Re: [boost] [config] [chrono] consexpr workaround

Subject: Re: [boost] [config] [chrono] consexpr workaround
From: Krzysztof Czainski (1czajnik_at_[hidden])
Date: 2013-03-19 12:00:31

2013/3/19 John Maddock <boost.regex_at_[hidden]>

>  Hi,
>>
>> Could/should BOOST_CONSTEXPR expand to inline in C++03?
>>
>> If not, could a macro BOOST_CONSTEXPR_OR_INLINE be added?
>>
>> For one thing, constexpr functions are usually (but maybe not always?)
>> good
>> candidates for inlines. But also please consider the following example.
>> Suppose we put the following code in a header file:
>>
>> struct X { BOOST_CONSTEXPR int fun() const; };
>>
>> BOOST_CONSTEXPR int X::fun() const { return 0; }
>>
>> Now if this file is included in multiple .cpp files, this will fail at
>> link
>> time in C++03 with message "multiple definition of `X::fun()", and it will
>> link fine in C++11. But when BOOST_CONSTEXPR is inline in C++03, the above
>> works.
>>
>> Would making BOOST_CONSTEXPR expand to inline in C++03 do any harm?
>>
>
> Yes, it would break lots of Boost.Multiprecision code that's declared [1]:
>
> inline BOOST_CONSTEXPR T foo();
>

Oh yeah, I didn't think about code, that already uses both BOOST_CONSTEXPR
together with inline ;-)

which begs the question why can't you do the same?
>
> Particularly for templates where the constexpr-ness depends on the
> template argument, this is exactly what you want anyway.
>

Right. So you pretty much answered my initial question, John ;-) Thanks.

My initial reasoning was based on this: I analysed performance of some
C++03 code, that uses Boost.Chrono. And I discovered, that a lot of trivial
functions didn't get inlined. So now, narrowing the question to use of
BOOST_CONSTEXPR in Boost.Chrono:
1. Could the functions marked BOOST_CONSTEXPR be also marked inline (and
this question probably applies to other Boost libraries)?
2. This is probably related only to the TI compiler I use: marking
constructors inline doesn't work for me, they need to be marked
BOOST_FORCEINLINE (I've configured it), so could the constexpr constructors
in Boost.Chrono be marked with BOOST_FORCEINLINE as well?

Cheers,
Kris