Re: Recursion, Yo

On 10/04/2024 03:50, Lawrence D'Oliveiro wrote:

On Tue, 09 Apr 2024 11:44:23 +0100, Ben Bacarisse wrote:
 

It's significant (or at least note worthy) that the code in the
original post was not ISO C ...

 Interesting that GCC’s C compiler allows nested routine definitions,
but the C++ compiler does not.

It is an old extension, going back to when gcc barely (if at all) supported C++.  The compiler middle-end had to have support for nested functions for languages like Pascal, Module 2 and Ada (I believe Ada is the only one of these that stuck around in gcc mainline, but other language front-ends have been made outside the main tree).  Someone thought it might be a useful feature in C too, and perhaps something that would catch on in the standards (several early gcc extensions ended up standardised in C99).
It is not much used in practice, AFAIK.  For some cases the code generation for nested functions was fine and straight-forward.  In other cases, however, it required a trampoline generated on the stack, and that became a real pain once non-executable stacks came into fashion.
Nested functions were never as interesting for C++ as you already have better mechanisms for controlling scope and data access - classes and their methods, including nested classes.  And once lambdas joined the party in C++11 there was absolutely no reason to have nested functions - there is nothing (AFAIK) that you could do with nested functions that you can't do at least as well, and often better, with lambdas.