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From: Paavo Helde <eesnimi@osa.pri.ee>
Newsgroups: comp.lang.c++
Subject: Re: ({
Date: Sat, 22 Mar 2025 17:18:58 +0200
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On 22.03.2025 12:53, Alan Mackenzie wrote:
> Paavo Helde <eesnimi@osa.pri.ee> wrote:
>> It appears the statement expressions are a gcc extension which does not
>> even compile in standard C++, and is probably not needed for anything in
>> C++ as there are better options like templated and inlined functions.
>> In C there might be some usage case for it.
> 
> I'm not sure what you meant by templated functions here, but an inline
> function has the disadvantage of fragmenting the code.  Rather than have
> a few lines of code where they're used, you need to look somewhere else
> to see what they do.

At the GCC page for statement expressions they say "This feature is 
especially useful in making macro definitions “safe” (so that they 
evaluate each operand exactly once)" and most of their examples are 
about macros.

A macro is fragmenting the code exactly in the same way as an inline 
function, except the macros are worse than functions in multiple ways. 
In C one might argue macros are needed for supporting multiple types at 
the same time, but in C++ this is solved much better by function templates.

Ergo, a C-style macro using statement expressions can easily replaced 
with a more regular and better behaving function or function template in 
C++, with no increase in code "fragmentation".

For "in-place" use Bonita is right a lambda can be used to the same 
effect, but the readability probably does not go better. As per the 
first example on the GCC page (poor man implementation of abs()):

int a = ({ int y = foo (); int z; if (y > 0) z = y; else z = - y; z; });

can be written with a lambda in C++:

int a = [](){int y = foo (); int z; if (y > 0) z = y; else z = - y; 
return z; }();

I suspect Bonita might like this. I still prefer the more mundane

int a = std::abs(foo());