What is the #include directive in C/C++?
The #include directive in C is interpreted by the preprocessor to include the header file’s content, and can be used to include code from user-defined or compiler-based header files into a program. The #include directive is written at the beginning of a file to ensure that all header files are included before compiling the rest of the code. There are two ways that this directive can be used:
1) Compiler based header files
Compiler based header files contain standard library functions that can be used by a developer to help write their program. These files include the '.h' extension at the end and are written within <> brackets.
For example, the following code demonstrates that including 'stdio.h' enables you to use the printf() function for printing outputs onto a screen:
#include <stdio.h>int main() {// your code goes hereprintf("hello world");return 0;}
Additionally, the C language has many other library functions that can be implemented through the use of header files. Some common examples are:
-
The
<iostream>header file can be used to receive input usingcin>>, and print output of a program usingcout<<. -
<time.h>can be used to retrieve the duration of time in between two events using thetime()function. -
<string.h>can be used for string manipulation using functions likestrlen()to find the length of a string,strcpy()to copy one string into another, andstrcat()to append one string to another. -
<math.h>is a very important header file as it includes functions such aspow(x,y)to raisexto the power ofy,sin(x), andcos(x).
2) User defined header files
User defined header files are defined by the user beforehand and are used in large projects where breaking code down into files with helper functions helps to better organize the code. When the compiler runs the program, it searches for this file in its own directory to include it in the code. These files have the '.h' extension at the end and are written within quotation marks "".
For example, the following code adds two numbers together by including the user-defined 'adder.h'
#include <stdio.h>void adder(int a, int b){int c = a + b;printf("Your sum is: %d\n", c);}
The difference between compiler based header files and user defined header files lies in the syntax you use (i.e., "" or <>).
It is important to use the correct syntax in both cases.
The use of
""indicates that the mentioned header file is available in the same directory and the compiler can look for it. However, it is imperative that the header files are named correctly, both in the directory as well as within the code.
The use of
<>indicates that the compiler must search for the header file in its pre-defined header files.
Free Resources
