutions ).
- C++ supports function overloading while C does not support it.
- C++ allows use of functions in structures, but C does not permit that.
- C++ supports reference variables( two variables can point to same memory location ). C does not support this.
- C does not have a built in exception handling framework, though we can emulate it with other mechanism. C++ directly supports exception handling, which makes life of developer easy.
2.What is a class?
[Probably this would be the first question for a Java/c++ technical interview for freshers and sometimes for experienced as well. Try to give examples when you answer this question.]
Class defines a datatype, it’s type definition of category of thing(s). But a class actually does not define the data, it just specifies the structure of data. To use them you need to create objects out of the class. Class can be considered as a blueprint of a building, you can not stay inside blueprint of building, you need to construct building(s) out of that plan. You can create any number of buildings from the blueprint, similarly you can create any number of objects from a class.
class Vehicle
{
public:
int numberOfTyres;
double engineCapacity;
void drive(){
// code to drive the car
}
};
3.What is an Object/Instance?
Object is the instance of a class, which is concrete. From the above example, we can create instance of class Vehicle as given below
Vehicle vehicleObject;
We can have different objects of the class Vehicle, for example we can have Vehicle objects with 2 tyres, 4tyres etc. Similarly different engine capacities as well.
4.What do you mean by C++ access specifiers ?
[Questions regarding access specifiers are common not just in c++ interview but for other object oriented language interviews as well.]
Access specifiers are used to define how the members (functions and variables) can be accessed outside the class. There are three access specifiers defined which are public, private, and protected
- private:
Members declared as private are accessible only with in the same class and they cannot be accessed outside the class they are declared. - public:
Members declared as public are accessible from any where. - protected:
Members declared as protected can not be accessed from outside the class except a child class. This access specifier has significance in the context of inheritance.
5.What are the basics concepts of OOPS?
- Classes and Objects
Refer Questions 2 and 3 for the concepts about classes and objects
- Encapsulation
Encapsulation is the mechanism by which data and associated operations/methods are bound together and thus hide the data from outside world. It’s also called data hiding. In c++, encapsulation achieved using the access specifiers (private, public and protected). Data members will be declared as private (thus protecting from direct access from outside) and public methods will be provided to access these data. Consider the below class
class Person
{
private:
int age;
public:
int getAge(){
return age;
}
int setAge(int value){
if(value > 0){
age = value;
}
}
};
In the class Person, access to the data field age is protected by declaring it as private and providing public access methods. What would have happened if there was no access methods and the field age was public? Anybody who has a Person object can set an invalid value (negative or very large value) for the age field. So by encapsulation we can preventing direct access from outside, and thus have complete control, protection and integrity of the data.
- Data abstraction
Data abstraction refers to hiding the internal implementations and show only the necessary details to the outside world. In C++ data abstraction is implemented using interfaces and abstract classes.
class Stack
{
public:
virtual void push(int)=0;
virtual int pop()=0;
};
class MyStack : public Stack
{
private:
int arrayToHoldData[]; //Holds the data from stack
public:
void push(int) {
// implement push operation using array
}
int pop()
{
// implement pop operation using array
}
};
In the above example, the outside world only need to know about the Stack class and its push, pop operations. Internally stack can be implemented using arrays or linked lists or queues or anything that you can think of. This means, as long as the push and pop method performs the operations work as expected, you have the freedom to change the internal implementation with out affecting other applications that use your Stack class.
- Inheritance
Inheritance allows one class to inherit properties of another class. In other words, inheritance allows one class to be defined in terms of another class.
class SymmetricShape
{
public:
int getSize()
{
return size;
}
void setSize(int w)
{
size = w;
}
protected:
int size;
};
// Derived class
class Square: public SymmetricShape
{
public:
int getArea()
{
return (size * size);
}
};
In the above example, class Square inherits the properties and methods of class SymmetricShape. Inheritance is the one of the very important concepts in C++/OOP. It helps to modularise the code, improve reusability and reduces tight coupling between components of the system.
6.What is the use of volatile keyword in c++? Give an example.
Most of the times compilers will do optimization to the code to speed up the program. For example in the below code,
- int a = 10;
- while( a == 10){
- // Do something
- }
compiler may think that value of ‘a’ is not getting changed from the program and replace it with ‘while(true)’, which will result in an infinite loop. In actual scenario the value of ‘a’ may be getting updated from outside of the program.
Volatile keyword is used to tell compiler that the variable declared using volatile may be used from outside the current scope so that compiler wont apply any optimization. This matters only in case of multi-threaded applications.
In the above example if variable ‘a’ was declared using volatile, compiler will not optimize it. In shot, value of the volatile variables will be read from the memory location directly.
Volatile keyword is used to tell compiler that the variable declared using volatile may be used from outside the current scope so that compiler wont apply any optimization. This matters only in case of multi-threaded applications.
In the above example if variable ‘a’ was declared using volatile, compiler will not optimize it. In shot, value of the volatile variables will be read from the memory location directly.
7.In how many ways we can initialize an int variable in C++?
In c++, variables can be initialized in two ways, the traditional C++ initialization using “=” operator and second using the constructor notation.
Traditional C++ initilization
- int i = 10;
variable i will get initialized to 10.
Using C++ constructor notation
- int i(10);
Implicit conversions are performed when a type (say T) is used in a context where a compatible type (Say F) is expected so that the type T will be promoted to type F.
short a = 2000 + 20;
In the above example, variable a will get automatically promoted from short to int. This is called implicit conversion/coercion in c++.
8.What is implicit conversion/coercion in c++?
Implicit conversions are performed when a type (say T) is used in a context where a compatible type (Say F) is expected so that the type T will be promoted to type F.
short a = 2000 + 20;
In the above example, variable a will get automatically promoted from short to int. This is called implicit conversion/coercion in c++.
9.What are C++ inline functions?
C++ inline functions are special functions, for which the compiler replaces the function call with body/definition of function. Inline functions makes the program execute faster than the normal functions, since the overhead involved in saving current state to stack on the function call is avoided. By giving developer the control of making a function as inline, he can further optimize the code based on application logic. But actually, it’s the compiler that decides whether to make a function inline or not regardless of it’s declaration. Compiler may choose to make a non inline function inline and vice versa. Declaring a function as inline is in effect a request to the compiler to make it inline, which compiler may ignore. So, please note this point for the interview that, it is upto the compiler to make a function inline or not.
inline int min(int a, int b)
{
return (a < b)? a : b;
}
int main( )
{
cout << “min (20,10): ” << min(20,10) << endl;
cout << “min (0,200): ” << min(0,200) << endl;
cout << “min (100,1010): ” << min(100,1010) << endl;
return 0;
}
If the complier decides to make the function min as inline, then the above code will internally look as if it was written like
int main( )
{
cout << “min (20,10): ” << ((20 < 10)? 20 : 10) << endl;
cout << “min (0,200): ” << ((0 < 200)? 0 : 200) << endl;
cout << “min (100,1010): ” << ((100 < 1010)? 100 : 1010) << endl;
return 0;
}
10.What do you mean by translation unit in c++?
We organize our C++ programs into different source files (.cpp, .cxx etc). When you consider a source file, at the preprocessing stage, some extra content may get added to the source code ( for example, the contents of header files included) and some content may get removed ( for example, the part of the code in the #ifdef of #ifndef block which resolve to false/0 based on the symbols defined). This effective content is called a translation unit. In other words, a translation unit consists of
- Contents of source file
- Plus contents of files included directly or indirectly
- Minus source code lines ignored by any conditional pre processing directives ( the lines ignored by #ifdef,#ifndef etc)
11.What do you mean by internal linking and external linking in c++?
A symbol is said to be linked internally when it can be accessed only from with-in the scope of a single translation unit. By external linking a symbol can be accessed from other translation units as well. This linkage can be controlled by using static and extern keywords.
12.What do you mean by storage classes?
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.
13. 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++
Following storage classes are available in C++
- auto
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 ;
i++;
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;
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
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 |
14.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.
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{
public:
int* ptr;
SampleClass();
// 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”;
}
15.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.
16.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.
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.
17.What do you mean by persistent and non persistent objects?
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. ]
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. ]
18.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.
19.What are virtual functions and what is its use?
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;
public:
Base()
{
a = 1;
}
virtual void method()
{
cout << a;
}
};
class Child: public Base
{
int b;
public:
Child()
{
b = 2;
}
virtual void method()
{
cout << b;}
};
int main()
{
Base *pBase;
Child oChild;
pBase = &oChild;
pBase->method();
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.
20.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++).
