Introduction

With the introduction of C++20, modules have become an official part of the C++ language. Modules provide a more efficient way to organize and manage code dependencies, resulting in faster build times and improved code readability. One important feature that modules bring is the ability to use forward declarations, allowing us to declare types and functions before they are defined. In this blog post, we will explore how to implement forward declarations in C++ modules.

Why Use Forward Declarations?

Forward declarations help reduce compilation time by avoiding the need to include large header files that define the complete structure of a class. When using traditional header files, every time a header file is included, the compiler has to process the entire content of that file, even if only a small part of it is required. By using forward declarations, we can provide the compiler with enough information to generate code without needing to include the complete definition.

Forward Declarations in C++ Modules

To implement forward declarations in a C++ module, we follow a similar approach to forward declarations in traditional header files. However, with modules, we need to consider some additional steps to ensure proper usage.

1. Create Module Interface Files

In your module, you will have a module implementation file (*.cppm) and a corresponding module interface file (*.ixx). The module interface file should contain the forward declarations of the types and functions that you want to expose to other modules. For example:

// mymodule.ixx
export module mymodule;

export namespace mymodule {
    export class MyClass;
    export void myFunction();
}

2. Include the Module Interface File

In other modules or translation units that need to use the forward-declared types or functions, we include the module interface file using the import directive. For example:

import mymodule;

// Use the forward-declared types and functions
mymodule::MyClass* obj;
mymodule::myFunction();

3. Define the Forward-Declared Types and Functions

In the module implementation file (*.cppm), include the corresponding module interface file and provide the definition for the forward-declared types and functions. For example:

// mymodule.cppm
#include <mymodule.ixx>

export module mymodule;

export namespace mymodule {
    class MyClass {
        // Class implementation
    };

    void myFunction() {
        // Function implementation
    }
}

4. Compile and Use the Module

When compiling your code, make sure to enable C++20 and the module support flag (e.g., -std=c++20 -fcxx-modules). Compile all the module implementation files separately, producing their corresponding compiled module files (*.pcm). Finally, compile your main program/source file, including the required modules using the import directive.

Conclusion

Using forward declarations in C++ modules can greatly improve build times by reducing the need to include heavy header files. By following the steps outlined in this blog post, you can effectively use forward declarations in your C++ modules. Take advantage of this feature to improve your code organization and optimize your build workflows.

#C++ #Modules