{"id":5219,"date":"2025-04-01T23:01:32","date_gmt":"2025-04-02T03:01:32","guid":{"rendered":"https:\/\/ikriv.com\/blog\/?p=5219"},"modified":"2025-04-01T23:01:32","modified_gmt":"2025-04-02T03:01:32","slug":"c-regular_invocable-and-generative-ai","status":"publish","type":"post","link":"https:\/\/ikriv.com\/blog\/?p=5219","title":{"rendered":"C++ regular_invocable and generative AI"},"content":{"rendered":"

I took interest in C++ random number generation<\/a> because of the concept of regular_invocable<\/a>. To my understanding, regular_invocable<\/code> is C++’s notion of a “pure function”, or something really close to it.<\/p>\n

I was talking to ChatGPT about C++ concepts, and at some point asked this question:<\/p>\n

“Does the compiler enforce regular_invocable? E.g. if I create a function that returns a random value, and then pass it to a template requiring a regular_invocable, would it cause a compilation error?<\/em>”<\/p>\n

ChatGPT incorrectly responded that the program below will fail to compile:<\/p>\n

\r\n#include <concepts>\r\n#include <iostream>\r\n#include <random>\r\n#include <functional>  \/\/ Include this for std::invoke\r\n\r\n\/\/ Example of a non-deterministic function\r\nint random_func(int x) {\r\n    static std::random_device rd;\r\n    static std::mt19937 gen(rd());\r\n    std::uniform_int_distribution<int> dis(1, 100);\r\n    return dis(gen);  \/\/ Random value returned\r\n}\r\n\r\ntemplate <std::regular_invocable<int> F>\r\nvoid test(F&& f) {\r\n    std::cout << "Result: " << std::invoke(std::forward<F>(f), 10) << std::endl;\r\n}\r\n\r\nint main() {\r\n    \/\/ IVK -- the comments below, generated by ChatGPT, are NOT CORRECT -- \r\n    \/\/ The following line will cause a compilation error\r\n    \/\/ because random_func is not deterministic (it produces a random number)\r\n    test(random_func);  \/\/ Compilation error\r\n}\r\n<\/pre>\n
In fact, it initially responded with an example that was missing an include and a few std::<\/code> snippets, but it was fixed when I complained.<\/p>\n
This ChatGPT response is incorrect. In reality, that compiler does not check that the function in question is pure. At least in Visual std::regular_invocable<\/code> is defined as an alias of std::invocable<\/code>.<\/p>\n
\r\n\/\/ C:\\Program Files\\Microsoft Visual Studio\\2022\\Community\\VC\\Tools\\MSVC\\14.35.32215\\include\\concepts\r\n_EXPORT_STD template <class _FTy, class... _ArgTys>\r\nconcept regular_invocable = invocable<_FTy, _ArgTys...>;\r\n<\/pre>\n
Interestingly, when I asked ChatGPT this question on its own, and not in the middle of a conversation, ChatGPT gave the correct answer. So did Claude, Perplexity and Gemini. There must have been something in the context of our conversation that threw it off.<\/p>\n
Morals of the story:<\/p>\n