The #include directive in C and C++ is important for including the code from the other files into a source file. There are two main forms of this directive: #include <filename> for including standard library headers and #include “filename” for including user-defined headers, and understanding the differences between these two forms is important for C and C++ programming. So, in this article, we will discuss the #include <filename> and #include “filename” and their differences.
Table of Contents:
Understanding #include <filename> in C/C++
The #include directive in C and C++ is used to include the contents of a file in the source code before the compilation, and when the angle brackets are used with this #include directive (#include <filename>), then the compiler considers it a standard library header file.
Example in C:
Output:
The code shows how the standard input/output library <stdio.h> is included in C to use the printf function.
Example in C++:
Output:
The code shows how the standard input/output library < iostream> is included in C++ to use the std::cout function.
Understanding #include “filename” in C/C++
The #include “filename” directive in C and C++ is used to include the user-defined header files. While the #include <filename> directive searches in system directories, this directive form searches in the working directory before looking anywhere else.
Note: When you are using the user-defined header files, please check that the compiler doesn’t look in additional included directories through the compiler flags.
Example in C:
File: mathutils.h
File: main.c
Output:
The example shows how the mathutils.h header in C defines a function add(int a, int b) and main.c includes this header to implement the function add and prints the sum of 3 and 5.
Example in C++:
File: utilities.h
File: utilities.cpp
File: main.cpp
Output:
The example shows how the utilities.h is used in C++ to declare the greet function, utilities.cpp is used to define and print the greeting, and the main.cpp file implements the add function.
Learn how to use the printf function in C++ in this blog.
#include <filename> vs #include “filename”
| Aspect |
#include <filename> (Angle Brackets) |
#include “filename” (Quotes) |
| Usage |
Standard library headers |
User-defined/project headers |
| Search Path |
System directories first |
Current directory first, then system directories |
| File Location |
Predefined system paths |
Project directory or specified paths |
| Customization |
Cannot include custom files |
Can include both standard and custom headers |
| Example |
#include <iostream> |
#include “myheader.h” |
| Risk of Conflict |
Low (system headers are unique) |
Higher (if a local file has the same name as a system header) |
| Compiler Behavior |
Skips current directory, searches system paths directly |
Checks current directory first, then system paths |
Conclusion
For efficient C and C++ programming, it is important to understand the differences between the #include <filename> and #include “filename”. The #include <filename> is used for the standard library headers, which searches the system directories, while the #include “filename” is used for the user-defined headers, which prioritizes the current directory. So, by understanding the differences and with the help of best practices, you can easily write a C or C++ program.
FAQs on Difference between #include <filename> and #include “filename” in C/C++
1. What does #include do?
The #include includes standard library headers and searches system directories.
2. What does #include "filename" do?
The #include “filename” includes user-defined headers and searches the current directory first.
3. Can I use #include "filename" for standard headers?
Yes, you can use #include “filename” for standard headers, but it may lead to naming conflicts with the local files.
4. What if a file isn't found with #include "filename"?
If a file isn’t found with #include “filename”, then the compiler will search system directories.
5. How can I avoid conflicts with standard headers?
You can avoid conflicts with the standard headers by using unique names for user-defined headers and organizing them into specific directories.