C++

How to Write and Use `C++ Libraries`: A Step-by-Step Guide

Creating and using libraries in C++ is an essential skill for developers who want to build modular and reusable code. This blog post will guide you through the process of writing, compiling, and integrating C++ libraries, specifically focusing on shared libraries for both Linux and Windows environments. Read on to uncover simple examples and clear explanations that will make this topic easy to understand.

Understanding C++ Libraries

What are C++ Libraries?

C++ libraries are collections of precompiled code that you can include in your projects to reuse functionality without rewriting code. There are two primary types of libraries in C++:

Static Libraries: These are linked at compile-time and become part of the executable. These typically have a .a extension on Linux and .lib on Windows.
Dynamic Libraries: These are linked at runtime. On Linux, they have a .so extension, while on Windows, the equivalent files are .dll.

In this post, we’ll focus on creating and using dynamic libraries.

Creating a Dynamic Library in C++

Step 1: Write Your Library Code

Let’s begin with a simple example. We want to create a library containing a function that will print a message.

Create the header file (t.h):
```
void f();
```

Create the source file (t.cc):

#include <iostream>
#include "t.h"    

void f() {
    std::cout << "OH HAI.  I'M F." << std::endl;
}

Step 2: Compile the Code into a Shared Library

Now that we have our library code ready, we will compile it into a shared library. Here’s how to do that:

Compile the source code into object code:

g++ -fpic -c t.cc         # Produces t.o

Create the shared library:

g++ -shared -o t.so t.o   # Produces t.so

Step 3: Use the Library in Your Main Program

Now, let’s create a main application that will use the library we just created.

Create the main file (r.cc):

#include "t.h"

int main() {
    f(); // Call the function from the library
    return 0;
}

Step 4: Compile and Link Your Application with the Library

To compile the main program along with the shared library, use the following command:

g++ r.cc t.so -o mainApp

Step 5: Set Up the Library Path (Linux Only)

In Linux, make sure the program can find the shared library at runtime. You can do this by setting the LD_LIBRARY_PATH to include the current directory:

export LD_LIBRARY_PATH="."

You don’t need to run this step if you install the shared library to a global library directory.

Final Step: Run Your Application

After compilation, you can run your application:

./mainApp

You should see the output:

OH HAI.  I'M F.

Summary of Key Steps

Create a header file for your library functions.
Write the implementation in a source file.
Compile the source file into an object file.
Create a shared library from the object file.
Write a main application and link against the library.
Ensure your application can find the library at runtime.

By following these steps, you can successfully create and utilize your own C++ libraries, paving the way for organized and reusable code. This skill is invaluable for any developer aiming to enhance their programming practices in C++.

Now you are equipped to explore and create your own dynamic libraries in C++. Happy coding!

How to Write and Use C++ Libraries: A Step-by-Step Guide