`std::alignment_of` versus `alignof`

Question

I've just stumbled upon the std::alignment_of type trait, and its soon-to-be friend std::alignment_of_v. They seem to have been specifically designed to be equivalent to a plain call to alignof, and the future addition of the _v helper indicates that it's not just a legacy bit.

What is the use of std::alignment_of (_v), when we have alignof ?

Fulmination

To have true error when use with expression instead of use gnu-alignof-expression :-). – Botulism

Answer 1

They are almost completely redundant. As @Revolver noted, they were introduced in different papers, and alignment_of comes from boost nearly verbatim.

But that does not mean the trait is useless.

A template<class...>class can be passed to other templates and used with metaprogramming. Operators like alignof cannot: you would have to write the template<class>class alignment_of before you could pass it to metaprogramming facilities.

Now the same could be said of sizeof needing a std::size_of<class> template.

...

The addition of _v was because they swept every ::value integral_constant-type template in std and added a _v variable template. Considering which ones where worthy and which not would be bikeshed painting and nearly pointless: it is easier to do every one than spend effort picking the worthy ones to do. It being done is not evidence the feature is not obsolete.

Andro

The usefulness of the metafunction itself hadn't crossed my mind. That is a very good point, thank you ! – Fulmination

Answer 2

The std::alignment_of is introduced as port of Boost type traits library. It predates C++11 and alignof keyword. That trait was superseded by alignof operator, but it was kept mainly for compatibility purposes (so you could just replace boost:: with std::) and _v variable alias is introduced for consistency with other parts of the library.

Rhombohedral

From your answer it seems as if they were introduced separately, when they were not (both since C++11). – Bennybenoit

@black technically they were introduced separately (in different papers). The point of my answer was that it was a part of pre-C++11 library before inclusion in standard. – Rhombohedral

alignment_of was in TR1. – Busywork

Answer 3

One nice thing I found in practice is that alignof is simply syntactically invalid for old code, whereas alignment_of is just a standard template. So you can stay compatible with older compilers by providing an implementation of alignment_of using whatever alternative syntax your compiler accepts, then use that everywhere.

Shammy

Can you give an example where alignof would've been syntactically incorrect but alignment_of would've worked? – Revisionism

@AyxanHaqverdili: Anywhere in old (pre-C++11) code. – Shammy

Oh, if you're bringing in your own implementation that just masks compiler-specific code in pre-C++11 code, you could also implement alignof as a macro. – Revisionism

@AyxanHaqverdili: Sure, you can implement lots of things as a macro. – Shammy