In today's post, I would like to continue talking about forward declared or better incomplete types in C++. I discussed some scenarios...

Andreas Fertig's blog offers insights into C++ development, particularly focusing on the C++ Standard Library and language features. Readers can find tutorials, code examples, and explanations covering topics such as lambdas, smart pointers, and C++20 enhancements. Additionally, they can learn about best practices for writing modern, idiomatic C++ code and leveraging the full power of the language.

Andreas Fertig

Explores how C++20 concepts behave with forward-declared (incomplete) types, revealing a subtle gotcha: using `auto` return types forces eager template instantiation, causing concepts to evaluate before the complete type definition is available. This can lead to unexpected behavior where the same concept produces different results depending on when it's evaluated during compilation. The article demonstrates this through practical examples showing how a concept checking for a member function can fail when the type is still incomplete.

C++20s concepts with a forward declared type