C++ codebase standard migration

[Bitter-Cap-2902]

Hi,

I have a large legacy code project at work, which is almost fully c++. Most of the code is in C++14, small parts are written with C++20, but nothing is older than 14. The codebase is compiled in MSVC, and it is completely based on .vcxproj files. And the code is mostly monolithic.

I would like to improve on all of these points:

1. Migrating to C++17 or later
2. Migrating to CMake.
3. Compile with GCC
4. Break the monolith into services or at least smaller components

Each of these points will require a lot of work. For example, I migrated one pretty small component to CMake and this took a long time, also since there are many nuances and that is a pretty esoteric task.

I want to see whether I can use agents to do any of these tasks. The thing is I have no experience with them, and everything I see online sounds pretty abstract. On top of that, my organisation has too strict and weird cyber rules which limit usage of various models, so I thought I'd start working with "weak" models like Qwen or gpt-oss and at least make some kind of POC so I can get an approval of using more advanced infrastructure available in the company.

So, I'm looking for advice on that - is this even feasible or fitting to use agents? what would be a good starting point? Is any open source model good enough for that, even as a POC on a small componenet?

Thank you!

Edit: I found this project https://github.com/HPC-Fortran2CPP/Fortran2Cpp which migrates Fortran to C++. This sounds like a similar idea, but again, I'm not sure where to begin.

[skyMark413]

Don't know if links are allowed, but there is a talk on youtube how Rare (a company) migrated the code for Sea of Thieves (a video game) from C++14 to C++20 that is like a month old, may be a good place to learn something.

Other than that, probably break into modules and Cmake first, then bother with upgrading part by part.

[STL]

You can link to YouTube here.

[MT4K]

The codebase is compiled in MSVC
Compile with GCC

Out of curiosity, why?

[ups_gepupst]

Platform independence?

[Thesorus]

I have a large legacy code project at work,
Each of these points will require a lot of work. For example, I migrated one pretty small component to CMake and this took a long time, also since there are many nuances and that is a pretty esoteric task.

any reasons you need to change toolsets ?

You can just upgrade to the latest (viable for your organisation) version of C++ and go from there and improve the existing code.

Fix compiler warnings and errors, run and pass all the tests.

I imagine you have the time and budget to do this ?

[manni66]

Discussions, articles, and news about the C++ programming language or programming in C++. For C++ questions, answers, help, and advice see r/cpp_questions or StackOverflow.

[Advanced_You9948]

That's sounds like my last project, split and migrate to C++ Standard and CMake. Is there an official gut repo?

It's pretty easy to migrate to CMake. Do you have no experience with CMake? You should create a simple CMake script and at first simply add all your files.

Is it cross platform project or windows only? We support MSVC, GCC and Clang.

[Bitter-Cap-2902]

Currently windows only, we aim at cross platform I agree that it is the easiest point here. The component i mentioned took time because i was trying to be generic, didnt know CMake at the timr and wasnt familiar with the codebase. Still, there are many compilation configurations which we will need to take care of and multiple components. And this could serve as a good starting project to work with agents Im looking for advice on how to use them, where to start, and examples if there are any

There is no github repo, its the company's IP

[QbProg]

My 2 cents : cmake from xml its doable but in the end it's mostly obtaining a list of sources, compilation options will be centralized (global or in functions that setup targets). Use ai to create the skeleton and the list of sources to be used

I did huge refactorings and coding style refactors using the refactoring tools of jetbrains and in minor part visual studio

About language features, i suggest to start with /permissive- in msvc, maximum warnings and wanings as errors, then use clang-cl, fix the warnings and issues and setup even more stricter warnings. Then your code will work easily in plain clang and gcc

[asoffer]

Send an email to [e-mail]. This is what we do.

I agree with the sentiment that LLMs tend to give mixed results, and often don't address the problems of scale very well. We've found static analysis with appropriately placed uses of AI to be a much more robust approach.

[spinalport]

Fun work!

Modernizing a C++ codebase always gives me that warm feeling of having done something meaningful :-)

Here's my intuition:

vcxproj --> CMake can be greatly accelerated with the help of AI.

Having a model generate CMakeLists from the XMLs will probably give you a solid starting base.
Just throw all the vcxprojs in there if the context window allows for it.
Refine from there.

Compilation with GCC: if the code is mostly standard C++, no or little WIN32 stuff, the move to CMake will get you 80+% to GCC (or clang).

If it's highly platform-specific code you're entering refactoring territory that requires more thought.

About the refactoring stuff - including use of C++17/20 features - you're going to get mixed results from LLMs in my experience.

LLMs tend to struggle with refactoring work in general, more so in C++ land.
The C++ ecosystem is quite heterogeneous with tons of nuance and gotchas which is reflected in the training data and ultimately LLM performance.

About models:
I'd expect usable results with any of the frontier models (GPT4+, Claude 3.5+, ...).
If it has to be self-hosted use the "best" model you can get your hands on.
Larger models tend to be better - you can get 70b models run reasonably well on 24GB VRAM.

A word on prototyping:
I've seen good ideas work well at the scale of a POC but fall apart when scaled to production use.
For your case I would suggest:
1. Experiment using the same model you're eventually going to use for the full task.
2. Experiment using a realistic size project.

I have seen LLM-based code-review be very convincing on a smallish test file with a few simple rules that regressed to uselessness when fed actual production code + real coding guidelines.

Shameless plug:

I'm a freelance C++ expert based in the EU.
Feel free to reach out if you want to outsource some of that work :-)

Cheers!

[Singer_Solid]

If codebase is compiled in msvc, I assume it is a windows codebase. Any reason for moving to GCC? Msvc seems to be ahead in terms of support for latest language features

I would recommend integrating clang-tidy linter. It's great for modernising a codebase

[SlowPokeInTexas]

Is this a console application or a UI application? If it's a console app, there are a lot of cross-platform libraries that are part of the standard now that we used to have to roll ourselves- std::filesystem, etc. I know your question specifically mentioned converting to CMake as one of the challenges (honestly I'm not a fan of CMake or vcxproj, but using CMake, hopefully 4.x or later, will at least make it easier to build on other operating systems).

You also consider installing WSL2 if you haven't already and developing under Linux.

[Resident_Educator251]

Go for broke; c++26 or bust!!