Adventures in C++

Adventures in C++

My experiences in learning C++


C++20 and Modules

One of the 4 big new features introduced in C++20 is the use of modules. modules are to replace the old #include method.


there is basic modules support since gcc-11 (when using the -fmodules-ts compiler option like:

g++ -fmodules-ts -std=c++20 helloworld.cpp -o helloworld

however this will fail since the module for <print> is not compiled yet.

g++ -fmodules-ts -std=c++20 -c -x c++-system-header print

This should generate a directory called gcm.cache and buried deep in that folder structure will be the print.gcm file (but you can leave it there. The compiler knows where to look.

if you compile a module you've written yourself it will be placed in gcm.cache as-well.


Very basic Example using C++20 modules

The module interface file several sources use the file extension cppm (clang) or ixx (Visual Studio), the gcc compiler (at this moment) does not support these extensions because they feel there is no need for another set of file extension. to distinguish between the interface and implementation files I use the file extension cxx for the module interface file and just cpp for the implementation.

The module implementation file contains the actual code.

the main source file imports the required module and uses it's sayHello function.

These files need to be compiled in the correct order to avoid errors during compilation, My workflow is as follows

  1. /opt/gcc-15/bin/g++ -fmodules-ts -std=c++23 -c -x c++-system-header -static-libstdc++ print
  2. /opt/gcc-15/bin/g++ -fmodules-ts -std=c++23 -c -static-libstdc++ hello.cxx
  3. /opt/gcc-15/bin/g++ -fmodules-ts -std=c++23 -static-libstdc++ hello.cxx hello.cpp main.cpp -o helloworld

In this example I use -std=c++23 and not -std=C++20, the reason for this is that print is unknown (lat least at this moment) by the std library. If you want to use c++20 you can import <iostream>.

The spaceship operator

The three-way-comparison a.k.a. the Spaceship operator ( <=> ) is another new thing in the language since C++20. It basically does the three size comparison operations in one. This is especially useful in situations where doing the comparisons separately is resource heavy.

import <compare>;
import <print>;
int main() {
a simple example:
int value{42};
std::strong_ordering result { value <=> 19 }
std::println("value < 19: {}", result == std::strong_ordering::less};
std::println("value < 19: {}",result == std::strong_ordering::greater);
std::println("value = 19: {}",result == std::strong_ordering::equal);
return 0;
}

C++23 and Modules

C++23 introduces a new way to import modules, you no longer need to import individual modules (like <print> for example but you can just import std; and get the entire Standard Library.


(At the moment of posting, this is not yet possible with my compiler [a snapshot of the most recent GCC-15] so at the moment we will still need to import the modules separately..

About

I'm trying to learn Modern C++ by several books and websites.


The most common sources are:

Beginning C++20 (978-1-4842-5884-2)
- Ivor Horton
- Peter van Weert

Professional C++ 6th Edition (978-1-394-19317-2)
- Marc Gregoire

CloudyWizzards

Adventures in C++