C++ class constructors qualified as attribute((pure)) or attribute((const))

Question

Can and should C++ class constructors be declared __attribute__((pure)) if they only can reach data via its parameters? And in which cases should they be qualified as __attribute__((const))?

Err, what's the point? When will an average well-formed program going to invoke a c'tor for the same object twice? — StoryTeller - Unslander Monica, Nov 19 '18 at 13:09
Ahh, I didn't think of the fact that it creates/allocates a new instance. I guess it's not relevant to qualify constructors then. — Nordlöw, Nov 19 '18 at 13:11

score 2 · Accepted Answer · answered Nov 19 '18 at 13:34

GCC warns when you qualify constructors as pure or const. This is because a constructor does not return anything (returns void) and it does not make much sense to have a pure or const attributes on such functions.

See godbolt demo here.

<source>:3:30: warning: 'pure' attribute on function returning 'void' [-Wattributes]
     A()  __attribute__((pure));

                              ^
<source>:8:31: warning: 'const' attribute on function returning 'void' [-Wattributes]
     B()  __attribute__((const));                               ^

From GCC documentation:

const
...
Because a const function cannot have any side effects it does not make sense for such a function to return void. Declaring such a function is diagnosed.

pure
...
Because a pure function cannot have any side effects it does not make sense for such a function to return void. Declaring such a function is diagnosed.

C++ class constructors qualified as __attribute__((pure)) or __attribute__((const))

1 Answers1

C++ class constructors qualified as attribute((pure)) or attribute((const))