Top 50 C++ Interview Questions and Answers (2024)

‘std’ is also known as Standard or it can be interpreted as a namespace. The command “using namespace std” informs the compiler to add everything under the std namespace and inculcate them in the global namespace. This all inculcation of global namespace benefits us to use “cout” and “cin” without using “std::_operator_”.

When a variable is described as a reference it becomes an alias of the already existing variable. In simple terms, a referenced variable is another named variable of an existing variable keeping in mind that changes made in the reference variable will be reflected in the already existing variable. A reference variable is preceded with a ‘&’ symbol.

Syntax:

int GFG = 10;

// reference variable
int& ref = GFG;

In this programming language to call a function we have 2 methods: Call by Value and Call by Reference

A token is the smallest individual element of a program that is understood by a compiler. A token comprises the following:

1. Keywords – That contain a special meaning to the compiler
2. Identifiers – That hold a unique value/identity 
3. Constants – That never change their value throughout the program 
4. Strings – That contains the homogenous sequence of data 
5. Special Symbols – They have some special meaning and cannot be used for another purpose; eg: [] () {}, ; * = # 
6. Operators – Who perform operations on the operand

A class is a user-defined data type where all the member functions and data members are tailor-made according to demands and requirements in addition to which these all can be accessed with the help of an object. To declare a user-defined data type we use a keyword class.

An object is an instance of a class and an entity with value and state; In simple terms, it is used as a catalyst or to represent a class member. It may contain different parameters or none.

Note: A class is a blueprint that defines functions which are used by an object.

When a function of the same name, same arguments or parameters, and same return type already present/declared in the base class is used in a derived class is known as Function Overriding. It is an example of Runtime Polymorphism or Late Binding which means the overridden function will be executed at the run time of the execution.

• Classes: It is a user-defined datatype
• Objects: It is an instance of a class
• Abstraction: It is a technique of showing only necessary details
• Encapsulation: Wrapping of data in a single unit
• Inheritance: The capability of a class to derive properties and characteristics from another class
• Polymorphism: Polymorphism is known as many forms of the same thing

The capability or ability of a class to derive properties and characteristics from another class is known as inheritance. In simple terms, it is a system or technique of reusing and extending existing classes without modifying them.

Multiple inheritances mean that a derived class can inherit two or more base/parent classes. It is useful when a derived class needs to combine numerous attributes/contracts and inherit some, or all, of the implementation from these attributes/contracts. To take a real-life example consider your Parents where Parent A is your DAD Parent B is your MOM and Chid C is you.

Multiple Inheritances

Virtual inheritance is a technique that ensures only one copy of a base class’s member variables is inherited by grandchild-derived classes. Or in simple terms, virtual inheritance is used when we are dealing with a situation of multiple inheritances but want to prevent multiple instances of the same class from appearing in the inheritance hierarchy.

Polymorphism is known as many forms of the same thing. In simple terms, we can say that Polymorphism is the ability to display a member function in multiple forms depending on the type of object that calls them. 

In other words, we can also say that a man can be an employee to someone, a son of someone, a father of someone, and a husband of someone; this is how polymorphism can be projected in real life.

There is 2 type of polymorphism:

1. Compile Time Polymorphism
   • Function Overloading
   • Operator Overloading
2. Run Time Polymorphism
   • Function Overriding
   • Virtual Function

There is 2 type of polymorphism

Compile Time Polymorphism or Static Binding

This type of polymorphism is achieved during the compile time of the program which results in it making a bit faster than Run time. Also, Inheritance is not involved in it. It is comprised of 2 further techniques:

Function Overloading: When there are multiple functions with the same name but different parameters then this is known as function overloading.

Operator Overloading: It is basically giving practice of giving a special meaning to the existing meaning of an operator or in simple terms redefining the pre-redefined meaning

Run-Time Polymorphism or Late Binding

Run-time polymorphism takes place when functions are invoked during run time. 
 
Function Overriding: Function overriding occurs when a base class member function is redefined in a derived class with the same arguments and return type.

Output:

Function of derived class

A constructor is a special type of member function of a class, whose name is the same as that of the class by whom it is invoked and initializes value to the object of a class. 

There are 3 types of constructors:

A. Default constructor: It is the most basic type of constructor which accepts no arguments or parameters. Even if it is not called the compiler calls it automatically when an object is created.

Example:

B. Parameterized constructor: It is a type of constructor which accepts arguments or parameters. It has to be called explicitly by passing values in the arguments as these arguments help initialize an object when it is created. It also has the same name as that of the class. 

Also, It is used to overload constructors.

Example:

Output

G.x = 10, G.y = 15

C. Copy Constructor: A copy constructor is a member function that initializes an object using another object of the same class. Also, the Copy constructor takes a reference to an object of the same class as an argument.

Example:

Destructors are members of functions in a class that delete an object when an object of the class goes out of scope. Destructors have the same name as the class preceded by a tilde (~) sign. Also, destructors follow a down-to-top approach, unlike constructors which follow a top-to-down.

Syntax:

~constructor_name(); // tilde sign signifies that it is a destructor

When destroying instances or objects of a derived class using a base class pointer object, a virtual destructor is invoked to free up memory space allocated by the derived class object or instance.

Virtual destructor guarantees that first the derived class’ destructor is called. Then the base class’s destructor is called to release the space occupied by both destructors in the inheritance class which saves us from the memory leak. It is advised to make your destructor virtual whenever your class is polymorphic.

The simple answer is NO we cannot overload a destructor. It is mandatory to only destructor per class in C++. Also to mention, Destructor neither take arguments nor they have a parameter that might help to overload.

Pointers are the variables that are used to store the address location of another variable. Operations that are permitted to a pointer are:

1. Increment/Decrement of a Pointer
2. Addition and Subtraction of integer to a pointer
3. Comparison of pointers of the same type

The delete operator is used to delete/remove all the characteristics/properties from an object by deallocating its memory; furthermore, it returns true or false in the end. In simple terms, it destroys or deallocates array and non-array(pointer) objects which are created by new expressions.

A friend class is a class that can access both the protected and private variables of the classes where it is declared as a friend.

Example of friend class:

A friend function is a function used to access the private, protected, and public data members or member functions of other classes. It is declared with a friend keyword. The advantage of a friend function is that it is not bound to the scope of the class and once it is declared in a class, furthermore to that, it cannot be called by an object of the class; therefore it can be called by other functions. Considering all the mentioned points we can say that a friend function is a global function.

Example of friend function:

Overflow Error occurs when the number is too large for the data type to handle. In simple terms, it is a type of error that is valid for the defined but exceeds used the defined range where it should coincide/lie. 

For example, the range of int data type is –2,147,483,648 to 2,147,483,647 and if we declare a variable of size 2,247,483,648 it will generate a overflow error.

A scope resolution operator is denoted by a ‘::‘ symbol. Just like its name this operator resolves the barrier of scope in a program. A scope resolution operator is used to reference a member function or a global variable out of their scope furthermore to which it can also access the concealed variable or function in a program.

Scope Resolution is used for numerous amounts of tasks:

1. To access a global variable when there is a local variable with the same name
2. To define the function outside the class
3. In case of multiple inheritances
4. For namespace

The access restriction specified to the class members( whether it is member function or data member) is known as access modifiers/specifiers. 

Access Modifiers are of 3 types:

1. Private – It can neither be accessed nor be viewed from outside the class 
2. Protected – It can be accessed if and only if the accessor is the derived class
3. Public – It can be accessed or be viewed from outside the class 

Yes, it is absolutely possible to compile a program without a main(). For example Use Macros that defines the main

STL is known as Standard Template Library, it is a library that provides 4 components like container, algorithms, and iterators.
 

C++ STL

An inline function is a form of request not an order to a compiler which results in the inlining of our function to the main function body. An inline function can become overhead if the execution time of the function is less than the switching time from the caller function to called function. To make a function inline use the keyword inline before and define the function before any calls are made to the function.

Inline Function Explanation

Syntax:

inline data_type function_name()
{
Body;
}

The answer is No; It cannot be recursive.

An inline function cannot be recursive because in the case of an inline function the code is merely placed into the position from where it is called and does not maintain a piece of information on the stack which is necessary for recursion.

Plus, if you write an inline keyword in front of a recursive function, the compiler will automatically ignore it because the inline is only taken as a suggestion by the compiler.

An abstract class is a class that is specifically designed to be used as a base class. An abstract class contains at least one pure virtual function. You declare a pure virtual function by using a pure specifier(= 0) in the declaration of a virtual member function in the class declaration

You cannot use an abstract class as a parameter type, a function return type, or the type of an explicit conversion, nor can you declare an object of an abstract class. However, it can be used to declare pointers and references to an abstract class. 

An abstract class is used if you want to provide a common, implemented functionality among all the implementations of the component. Abstract classes will allow you to partially implement your class, whereas interfaces would have no implementation for any members whatsoever. In simple words, Abstract Classes are a good fit if you want to provide implementation details to your children but don’t want to allow an instance of your class to be directly instantiated.

The static data member of a class is a normal data member but preceded with a static keyword. It executes before main() in a program and is initialized to 0 when the first object of the class is created. It is only visible to a defined class but its scope is of a lifetime.

Syntax:

static Data_Type Data_Member;  

The static member function is the member function that is used to access other static data members or other static member functions. It is also defined with a static keyword. We can access the static member function using the class name or class objects.

Syntax:

classname::function name(parameter);

Just like its name, things can change suddenly and unexpectantly; So it is used to inform the compiler that the value may change anytime. Also, the volatile keyword prevents the compiler from performing optimization on the code. It was intended to be used when interfacing with memory-mapped hardware, signal handlers, and machine code instruction.

Storage class is used to define the features(lifetime and visibility) of a variable or function. These features usually help in tracing the existence of a variable during the runtime of a program.

Syntax:

storage_class var_data_type var_name; 

Some types of storage classes:

Examples of storage class

Just like its name, the mutable storage class specifier is used only on a class data member to make it modifiable even though the member is part of an object declared as const. Static or const, or reference members cannot use the mutable specifier. When we declare a function as const, this pointer passed to the function becomes const.

So the scope of a variable is a region where a variable is accessible. There are two scope regions, A global and block or local. 

A block scope variable is also known as a local scope variable. A variable that is defined inside a function (like main) or inside a block (like loops and if blocks) is a local variable. It can be used ONLY inside that particular function/block in which it is declared. a block-scoped variable will not be available outside the block even if the block is inside a function.

The auto keyword may be used to declare a variable with a complex type in a straightforward fashion. You can use auto to declare a variable if the initialization phrase contains templates, pointers to functions, references to members, etc. With type inference capabilities, we can spend less time having to write out things the compiler already knows. As all the types are deduced in the compiler phase only, the time for compilation increases slightly but it does not affect the runtime of the program. 

Namespaces enable us to organize named items that would otherwise have global scope into smaller scopes, allowing us to give them namespace scope. This permits program parts to be organized into distinct logical scopes with names. The namespace provides a place to define or declare identifiers such as variables, methods, and classes. 

Or we could say that A namespace is a declarative zone that gives the identifiers (names of types, functions, variables, and so on) within it a scope. Namespaces are used to arrange code into logical categories and to avoid name clashes, which might happen when you have many libraries in your code base.

The void keyword, when used as a function return type, indicates that the function does not return a value. When used as a parameter list for a function, void indicates that the function takes no parameters. Non-Value Returning functions are also known as void functions. They’re called “void” since they’re not designed to return anything. True, but only partially. We can’t return values from void functions, but we can certainly return something. Although void functions have no return type, they can return values.

Yes, we can call a virtual function from a constructor. But it can throw an exception of overriding.

Just like its name a void pointer is a pointer that is not associated with anything or with any data type. Nevertheless, a void pointer can hold the address value of any type and can be converted from one data type to another.