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In this reply https://stackoverflow.com/a/58737595 please, can you explan how it is possible to write : 

return f( f, std::forward<Args>(args)... );

whereas f is only declared as : F f; and we don't know that f is a function accepting 2 parameters a priori. Thank you for your explanation.

I would be grateful for the deep explanations

Dev
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1 Answers1

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It looks like you need some good C++ tutorial/book, specifically about templates.

The piece of code you're referring to is a templated struct.

template<class F>
struct y_combinator {
  F f;
 /* more code */
}

It means that whenever we want to use it we have to specify the template parameter.

y_combinator<int> y_int;

When we use our templated struct like that, the compiler will generate a new struct, let's call it y_combinator_int, and replace all occurences of F with int.

The important thing is that templates use duck typing, if you do something that's illegal/makes no sense, you will get a compile time error (currently such errors are really hard to read, but with C++20 we will probably get more human readable errors).

Overall templates are a very complicated subject, if you want to learn them you need to find a good tutorial/book.

Kaldrr
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  • I have updated the line of code of my first message – Dev Nov 19 '19 at 11:13
  • The anserw remains the same, templates use duck typing. We don't know if F has such operator, but when we specify the type of F, like in my code example, the compiler will generate needed code and functions, which can result in a compilation error – Kaldrr Nov 19 '19 at 11:18
  • [This error message](http://coliru.stacked-crooked.com/a/fbecc120ce01e971) isn't that cryptic: – Caleth Nov 19 '19 at 11:19