C++ Library and Header File Writing Tutorial

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“This article introduces how to generate library files under the Linux system and how to write header files to use the library functions.”

We know that during the compilation of a C++ project, files containing the main() function will generate an executable program. Code that does not contain the main() function is generally called by other programs, so we can package them into a “thing”, which is a library.

Libraries are generally a collection of many programs and algorithms. For example, the OpenCV library contains many algorithms related to computer vision, and the Eigen library provides many algorithms for matrix algebra calculations.

Let’s take a simple C++ file as an example to demonstrate how to write a library.

Create a new folder named cppSpace in the root directory, and within that folder, create a file named libHelloWorld.cpp:

// Library file without main() function#include <iostream>using namespace std;void printHello(){ cout << "Hello world!" << endl;}

This library file is quite simple and only provides a printHello() function, which outputs a message when called.

According to the principles of CMake, we add the following instructions in the CMakeLists.txt file:

# The instruction syntax format: add_library( program_name source_file ) add_library( hello libHelloWorld.cpp )

This command tells CMake that we want to compile this file into a library named “hello”. Next, use CMake to compile this project, entering the following in the terminal:

cd buildcmake ..make

After running the above commands, a file named libhello.a will be generated in the directory, which is our library. This library is a static library, which generates a copy each time it is called, taking up more space. In contrast, a shared library only produces one copy during multiple calls, saving space. To create a shared library, we add the following instruction in the CMakeLists.txt file:

add_library( hello_shared SHARED libHelloWorld.cpp )

By compiling the project with CMake again, we will obtain the file libhello_shared.so, which is the shared library file.

If we only have these two formats of files, we won’t know what the functions inside are or how to call them. To allow users to utilize these library functions, we need to write a header file that specifies what is included in the library. For users, as long as they declare this header file in their program, they can call this library.

Header files are important directives at the beginning of C++ programs. Below is an example of writing the header file for libhello.a, explaining its writing and usage process.

In the cppSpace folder, create a header file named libHelloWorld.h:

#ifndef LIBHELLOWORLD_H_#define LIBHELLOWORLD_H_// Macro definition to prevent redefinition errors when including the header filevoid printHello();   // Declare the included library function#endif

We have written our header file libHelloWorld.h. Next, we will write an executable program useHello.cpp to call it:

#include "libHelloWorld.h"    // Declare the header fileint main(){ printHello(); return 0;}

To successfully call this library function, we also need to link to the library file libhello.a during compilation, which involves adding the following instructions in the CMakeLists.txt file:

# Compile useHello.cpp normallyadd_executable( useHello useHello.cpp )# Link to the library file (Note! Add the library file name, not the header file name)target_link_libraries( useHello hello )

Thus, the file useHello.cpp can use the library functions from hello.a.

In conclusion, the writing and usage process of library files and header files has been introduced.

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