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cpp-modulos

What are modules in C++ and how to use them

The modules offer a simpler, more efficient, and safer alternative to the traditional system of including files via the preprocessor.

Traditionally, C++ code has been organized in header files .h and implementation files .cpp. But this has frankly always been a hassle inherited from C 🤷.

The introduction of modules in C++11 and their formalization in C++20 aims to completely change the way C++ projects are structured and managed.

The concept of a module exists in many languages and has become a standard in programming. It represents a huge improvement for C++.

What is a module?

In simple terms, a module in C++ is a unit of code that consists of a declaration and definition of functions, classes, all in the same file.

Key Features

  • Encapsulation: Modules allow you to define what is accessible from the outside (what is exported) and what is private to the module (what is kept internal).
  • Compilation Optimization: Modules allow compiling once and reusing that result, reducing compilation times.
  • Elimination of Circular Dependencies: Modules allow importing only the necessary components, avoiding unnecessary dependencies.

If you are a regular programmer, by now you should be crying tears of joy to see all these advantages (I’m still crying 😭).

Basic Syntax of Modules

Declaring a Module

A module in C++ is declared using the keyword module followed by the module name.

The declaration is generally placed in a separate file with the extension .cppm (or sometimes .ixx, although .cppm is more common and standard in most compilers).

module mymodule;  // We declare a module called "mymodule"

export void foo() {  // We export the function foo
    std::cout << "Hello from foo!" << std::endl;
}

export class MyClass {  // We export a class MyClass
public:
    void sayHello() {
        std::cout << "Hello from MyClass!" << std::endl;
    }
};

In this example:

  • module mymodule; declares the module mymodule.
  • Functions and classes prefixed with export become accessible from outside the module.

Importing a Module

To use a module in another file, we simply use the keyword import, which replaces the traditional #include.

import mymodule;  // We import the module "mymodule"

int main() {
    foo();  // We call the exported function from mymodule
    MyClass obj;
    obj.sayHello();  // We use the class exported from mymodule
}

In this example,

  • The file main.cpp imports the module mymodule and uses the entities that have been exported (the function foo and the class MyClass).

Example with and without modules

Let’s look at the advantages of modules by doing a simple example with the traditional header system, and with the new module system.

Example using headers

First, let’s look at the traditional option, with implementation files and headers. We would have,

This file includes the header and uses the function sum.

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

int main() {
    int result = sum(3, 4);
    std::cout << "The sum is: " << result << std::endl;
    return 0;
}

This file contains the declaration of the function sum.

#ifndef SUM_H
#define SUM_H

int sum(int a, int b);

#endif

This file defines the function sum.

#include "sum.h"

int sum(int a, int b) {
    return a + b;
}

To compile this program, you would typically use a command like:

g++ main.cpp sum.cpp -o program

Example using modules

Now we implement the same example using modules. Instead of having a separate header file and an implementation file, we create a single module file.

This file contains both the declaration and definition of the function sum, and it is exported for other files to use.

export module sum;

export int sum(int a, int b) {
    return a + b;
}

To use the module, we simply import it in the main.cpp file.

import sum;
#include <iostream>

int main() {
    int result = sum(3, 4);
    std::cout << "The sum is: " << result << std::endl;
    return 0;
}

To compile the program with modules, the command would be something like:

g++ -fmodules-ts sum.cppm main.cpp -o program

The option -fmodules-ts is used in some compilers like GCC to enable C++20 modules, although options may vary by compiler.

Benefits of Modules in C++

Modules in C++ are largely intended to replace the use of header files in the language.

Modules introduce a more modern, efficient, and safe way to organize and reuse code, solving several problems associated with the traditional header system.

In case I haven’t convinced you yet, let’s look at some of their advantages 👇

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