C++14 How often should I use constexpr?

Question

I've been reading a book about C++14/11. I just finished reading a chapter about the constexpr keyword. I know what it's used for, but how often should I use constexpr? Should I use it even in code for classes I know will never be used to create contstexpr objects? (Just in case, because it doesn't cost me anything, right?)

Primogeniture

You ask for opinions. From my point if view, you should use it as often as you can. Someone might have different thoughts. – Dayfly

@GeorgeSovetov Problem is that 'opinions' are off-topic here (primarily opinion based category). – Sabrinasabsay

Early and often. And twice on Sundays. – Indoeuropean

@black That's what I'm talking about. – Dayfly

I know this is not the time nor place, but let me just say: I find it dumb that this site considers it off-topic to ask for opinions (even opinions like this), because, why wouldn't i want to hear an opinion of someone who is much better and more experienced than me? – Primogeniture

@TheodorosChatzigiannakis "Opinions aren't off-topic" Huh? – Fulmination

@TheodorosChatzigiannakis Of course every answer is subjective - humans can't reason otherwise. But there is at least a minimum of objectiveness at the bottom: facts rather than opinions. Here I wouldn't know how to write an answer. It is way too dependent on OP's code, something we can't control. – Sabrinasabsay

IOW, how would you say flatmouse's answer is wrong? – Sabrinasabsay

Answer 1

C++14 How often should I use constexpr?

There's an extensive discussion in item 15 (Use constexpr whenever possible) of the Scott Meyer's book Effective Modern C++.

The outline of this item is that constexpr should be used whenever possible, due to the fact that constexpr functions and objects can be used in a wider range of contexts than the non-constexpr ones.

However, in the same item the author mentions a caveat of overusing constexpr. That is, if you decide to qualify an object or a function as constexpr your clients will be allowed to use it in constexpr contents. However, if you later decide that this code must not be constexpr and remove constexpr, this can cause your code not to compile including the side effects that this will have to your clients.

Quoting from original text:

“Use constexpr whenever possible” is your willingness to make a long-term commitment to the constraints it imposes on the objects and functions you apply it to.

Desman

Answer 2

Google's C++ Style Guide suggests:

Use constexpr to specify true constants and the functions that support their definitions. consteval may be used for code that must not be invoked at runtime. Avoid complexifying function definitions to enable their use with constexpr. Do not use constexpr or consteval to force inlining.

It is important to note that without the static keyword, using constexpr on local variables can lead to some performance issues with large arrays. They might be initialized at runtime rather than storing them as data in the executable. constexpr doesn't imply static, only const.

A white paper called Don't constexpr All The Things mentions further drawbacks including the complex set of rules necessary to mark a function as constexpr.

A pragmatic approach might be to limit the usage of constexpr keyword to only compile time functions and variable declarations that support those functions. Marking variables not used in compile time functions as constexpr instead of const has limited usefulness, unless it can be proven that the compiler will output better assembly.

Pacificia