C++ interview questions

What are the difference between reference variables and pointers in C++?

Answer:

[This question is usually asked in a twisted way during c++ interviews. Sometimes the interviewer might use examples and ask you to find the error.]

What is meant by reference variable in C++?

Answer:

In C++, reference variable allows you create an alias (second name) for an already existing variable. A reference variable can be used to access (read/write) the original data. That means, both the variable and reference variable are attached to same memory location. In effect, if you change the value of a variable using reference variable, both will get changed (because both are attached to same memory location).

How to create a reference variable in C++
Appending an ampersand (&) to the end of datatype makes a variable eligible to use as reference variable.

int a = 20;
int& b = a;

The first statement initializes a an integer variable a. Second statement creates an integer reference initialized to variable a 
Take a look at the below example to see how reference variables work.

int main ()
{
	int   a;
	int&  b = a;
	a = 10;
	cout << "Value of a : " << a << endl;
	cout << "Value of a reference (b) : " << b  << endl;
	b = 20;
	cout << "Value of a : " << a << endl;
	cout << "Value of a reference (b) : " << b  << endl;
	return 0;
}

Above code creates following output.

Value of a : 10
Value of a reference (b) : 10
Value of a : 20
Value of a reference (b) : 20

What you mean by early binding and late binding? How it is related to dynamic binding?

Answer:

[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.

What is virtual destructors? Why they are used?

Answer:

[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.

Why pure virtual functions are used if they don't have implementation / When does a pure virtual function become useful?

Answer:

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.

What do you mean by pure virtual functions in C++? Give an example?

Answer:

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++).

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 {
    public:
        // draw() is a pure virtual function.
        virtual void draw() = 0;
};

What are virtual functions and what is its use?

Answer:

[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; 
    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.

Is it possible to get the source code back from binary file?

Answer:

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.

What do you mean by persistent and non persistent objects?

Answer:

[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. ]

What is difference between shallow copy and deep copy? Which is default?

Answer:

[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.

What is realloc() and free()? What is difference between them?

Answer:

• 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.

What is 'Copy Constructor' and when it is called?

Answer:

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{
    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";
}

How many storage classes are available in C++?

Answer:

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++

• 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;
    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

What do you mean by storage classes?

Answer:

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.

What do you mean by internal linking and external linking in c++?

Answer:

[This interview question is related to questions on "translation unit" and "storage classes"]

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.

What do you mean by translation unit in c++?

Answer:

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)

What are C++ inline functions?

Answer:

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;
}

What is implicit conversion/coercion in c++?

Answer:

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++.

In how many ways we can initialize an int variable in C++?

Answer:

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);

What is the use of volatile keyword in c++? Give an example.

Answer:

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.