Header Files in C++

In this lesson: Standard C++ headers, creating your own .h files and include guards / #pragma once — with examples. In Hindi & English.

What is a header file?

A header file holds *declarations* that more than one part of a program needs: function prototypes, class definitions, constants and templates. When you write #include <iostream>, the preprocessor copies the whole text of that header into your file before the compiler sees it. That is how the compiler knows what std::cout is.

Two ways to include

#include <vector>        // angle brackets: search the standard/system folders
#include "student.h"     // quotes: search your project folder first

Use < > for standard library and installed library headers, and " " for headers you wrote.

Most-used standard headers

C library headers have a C++ version with a c prefix and no .h: use <cmath> instead of <math.h>, <cstring> instead of <string.h>.

#include <iostream>
#include <iomanip>
#include <cmath>

int main() {
    double r = 2.5;
    std::cout << std::fixed << std::setprecision(2)
              << "Area = " << M_PI * std::pow(r, 2) << "\n"
              << "sqrt(2) = " << std::sqrt(2.0) << "\n";
    return 0;
}

(M_PI is provided by GCC/Clang's <cmath>; on MSVC define _USE_MATH_DEFINES first or use your own constexpr double PI.)

Writing your own header

Split a program into three files: the header declares, one .cpp defines, and main.cpp uses.

// student.h
#ifndef STUDENT_H
#define STUDENT_H

#include <string>

class Student {
public:
    Student(std::string name, int marks);
    void show() const;
    bool passed() const;
private:
    std::string name_;
    int marks_;
};

#endif // STUDENT_H

// student.cpp
#include "student.h"
#include <iostream>

Student::Student(std::string name, int marks)
    : name_(name), marks_(marks) {}

void Student::show() const {
    std::cout << name_ << ": " << marks_
              << (passed() ? " (Pass)" : " (Fail)") << "\n";
}

bool Student::passed() const { return marks_ >= 33; }

// main.cpp
#include "student.h"

int main() {
    Student a("Aman", 78), b("Ravi", 29);
    a.show();
    b.show();
    return 0;
}

g++ main.cpp student.cpp -o result
./result

Only .cpp files go on the compile command; headers are pulled in by #include.

Include guards and #pragma once

If student.h gets included twice (directly and through another header), class Student would be defined twice — an error. The #ifndef / #define / #endif lines above prevent that: the second time, STUDENT_H is already defined, so the content is skipped.

Most compilers also accept a one-line alternative at the top of the header:

#pragma once

It is not part of the official standard but is supported by GCC, Clang and MSVC. Either approach is fine; just use one in every header you write.

What goes where

iostream.h, conio.h and bits/stdc++.h

• <iostream.h> and <conio.h> (clrscr(), getch()) come from Turbo C++. They do not exist in standard C++ — use <iostream> and drop clrscr()/getch().
• <bits/stdc++.h> includes the entire library. It works only on GCC and slows compilation; fine in competitive programming, but avoid it in school projects and real code.

Common mistakes

• Writing #include <iostream.h> or #include "iostream" for standard headers.
• Forgetting include guards and getting "redefinition of class" errors.
• Defining a normal function in a header included by two .cpp files — "multiple definition" at link time.
• Putting using namespace std; in a header, which leaks into every file that includes it.
• Compiling only main.cpp and getting "undefined reference" because student.cpp was not compiled.

Summary

• Headers contain declarations; #include copies them in before compilation.
• < > for standard/library headers, " " for your own.
• Use <cmath>-style C++ headers, never <iostream.h>.
• Protect every header with include guards or #pragma once.
• Declarations in .h, definitions in .cpp; templates and inline functions stay in the header.

Related: preprocessor directives, namespaces, structure of a C++ program.