Interview Questions Archives - Page 4 of 15

Network Address Translation Explained With Simple Example

Network Address Translation or NAT is a method to map multiple local private IP address to one or more public IP addresses. To achieve this generally a network device for eg: Router which is sitting in the private network, changes the source IP and port field of the IP header in a packet.

Since IP Addresses are limited (IPv4) and as internet devices are too large in number. Hence, NAT is used to apply one or more public IP addresses to a pool of devices present in a private network to save IP addresses.
Theoretically using NAT one public IP Address is sufficient for an ISP hosting a large number of subscribers.

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Difference Between Process and Threads

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A program in execution is called a process. There can be multiple instances of a program running in the system which are identified by a unique id called process id. Operating System ensures that each process has a unique id which is always a non-negative integer number.

A thread is a lightweight process which can only be used inside a process. Like process, thread also has a unique identifier to identify threads which are called thread ids. Unlike process ids, thread ids are only unique to a process and one process threads are unknown to other process.

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Process Cleanups Via Exit Handlers Registered Using atexit function

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Any normal Unix process can terminate in following ways:

Graceful exit – Normal termination of process. For eg:
- Returning from main, calling exit etc.
Ungraceful exit – Abnormal termination of process. For eg:
- Abort signal. Due to some fatal error etc.

_exit, _EXIT and exit functions can be used to terminate the process normally. Usually exit functions do the cleanup but if needed a programmer can also define its own exit handler functions to do some specific cleanup actions needed by the program.

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Dynamic Memory Allocation Explained With Simple Example

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Dynamic memory allocation is the process of allocating memory at run time by the program based on the need of the program. Dynamic Memory allocation is done from the heap memory available in the system and handling of memory (creation and deletion) is totally handled by the programmer. In case, programmer forgets to cleanup the allocated dynamic memory block, it will lead into memory leak and this memory block will be blocked for further use until program restarts.

Methods available to allocate dynamic memory at run time are

malloc () – allocates one memory block of requested size.
calloc () – allocates multiple blocks of memory and initialize it with 0.
realloc () – increases or decreases the size of previously allocated memory block.
free () – frees the memory allocated by above functions.

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Early vs Late Binding using Virtual Table and VPtr

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Early Binding	Late Binding
Happens at compile time.	Happens at run time.
Compiler has all information to invoke correct function version at compile time.	Compiler doesn’t have information to identify correct function version.
Normal function calls are example of Early Binding.	Virtual functions are example of Late Binding.
Functions invoked by object is example of early binding	Functions invoked by pointers can be example of late binding.
Early binding is efficient as no cost paid at run-time for function resolution.	Late Binding is slightly costly as function resolution happens at run time.
Early binding doesn’t provide flexibility of one method multiple interfaces.	Late Binding provide flexibility.
Doesn’t have any virtual table or virtual pointer.	It uses Virtual Table and Virtual Pointer.

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Virtual Functions in C++ Explained With Simple Example

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A virtual function is a member function which is defined in Base class and redefined by the derived class. “virtual” keyword must be added before the function declaration to define a function virtual. These functions gets inherited by Derived class from Base class and Derived class can choose to provide it’s own definition.
A pure virtual function is a virtual function which has no definition in Base class. Pure virtual function definition is used to ensure all derived class must override the base class function definition.

Normal syntax to define virtual function is as follows:

virtual int samplefun (int x, char c);

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C++ Casting Operators Explained With Simple Example

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Type casting is the method to convert variable (or expression) of a type to another type. Casting operators are special operators which are used to convert variable of one data type to another data type. Type casting can be done in two ways:

Implicit casting is being done automatically where values are copied to another compatible type. Eg: int to long int or float etc.

Explicit casting is done specifically by the programmer. In C++ there are four types of casting operator available.

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reinterpret_cast Casting Operator Explained With Simple Example

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This is one of the most complex and dangerous casting operators available in C++. The “reinterpret_cast” operator can convert any type of variable to fundamentally different type. This cast operator can convert an integer to a pointer and so on. This cast operator can also convert variable into totally incompatible type too.

Normal syntax to do reinterpret_cast is as follows:

reinterpret_cast <target-type> (expr)

target-type is the target of the cast whereas expr is being cast into the new target-type.

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static_cast Casting Operator Explained With Simple Example

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This is the simplest type casting operator available in C++. The “static_cast” operator performs a normal cast. This casting operator is basically a substitute for normal casting operator. “static_cast” operator doesn’t do any runtime checks. Hence programmer should consider whether casting is applicable or not.

Normal syntax to do static_cast is as follows:

static_cast <target-type> (expr)

target-type is the target of the cast whereas expr is being cast into the new target-type.

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const_cast Casting Operator Explained With Simple Example

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This is one of the most dangerous casting operators. The “const_cast” operator is used to remove the const or volatile property of a variable. The target-type and source-type must be of the same type here.

Normal syntax to do const_cast is as follows:

const_cast <target-type> (expr)

target-type and expr must be of same type pointer or reference.

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