D - Modules

Modules are the building blocks of D. It is based on a simple concept. Every source file is a module. Accordingly, the single files that we have been writing our programs in have all been individual modules. By default, the name of a module is the same as its filename without the .d extension.

D - Templates

Templates are the foundation of generic programming, which involves writing code in a way that is independent of any particular type.
A template is a blueprint or formula for creating a generic class or a function.

D - Immutable

We often use variables that are mutable but there can be many occasions mutability is not required. Immutable variables can be used in such cases. A few examples are given below where immutable variable can be used.

D - File I/O

Files are represented by the File struct of the std.stdio module.
A file represents a sequence of bytes, does not matter if it is a text file or binary file. D programming language provides access on high level functions as well as low level (OS level) calls to handle file on your storage devices.

D - Concurrency

Concurrency is making a program run on more than one thread at a time. An example of a concurrent program would be a web server responding more than one client at the same time. Concurrency is easy with message passing but very difficult to write if they are based on data sharing.

D - Exception Handling

An exception is a problem that arises during the execution of a program. A D exception is a response to an exceptional circumstance that arises while a program is running, such as an attempt to divide by zero.
Exceptions provide a way to transfer control from one part of a program to another. D exception handling is built upon three keywords: try, catch, and throw.

D - Contract Programming

Contract programming in D programming is focused on providing a simple and understandable means of error handling.Contract programming in D are implemented by three types of code blocks:

• body block
• in block
• out block
  

D - Conditional Compilation

Conditional Compilation:

Conditional compilation is the process of selecting which code to compile and which code to not compile similar to the #if / #else / #endif in C and C++. Any statement that is not compiled in still must be syntactically correct. Conditional compilation involves condition checks that are evaluable at compile time.

D - Classes & Objects

Classes are the central feature of D programming that supports object-oriented programming and are often called user-defined types.
A class is used to specify the form of an object and it combines data representation and methods for manipulating that data into one neat package. The data and functions within a class are called members of the class.

D - Inheritance

One of the most important concepts in object-oriented programming is that of inheritance. Inheritance allows us to define a class in terms of another class, which makes it easier to create and maintain an application. This also provides an opportunity to reuse the code functionality and fast implementation time.

D - Overloading

D allows you to specify more than one definition for a function name or an operator in the same scope, which is called function overloading and operator overloading respectively.
An overloaded declaration is a declaration that had been declared with the same name as a previously declared declaration in the same scope, except that both declarations have different arguments and obviously different definition (implementation).

D - Encapsulation

All D programs are composed of the following two fundamental elements:

• Program statements (code): This is the part of a program that performs actions and they are called functions.
• Program data: The data is the information of the program which affected by the program functions.