Header File/Library

In some code’s head, we can see statement like this.

#include <iostream>

#include <iostream>

#include <stdio.h>

#include <stdio.h>

using System;

using System;

import numpy

import numpy

They are called Header File or Library.

Header files/Library serve as fundamental building blocks in programming, particularly in compiled languages like C and C++, acting as interface declarations that enable code organization and reuse.

These files (typically with .h extensions) contain function prototypes, macro definitions, type declarations, and constant values that multiple source files can reference without exposing implementation details.

Header files and external libraries serve as specialized repositories for pre-built functional modules, akin to prefabricated components used in construction. Different programming languages manage these repositories through distinct mechanisms: some require explicit requests for “material manifests” (such as C’s #include), while others offer “intelligent delivery services” (like Python’s import).

In compiled languages like C/C++, header files explicitly declare interfaces (function prototypes, macros, type definitions) and rely on preprocessor directives (#include) to copy declarations into source files during compilation, acting as explicit blueprints for component integration. Conversely, interpreted languages like Python unify interface and implementation within .py files, using import to dynamically load modules at runtime—eliminating the need for separate header declarations.

This dichotomy reflects fundamental architectural differences: compiled languages depend on headers for early type checking and separate compilation, whereas interpreted languages leverage runtime flexibility to resolve dependencies dynamically. The evolution from manual inclusion (#include) to automated resolution (import) parallels advancements in language design toward abstraction and efficiency.

Python

Python offers multiple approaches to import external libraries, each serving distinct use cases. The standard import statement remains the most common method, allowing module-level access.

import math

import math

For selective imports, the from...import syntax targets specific components.

from datetime import date

from datetime import date

Large libraries often use aliasing via as to shorten references

import pandas as pd

import pandas as pd

C

In C programming, external libraries are integrated through a two-phase process involving ‌header declarations‌ and ‌linker resolution‌. Header files (.h) act as interface contracts, containing function prototypes, macros, and type definitions. Developers include them using #include directives (e.g., #include <stdio.h> for standard I/O or #include "mylib.h" for local headers). The preprocessor literally copies these declarations into source files before compilation.

#include <stdio.h>

#include <stdio.h>

#include "mylib.h"

#include "mylib.h"

C++

In C++, there are several ways to incorporate external libraries into your project. The most common method is using the #include preprocessor directive to include header files (.h or .hpp) that contain function declarations.

#include <iostream>

#include <iostream>

C#

using MyOtherLibrary;

using MyOtherLibrary;

Java

import com.example.MyModule;

import com.example.MyModule;

JavaScript

import * as myModule from "/modules/my-module.js";

import * as myModule from "/modules/my-module.js";